The most characteristic reactions of saturated hydrocarbons are the substitution reactions of hydrogen atoms. They follow a chain, free radical mechanism and usually occur in the light or when heated. The replacement of a hydrogen atom with a halogen occurs most easily at the less hydrogenated tertiary carbon atom, then at the secondary one, and lastly at the primary one. This pattern is explained by the fact that the binding energy of the hydrogen atom with the primary, secondary and tertiary carbon atoms is not the same: it is 415, 390 and 376 kJ/mol, respectively.
Let us consider the mechanism of the reaction of bromination of alkanes using the example of methylethyl isopropylmethane:

Under normal conditions, molecular bromine practically does not react with saturated hydrocarbons. Only in the atomic state is it capable of tearing out a hydrogen atom from an alkane molecule. Therefore, it is first necessary to break the bromine molecule into free atoms, which initiate a chain reaction. This rupture occurs under the influence of light, that is, when light energy is absorbed, the bromine molecule disintegrates into bromine atoms with one unpaired electron.

This type of decomposition of a covalent bond is called homolytic cleavage (from the Greek homos - equal).
The resulting bromine atoms with an unpaired electron are very active. When they attack an alkane molecule, a hydrogen atom is abstracted from the alkane and a corresponding radical is formed.

Particles that have unpaired electrons and therefore have unused valencies are called radicals.
When a radical is formed, a carbon atom with an unpaired electron changes the hybrid state of its electron shell: from sp 3 in the original alkane to sp 2 in the radical. From the definition of sp 2 - hybridization it follows that the axes of the three sp 2 - hybrid orbitals lie in the same plane, perpendicular to which the axis of the fourth atomic p - orbital, not affected by hybridization, is located. It is in this unhybridized p-orbital that the unpaired electron in the radical is located.
The radical formed as a result of the first stage of chain growth is further attacked by the original halogen molecule.

Taking into account the planar structure of the alkyl, the bromine molecule attacks it equally likely from both sides of the plane - from above and from below. In this case, the radical, causing homolytic cleavage in the bromine molecule, forms the final product and a new bromine atom with an unpaired electron, leading to further transformations of the initial reagents. Considering that the third carbon atom in the chain is asymmetric, depending on the direction of attack of the bromine molecule on the radical (from above or below), the formation of two compounds that are mirror isomers is possible. Superimposition of models of these resulting molecules on top of each other does not lead to their combination. If you change any two balls - connections, then the combination is obvious.
Chain termination in this reaction can occur as a result of the following interactions:

Similar to the considered bromination reaction, the chlorination of alkanes is also carried out.”

To study the reaction of chlorination of alkanes, watch the animated film “Mechanism of the reaction of chlorination of alkanes” (this material is available only on CD-ROM).

2) Nitration. Despite the fact that under normal conditions alkanes do not interact with concentrated nitric acid, when they are heated to 140°C with dilute (10%) nitric acid under pressure, a nitration reaction occurs - the replacement of a hydrogen atom with a nitro group (M.I. Konovalov’s reaction ). All alkanes enter into a similar liquid-phase nitration reaction, but the reaction rate and yields of nitro compounds are low. The best results are observed with alkanes containing tertiary carbon atoms.

The nitration reaction of paraffins is a radical process. The usual substitution rules discussed above apply here as well.
Note that vapor-phase nitration - nitration with nitric acid vapor at 250-500°C - has become widespread in industry.

3) Cracking. At high temperatures in the presence of catalysts, saturated hydrocarbons undergo splitting, which is called cracking. During cracking, carbon-carbon bonds are homolytically broken to form saturated and unsaturated hydrocarbons with shorter chains.

CH 3 –CH 2 –CH 2 –CH 3 (butane) –– 400° C ® CH 3 –CH 3 (ethane) + CH 2 =CH 2 (ethylene)

An increase in the process temperature leads to deeper decomposition of hydrocarbons and, in particular, to dehydrogenation, i.e. to the elimination of hydrogen. Thus, methane at 1500ºС leads to acetylene.

2CH 4 –– 1500° C ® H–C º C–H(acetylene) + 3H 2

4) Isomerization. Under the influence of catalysts, when heated, hydrocarbons of normal structure undergo isomerization - rearrangement of the carbon skeleton with the formation of branched alkanes.

5) Oxidation. Under normal conditions, alkanes are resistant to oxygen and oxidizing agents. When ignited in air, alkanes burn, turning into carbon dioxide and water and releasing large amounts of heat.

CH 4 + 2O 2 –– flame ® CO 2 + 2H 2 O
C 5 H 12 + 8O 2 –– flame ® 5CO 2 + 6H 2 O

Alkanes are valuable high-calorie fuels. The combustion of alkanes produces heat, light, and also powers many machines.

Application

The first in the series of alkanes, methane, is the main component of natural and associated gases and is widely used as industrial and household gas. It is processed industrially into acetylene, carbon black, fluorine and chlorine derivatives.
The lower members of the homologous series are used to obtain the corresponding unsaturated compounds by dehydrogenation reaction. A mixture of propane and butane is used as household fuel. The middle members of the homologous series are used as solvents and motor fuels. Higher alkanes are used to produce higher fatty acids, synthetic fats, lubricating oils, etc.

Unsaturated hydrocarbons (alkynes)

Alkynes are aliphatic unsaturated hydrocarbons, in the molecules of which there is one triple bond between the carbon atoms.

Hydrocarbons of the acetylene series are even more unsaturated compounds than their corresponding alkenes (with the same number of carbon atoms). This can be seen by comparing the number of hydrogen atoms in a row:

C 2 H 6 C 2 H 4 C 2 H 2

ethane ethylene acetylene

(ethene) (ethene)

Alkynes form their own homologous series with a general formula, like diene hydrocarbons

C n H 2n-2

Structure of alkynes

The first and main representative of the homologous series of alkynes is acetylene (ethyne) C 2 H 2. The structure of its molecule is expressed by the formulas:

Н-СºС-Н or Н:С:::С:Н

By the name of the first representative of this series - acetylene - these unsaturated hydrocarbons are called acetylene.

In alkynes, the carbon atoms are in the third valence state (sp-hybridization). In this case, a triple bond appears between the carbon atoms, consisting of one s-and two p-bonds. The length of the triple bond is 0.12 nm, and the energy of its formation is 830 kJ/mol.

Nomenclature and isomerism

Nomenclature. According to systematic nomenclature, acetylene hydrocarbons are named by replacing the suffix -an in alkanes with the suffix -in. The main chain must include a triple bond, which determines the beginning of numbering. If a molecule contains both a double and a triple bond, then preference is given to the double bond in numbering:

Н-СºС-СН 2 -СН 3 Н 3 С-СºС-СН 3 Н 2 С=С-СН 2 -СºСН

butine-1 butine-2 2-methylpentene-1-yne-4

(ethylacetylene) (dimethylacetylene)

According to rational nomenclature, alkyne compounds are called acetylene derivatives.

Unsaturated (alkyne) radicals have trivial or systematic names:

Н-СºС- - ethynyl;

NSºС-CH 2 - -propargyl

Isomerism. The isomerism of alkyne hydrocarbons (as well as alkene hydrocarbons) is determined by the structure of the chain and the position of the multiple (triple) bond in it:

N-CºC-CH-CH 3 N-CºC-CH 2 -CH 2 -CH 3 H 3 C-C=C-CH 2 -CH 3

3-methylbutin-1 pentine-1 pentine-2

Preparation of alkynes

Acetylene can be produced in industry and in the laboratory in the following ways:

1. High-temperature decomposition (cracking) of natural gas - methane:

2СН4 1500°C ® НСºСН + 3Н 2

or ethane:

С 2 Н 6 1200°C ® НСºСН + 2Н 2

2. By decomposing calcium carbide CaC 2 with water, which is obtained by sintering quicklime CaO with coke:

CaO + 3C 2500°C ® CaC 2 + CO

CaC 2 + 2H 2 O ® HCºCH + Ca(OH) 2

3. In the laboratory, acitylene derivatives can be synthesized from dihalogen derivatives containing two halogen atoms at one or adjacent carbon atoms by the action of an alcoholic alkali solution:

H 3 C-CH-CH-CH 3 + 2KOH ® H 3 C-CºC-CH 3 + 2KBr + 2H 2 O

2,3-dibromobutane butine-2

(dimethylacetylene)

Related information.

Alkynes - These are unsaturated hydrocarbons whose molecules contain a triple bond. Representative - acetylene, its homologues:

General formula - CnH 2 n -2 .

Structure of alkynes.

The carbon atoms that form a triple bond are in sp- hybridization. σ - the bonds lie in a plane, at an angle of 180 °C, and π -bonds are formed by overlapping 2 pairs of non-hybrid orbitals of neighboring carbon atoms.

Isomerism of alkynes.

Alkynes are characterized by isomerism of the carbon skeleton and isomerism of the position of the multiple bond.

Spatial isomerism is not typical.

Physical properties of alkynes.

Under normal conditions:

C 2 -C 4- gases;

From 5 to 16- liquids;

From 17 and more - solids.

The boiling points of alkynes are higher than those of the corresponding alkanes.

Solubility in water is negligible, slightly higher than that of alkanes and alkenes, but still very low. Solubility in non-polar organic solvents is high.

Preparation of alkynes.

1. The elimination of 2 hydrogen halide molecules from dihalohydrogen atoms, which are located either at neighboring carbon atoms or at one. Cleavage occurs under the influence of an alcoholic alkali solution:

2. The effect of haloalkanes on salts of acetylene hydrocarbons:

The reaction proceeds through the formation of a nucleophilic carbanion:

3. Cracking of methane and its homologues:

In the laboratory, acetylene is obtained:

Chemical properties of alkynes.

The chemical properties of alkynes are explained by the presence of a triple bond in the alkyne molecule. Typical reaction for alkynes- an addition reaction that occurs in 2 stages. At the first, the addition and formation of a double bond occurs, and at the second, the addition to the double bond occurs. The reaction of alkynes proceeds more slowly than that of alkenes, because the electron density of the triple bond is “spread out” more compactly than that of alkenes and is therefore less accessible to reagents.

1. Halogenation. Halogens add to alkynes in 2 stages. For example,

And in total:

Alkynes just as alkenes decolorize bromine water, so this reaction is also qualitative for alkynes.

2. Hydrohalogenation. Hydrogen halides are somewhat more difficult to attach to a triple bond than to a double bond. To accelerate (activate) the process, use a strong Lewis acid - AlCl 3 . From acetylene under such conditions it is possible to obtain vinyl chloride, which is used to produce the polymer - polyvinyl chloride, which is of great importance in industry:

If hydrogen halide is in excess, then the reaction (especially for unsymmetrical alkynes) proceeds according to Markovnikov’s rule:

3. Hydration (addition of water). The reaction occurs only in the presence of mercury (II) salts as a catalyst:

At the 1st stage, an unsaturated alcohol is formed, in which the hydroxy group is located at the carbon atom forming the double bond. Such alcohols are called vinyl or phenols.

A distinctive feature of such alcohols is instability. They isomerize into more stable carbonyl compounds (aldehydes and ketones) due to proton transfer from HE-groups to carbon at a double bond. Wherein π -the bond breaks (between carbon atoms), and a new one is formed π -bond between carbon atoms and oxygen atom. This isomerization occurs due to the higher density of the double bond C=O compared with C=C.

Only acetylene is converted into aldehyde, its homologues into ketones. The reaction proceeds according to Markovnikov's rule:

This reaction is called - Kucherov's reactions.

4. Those alkynes that have a terminal triple bond can abstract a proton under the action of strong acidic reagents. This process is due to strong bond polarization.

The reason for polarization is the strong electronegativity of the carbon atom in sp-hybridization, so alkynes can form salts - acetylenides:

Copper and silver acetylenides are easily formed and precipitate (when acetylene is passed through an ammonia solution of silver oxide or copper chloride). These reactions are quality to the terminal triple bond:

The resulting salts easily decompose when exposed to HCl, As a result, the starting alkyne is released:

Therefore, alkynes are easy to isolate from a mixture of other hydrocarbons.

5. Polymerization. With the participation of catalysts, alkynes can react with each other, and depending on the conditions, various products can be formed. For example, under the influence of copper (I) chloride and ammonium chloride:

Vinylacetylene (the resulting compound) adds hydrogen chloride, forming chlorprene, which serves as a raw material for the production of synthetic rubber:

6. If acetylene is passed through coal at 600 ºС, an aromatic compound is obtained - benzene. From acetylene homologues, benzene homologues are obtained:

7. Oxidation and reduction reaction. Alkynes are easily oxidized by potassium permanganate. The solution becomes discolored because the parent compound has a triple bond. During oxidation, the triple bond is cleaved to form a carboxylic acid:

In the presence of metal catalysts, reduction with hydrogen occurs:

Application of alkynes.

Alkynes are used to produce many different compounds that are widely used in industry. For example, isoprene is obtained - the starting compound for the production of isoprene rubber.

Acetylene is used for welding metals, because... its combustion process is very exothermic.

Transcript

1 147 UDC; Bromination and iodochlorination of acetylenes A.A. Selina, S.S. Karlov, G.S. Zaitseva (Department of Organic Chemistry) Literary data on the reactions of bromination and iodochlorination of acetylenes are discussed. The results of a study of the halogenation reactions of element(si, Ge, Sn) substituted phenylacetylenes are presented. To date, quite a large number of works have been accumulated in the literature, the subject of research of which is the preparation of vicinal 1,2-dihalogenalkenes. This class of compounds is interesting primarily from the point of view of synthesis, which is associated with wide possibilities for further functionalization of molecules by replacing the halogen atom. What is important is their potential in cross-coupling reactions currently widely used in organic synthesis. In the case of 1-iodo-2-chloroalkenes, due to the significant difference in the energies of the l and I bonds, such a replacement can be carried out selectively. 1. BROMINATION REACTIONS 1.1. Bromination of acetylenes with molecular bromine Most early works investigated the interaction of bromine with acetylenes in acetic acid. The choice of such a solvent can be explained by the possibility of direct comparison of the results obtained with data on the bromination of olefins, the electrophilic addition of bromine to which had been quite well studied by that time. Later, reports appeared in the literature about reactions of acetylenes with 2 /MeH, 2 /MeH/H 2, 2 /H 3 H 3 /H 2, 2 /Hl 3, 2 /lh 2 H 2 l. The role of the solvent is nucleophilic solvation, promoting charge separation in the resulting transition state, and selective electrophilic solvation of the leaving bromide ion, with the latter making a more significant contribution to the overall solvent participation. It turned out that the transition from a less polar to a more polar solvent is accompanied by a significant increase in the rate of interaction, regardless of the nature of the substituents at the triple bond. In addition, the nature of the solvent significantly affects not only the ease, but also the direction of the bromination process, so it makes sense to consider the patterns of this reaction in each individual case. Interaction of acetylenes with 2 in acetic acid As shown in Scheme 1, the interaction of bromine with substituted acetylenes in acetic acid can lead to the formation of a total of six compounds. Bromoacetylene 1 is obtained only in the case of terminal alkynes, i.e. at 2 = H. Bromoacetates 4 Scheme / AcH Ac Ac VMU, chemistry, 3

2 148 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY T and 5 are formed regiospecifically in accordance with Markovnikov's rule, so that for phenylacetylene derivatives the formation of only 1-acetoxy-1-phenyl products is observed. The stereochemistry of compounds 2 and 3 was established on the basis of their dipole moments, taking into account the fact that this value for the cis isomer is much higher than for the trans isomer. Dibromoketone (6) is formed as a result of bromination of bromoacetates 4 and 5 and therefore can be considered as a secondary product of the reaction. All compounds are formed under kinetic control conditions, since in control experiments under reaction conditions no isomerization or further conversion of 1,2-dibromo derivatives to form bromoacetates or tetrabromo derivatives was observed. The composition and percentage of reaction products depend primarily on the structure of the original acetylenes. For phenylacetylene and methylphenylacetylene, non-stereospecific formation of dibromides 2 and 3 with a predominance of the trans isomer is observed, as well as the formation of a large amount (14-31% depending on the concentration of bromine and acetylene) of products 4, 5, 6. The addition of Lil 4 to the solution has little effect on the ratio of trans and cis dibromides in these compounds. It should be noted the special behavior of 4-methylphenylacetylene under the same conditions. While bromine, as in the case of phenylacetylene and methylphenylacetylene, adds non-stereospecifically to form approximately equal amounts of trans- and cis-isomers (56:44), 4-methylphenylacetylene does not produce solvent insertion products and elimination product 1 at all. In addition, the addition Lil 4 noticeably changes the ratio of trans- and cis-dibromoalkenes in favor of the cis isomer (56:44 changes to 42:58 with the addition of 0.1 M Lil 4). The results obtained for alkylacetylenes differ significantly from the behavior of phenylacetylenes described above. When bromination of both 3-hexine and 1-hexine occurs, only transdibromides are formed. This is consistent with the results of studies where it is reported that treatment of acetylene itself, propine, 3-hydroxypropine and 3-hydroxy-3-methylbutyne with bromine gives exclusively trans-addition products under conditions favorable for the reaction to occur via the ionic mechanism. In addition to the structure of acetylenes, the composition of the medium can have a significant influence on the ratio of reaction products. Thus, when adding salts containing bromide ion (in particular, when adding Li), in the case of phenyl-substituted acetylenes, a noticeable decrease (up to complete disappearance) is observed in the amount of bromoacetates and a strong increase (up to 97-99%) in the amount of trans-dibromides. The structure of acetylenes has a significant impact not only on the stereochemistry of the resulting compounds, but also on the rate of electrophilic addition of bromine to the triple bond. The relationship between the structure and reactivity of alkynes is discussed in detail in the work, in which the kinetics of bromination in acetic acid at 25 C was studied for acetylene and 16 of its derivatives. The data obtained showed that the replacement of one hydrogen atom in acetylene with an alkyl group leads to an increase in the reaction rate in times depending on the introduced substituent. The replacement of both hydrogen atoms leads, as a rule, to a further increase in the rate of bromination. The opposite trend is observed only in the cases of di(tert-butyl)acetylene and diphenylacetylene. The effect of substitution of the second hydrogen atom in acetylene by a second tert-butyl group, leading to a decrease in the reaction rate, is attributed to the occurrence of steric hindrance, and a similar slowdown of the process in the case of diphenylacetylene compared to phenylacetylene may be due to the negative inductive effect of the second phenyl group of tolane. Although one of the first works noted that acetylene compounds containing substituents with pronounced I- and M-effects can add bromine via a nucleophilic mechanism, nevertheless, the bromination of most acetylenes in acetic acid is an electrophilic process and proceeds via an ionic mechanism. This mechanism includes at least two stages: 1) the formation of a charged intermediate, the structure of which is determined by the nature of the substituents at the triple bond, 2) the interaction of this intermediate with a nucleophile, leading to the formation of reaction products. Initially, it was believed that the transition state from which the intermediate is subsequently formed is different for alkyl- and phenyl-substituted acetylenes. This assumption was confirmed by data on the reactivity of alkynes and the stereochemistry of the final products.

3 The kinetic equation of the process under consideration contains terms of both first and second order in bromine. This means that the reaction mechanism can involve both bimolecular and trimolecular transition states, the contribution of each of which is determined by the concentration of bromine in the solution. d[ 2 ]/dt = k 2 [A] [ 2 ] + k 3 [A] [ 2 ] 2. Below is a more detailed description of the possible mechanisms of interaction of bromine with the triple bond of acetylenes. 1. Mechanism of electrophilic addition of bromine to phenylacetylenes It is assumed that in the case of bromination of phenylacetylenes, the limiting step is the formation of open vinyl cation 8, which occurs through transition state 7 (Scheme 2). This assumption is in accordance with the kinetic data presented in the work, from which it follows that the bromination rate changes slightly as a result of the replacement of the hydrogen atom at the triple bond in phenylacetylene with a methyl or ethyl group. In other words, the influence of β-substituents on the formation of a cation stabilized by a phenyl group is very small. This allows us to conclude that in the transition state the acetylene carbon atom -2 has a very small positive charge, which is in good agreement with the structure of the open vinyl cation. 149 In connection with the increased interest in the structure, reactivity and stability of vinyl cations in the late 60s and early 70s, data were obtained from which it follows that linear structures of type 8 with sp-hybridization at the cationic center are more preferable than any of the curved structures 9a or 9b with sp 2 hybridization (Scheme 3). This is supported by theoretical calculations of molecular orbitals, which show that the curved form is less stable than the linear form by kcal/mol. These results suggest that for reactions in which a vinyl cation is formed adjacent to a phenyl substituent, the phenyl ring is directly conjugated to the vacant p-orbital on the α-carbon atom, as in 10a, rather than to the remaining π-bond of the vinyl system. as in 10b (Scheme 4). The mechanism proposed for the process with third-order kinetics involves the formation of trimolecular transition state 11, in which a second bromine molecule acts as a catalyst promoting heterolytic bond cleavage (Scheme 5). From the above diagrams it is clear that intermediate 8, from which the reaction products are obtained, is the same for both bimolecular and trimolecular processes. This is in good agreement with experimental data, according to which varying bromine concentration over a wide range does not lead to a change in percentage Scheme 2 δ+ 2 / AcH 7 δ = Scheme 3 9a 9b 3 VMU, chemistry, 3

4 150 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY T Scheme 4 10a H 10b H Scheme 5 2 / AcH δ+ = δ δ 2 11 Scheme 6 = "2" " 12 13, " = H or alk th ratio of reaction products (within experimental error). In other words, both processes lead to the same distribution of bromination products. In a second rapid step, the vinyl cation reacts non-stereospecifically with either the bromideione or the solvent acetic acid to give 1,2-dibromide or bromoacetate, respectively, with the cis or trans configuration. 2. Mechanism of electrophilic addition of bromine to alkyl acetylenes As shown in Scheme 6, in the case of alkyl acetylenes, the rate-determining stage of the entire process is the formation of a cyclic bromyrenium ion (13) occurring through a bridging transition state (12). There are several factors in favor of such an intermediate. The works note that alkylvinyl cations are less stable than phenylvinyl cations, therefore, in the case of alkyl-substituted acetylenes, the participation of bromine in the delocalization of the positive charge is more preferable. The more negative activation entropy value for 3-hexine (40 e.u.) compared to phenylacetylenes (30 e.u.) corresponds to a more ordered transition state. Finally, from the kinetic data on the bromination of alkyl-substituted acetylenes, we can conclude that in the transition state the positive charge is uniformly distributed over both acetylene carbon atoms, which also corresponds to the bridging structure.

5 In the second fast step, the bromine ion reacts stereospecifically with the bromide ion to produce exclusively trans dibromide; This is quite consistent with the experimentally observed absence of cis-addition products and the formation of a trans-bromine addition product with almost 100% stereospecificity. 3. The mechanism of bromination of acetylenes in the presence of lithium bromide When a bromide ion is added to a solution, a tribromide anion is formed, and an equilibrium is established between these ions: This process leads to a decrease in the concentration of free bromine in the solution, therefore, in the presence of lithium bromide, the interaction of acetylene with molecular bromine According to the bimolecular mechanism, it makes only a minor contribution to the overall result of the reaction. Theoretically, two reaction paths are possible under the conditions under consideration: attack by molecular bromine, catalyzed by bromide ion, and direct electrophilic attack by tribromide anion. These two processes are described by the same reaction rate equation and are therefore kinetically indistinguishable. However, according to the authors of the works, the results of a study of the bromination of a number of phenyl-substituted acetylenes in acetic acid clearly indicate that in the case of acetylenes, the process catalyzed by bromide ion is more likely. As shown in Scheme 7, this process proceeds according to the mechanism of trimolecular electrophilic addition of Ad E 3 through a transition state (14). δ 14 This transition state is supported by the complete trans-stereospecificity of the formation of 1,2-dibromide and a noticeable decrease in the amount of bromoacetate when salts containing bromide ion are added to the solution. At the same time, the observed changes in the composition of the reaction products would be difficult to explain based on the δ Scheme of direct electrophilic attack of the substrate by tribromide ion. Taking into account the different structures of transition states (7) and (14) for direct electrophilic attack by molecular bromine and attack catalyzed by bromide ion, one should expect certain differences in the patterns of the influence of substituents on the reactivity of acetylenes. The transition state (14) implies the synchronous formation of a bond with both the electrophile (2) and the nucleophile (). It can be assumed that with an increase in the electron donating capacity of the substituent in the phenyl ring, the formation of a bond between the electrophile and the substrate will precede the formation of a bond between the nucleophile and the substrate, since the formation of a positive charge on the α-carbon atom is more preferable. For electron-withdrawing substituents, on the contrary, the formation of a nucleophile substrate bond occurs earlier. Thus, both types of substituents should speed up the reaction. Unfortunately, the analysis of experimental data raises some doubts about the correctness of such reasoning, since in the entire range of the studied substituents (4-Me, 3,4-benzo, 4-fluorine, 4-bromo, 3-chloro) the minimum reactivity was not achieved Bromination of acetylenes with bromine in alcohols The work reports that the bromination of 1-hexine leads to the production in high yield of only the corresponding 1,2-dibromo derivative, regardless of whether the reaction is carried out in l 4 or in methanol. Later, the authors refuted this statement by studying in detail the interaction of a number of substituted acetylenes with an equimolar amount of bromine at room temperature in methanol. It has been shown (Scheme 8) that the result of the reaction is the formation of dibromodimethoxyalkanes (16) in high yields (from 52 to 79%), while isomeric dibromoalkenes (15) are formed only in small quantities (from 0 to 37% depending on the conditions reaction and the nature of substituents at the triple bond). It was found that lowering the temperature to 60 C, using a twofold excess of bromine, and increasing the amount of solvent do not lead to significant changes in the ratio of reaction products. The absence of bromomethoxyalkenes is probably due to the fact that enol ethers are more reactive 4 VMU, chemistry, 3

6 152 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY T SCHEMA 8 "Me 2 / MeH " + + " =, n-bu, n-hex Me " = H, Me E-15 Z " more capable of electrophilic addition than the original acetylenes. Replacing methanol with ethyl alcohol leads to a noticeable increase in the amount of E-(15) (from 7 to 13% for phenylacetylene) and a noticeable decrease in the amount of compound (16) (from 79 to 39% for phenylacetylene).When using isopropanol or tert- butyl alcohol, the only reaction products are isomeric dibromoalkenes (15).Carrying out the reaction under consideration in ethylene glycol leads to the fact that the attack of the second alkoxy group of the alcohol occurs intramolecularly and for phenylacetylene the formation of only 2-(dibromomethyl)-2-phenyl-1,3-dioxolane is observed Dibromoalkenes (15) under these conditions are obtained in trace amounts Bromination of acetylenes with bromine in haloalkanes The work discusses the stereochemistry of the interaction of a number of acetylenes with molecular bromine in chloroform under conditions of kinetic and thermodynamic control. As shown in Scheme 9, the reaction product in this case is a mixture of two isomeric dibromoalkenes (17). The reaction proceeds almost quantitatively in the case of = and with good yield when = alk. The ratio of isomers, as in previous cases, strongly depends on the process conditions. Kinetic control conditions are realized with a relatively short reaction time, relatively low temperatures, and using equimolar amounts of bromine and acetylene. In these cases, almost all acetylenes give mainly trans-dibromide. The only exception is tert-butylphenylacetylene, for which selective cis-addition leads to the formation of cis-dibromide as the main or only reaction product. Longer reaction times, higher temperatures, and higher molar ratios of bromine and acetylene meet thermodynamic control conditions and lead to an increase in the proportion of cis isomer without significantly affecting the overall product yield. For tert-butylphenylacetylene, a reverse transition of the initially formed cis-isomer to the trans-isomer is observed, and in the case of isopropylphenylacetylene, when changing the kinetic control of the reaction to thermodynamic control, no significant changes in the isomer ratio occur. It has been established that a thermodynamically equilibrium mixture of isomeric dibromoalkenes is usually formed after 48 hours when using a 10-fold excess of 2, although in some cases only a small excess is sufficient. These experimental data are consistent with the known fact of isomerization of dihaloalkenes under the action of bromine as a catalyst. In the case of alkylphenylacetylenes, a thermodynamically equilibrium mixture of isomers can also be easily obtained by irradiating the reaction mixture with ultraviolet light, even if bromine is taken in an equimolar amount relative to acetylene. This method cannot be used for alkylacetylenes and dialkylacetylenes because the yield of reaction products is too low. However, a thermodynamically controlled isomer ratio for these acetylenes can still be obtained by irradiating a chloroform solution of already isolated compounds with UV light (17). In each case, equilibrium mixtures of reaction products are formed after irradiation for 30 minutes at room temperature of mixtures of cis- and trans-isomers of any composition; the total yield of the starting compounds is more than 80%. Expressed

7 153 Scheme 9 " " =, alk " = H, alk 2 / Hl 3 + " E-17 Z-17 Scheme 1 0 " δ+ 18 assumption that bromination of acetylenes with molecular bromine occurs through the formation of a reactive intermediate (18), which is an open vinyl cation in which bromine weakly interacts with the benzyl carbon atom (Scheme 10). This conclusion about the interaction of bromine with the neighboring carbocation center was made from the analysis of experimental data, according to which the stereospecificity of the formation of the trans-isomer in the case of phenylacetylenes, it naturally decreases upon halogenation with iodine, bromine and chlorine. This is explained by a decrease in the degree of interaction in the series I >> > l. If in the case of iodine a cyclic iodonium ion is formed, then in the case of bromine an open vinyl cation is obtained, in which bromine only weakly interacts with an adjacent carbon atom, and when halogenated with chlorine, the intermediate is an almost completely exposed vinyl cation.The reason for the rather unusual high cis-stereospecificity of the halogenation of tert-butylphenylacetylene may be the fact that the attack of the anion must occur in the plane that contains the bulky tert-butyl group. In the course of studying the interaction of a number of acetylenes H (19) (=, H 2, H 2 H, H(H)H 3, H 3) with bromine adsorbed on the surface of graphite, it turned out that the presence of graphite leads to stereoselective bromination with the formation of high yield (95%) of trans-1,2-dibromoalkenes (20). The ratio of E/Z-(20)-isomers in this case is practically independent of the reaction conditions. The authors believe that graphite suppresses the isomerization of E-dibromide into Z-dibromide. The work describes the bromination of a number of substituted acetylenes (21) (29) with molecular bromine in 1,2-dichloroethane. As a result of the reactions, the corresponding 1,2-dibromo derivatives were generally obtained in the form of a mixture of two isomers with the E- and Z-configuration (Scheme 11). The dependence of the distribution of products on the concentration of reagents can be excluded on the basis - 5 VMU, chemistry, 3

8 154 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY Th eme 1 1 X X X Z E- X H H H H H 3 H N N 2 H H Me Et n-pr n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu n-bu nii results obtained for compound (24): varying the concentrations of the reagents by two order did not lead to any significant changes in the E/Z ratio. Bromine has been reported to add to alkynes (27), (28), and to 2-hexine (30) stereospecifically, yielding a trans-dibromide (Table). This is consistent with the formation of a bridging bromyrenium cation during the reaction. It should be noted that for (27) and (30) positive values ​​of the apparent activation energy were found. The addition of bromine turned out to be stereoselective for compound (25) (95% trans isomer). Bromination of all other alkynes resulted in the formation of a mixture of cis- and trans-dibromoalkenes with a predominance of the trans product. The presence of both isomers among the reaction products during bromination (21), (24) and (26) indicates the formation of open vinyl cations as reaction intermediates. For all compounds leading to mixtures of isomers, negative apparent activation energies were found. During the bromination of diphenylacetylene (29), despite the positive activation energy, a mixture of E- and Z-products is formed, indicating that the reaction proceeds through an open intermediate. In addition to the steric and electronic effects of the second phenyl substituent, the following two factors may be the reasons for this non-stereoselective addition. First, there is a significant steric repulsion between the phenyl ring and the bromine atom at carbon C-2. The second, much more important, factor is the stabilization of (29) due to the conjugation of two phenyl rings with the tolane triple bond. During the formation of a positive charge on the C-1 carbon atom, this conjugation is disrupted, so the stage of formation of the cationic intermediate requires additional energy. In continuation of this study, the authors studied the kinetics of the interaction of compounds (21) (30) with bromine in 1,2-dichloroethane and showed that the reaction rate strongly depends on the size and electronic characteristics of the substituents at the triple bond. The introduction of a methyl group instead of the acetylene hydrogen atom in phenylacetylene leads to an increase in the bromination rate by 1.6 times. The effect of substitution is even more pronounced in the case of ethyl and propyl derivatives, for which the reaction is accelerated by 7 and 3.7 times, respectively, compared to unsubstituted phenylacetylene. It is assumed that alkyl substituents are capable of inductively stabilizing the adjacent carbocation center. However, in the case under consideration, an increase in the +I effect of substituents leads to an increase in the reaction rate by less than one order of magnitude. This very weak effect means that the acetylene carbon atom C-2 carries a slight positive charge. This is consistent with the structure of the open vinyl cation in the bromination reactions of compounds (21) (24), i.e. the positive inductive effect of the β-alkyl group has a weak stabilizing effect on the vinyl cation. The usual α-arylvinyl cation is stabilized mainly due to (α-aryl)-π-p + -conjugation, and the bromine atom in the β-position does not interfere with the stabilizing effect.

9 155 Results of a study of the interaction of alkynes (21)-(30) with bromine in 1,2-dichloroethane Acetylene k 3, M -2 s -1 E a, kcal/mol E:Z, % 21 11.10 0.13 ( 0.02) 57: .32 0.61 (0.08) 78: .7 0.67 (0.09) 70: .5 0.55 (0.07) 66: .73 (0.3) 95 : .28 (0.02) 72: .046 +8.71 (0.3) 100: :0 29 0.6 +4.34 (0.8) 60: .63 +7.2 (1.0 ) 100:0 due to the presence of the aryl group. Similar trends are observed during the bromination of alkyl-substituted phenylacetylenes in other solvents such as methanol, acetic acid and aqueous acetone. Thus, the available data indicate that the positive charge in the intermediate arises mainly on the C-1 carbon atom. Another confirmation of this conclusion is the influence of the electronic effects of the para substituent in the phenyl ring on the bromination rate in series (25) (28). Thus, the methoxy group causes an increase in the reaction rate by 6 orders of magnitude compared to unsubstituted acetylene (29), while the cyano group decreases the rate constant by 3 orders of magnitude. The slower interaction of bromine with diphenylacetylene compared to compounds (21) (25) is explained, as in previous works, by the negative inductive effect of the second phenyl group. Bromination of hexine-2 occurs slowly, as would be expected from dialkylacetylene, which does not form an open stabilized vinyl cation. In this case, the formation of a bridging bromyrenium ion is more preferable. The energy of the bromyrene ion is higher than the energy of its isomeric β-bromovinyl cation. Therefore, for aryl-substituted acetylenes, only when the electron-withdrawing substituent on the aromatic ring strongly destabilizes the positive charge of the α-arylvinyl cation can the bromyrenium ion become a reactive intermediate, especially in a non-polar solvent such as 1,2-dichloroethane. It should also be noted that the rate constant for bromination of alkyne (23), measured in chloroform, is one order of magnitude lower than the same constant measured in dichloroethane. This indicates a direct effect of solvent polarity on the reaction rate. Significantly less polar 6 VMU, chemistry, 3

10 156 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY T chloroform appears to reduce the rate of formation of a charged intermediate. In addition, when the reaction is carried out in Hl 3 medium, a noticeable change in activation parameters occurs. The apparent activation energy for compound (23) is positive in this solvent and is 1.8 kcal/mol higher than in dichloroethane. A more detailed consideration of the mechanism of addition of bromine to the triple bond. It is generally accepted that the mechanism of bromination of alkynes includes a limiting stage of the formation of a cationic intermediate, which then reacts with a nucleophile to give the final product. However, until recently, no considerations were made about the processes preceding the formation of the transition state. Recent studies of the reactions of electrophilic addition of bromine to acetylenes have significantly supplemented the existing information on the course of bromination of alkynes. The work suggested the participation of π-complexes of a 1:1 composition between a halogen and an acetylene molecule in halogenation reactions. The existence of several such complexes has been experimentally documented in the gas phase and at low temperatures using in-matrix spectroscopy. Thus, 2 alkyne π-complexes were described as reactive intermediate species in the general scheme of the reaction mechanism, and the reduced reactivity of alkynes in bromination reactions compared to similarly constructed alkenes was explained by the different stabilities of the corresponding bimolecular π-complexes. A recent paper provides direct evidence for the existence of a 1:1 charge transfer complex between bromine and acetylene. During the bromination of acetylene (22) with bromine in dichloroethane, a corresponding complex was detected, which absorbs much more strongly in the UV region of the spectrum than the starting compounds. The use of the stopped jet method made it possible to record absorption spectra a few milliseconds after the start of the reaction, i.e. even before the formation of the final products. Thus, after mixing methylphenylacetylene (22) with bromine, the difference optical density was measured in the nm range. Subtraction of the contributions from the absorption spectra of alkyne and 2 from the experimentally obtained curve led to the appearance of a new UV band centered at λ max = 294 nm, which clearly indicates the formation of a new intermediate particle, to which the structure of the 1:1 π-complex was assigned. Attempts to obtain the formation constant of this particle based on spectrophotometric data were unsuccessful, but the stability constant of such an intermediate complex was calculated based on the equilibrium concentration of free bromine in solution. The bromine concentration was determined spectrophotometrically at λ = 560 nm (the parent alkyne and the resulting complex do not absorb at this wavelength). The stability constant (K f) of the π-complex determined in this way at 25 C turned out to be equal to 0.065 ± 0.015 M 1. Based on this value, the equilibrium concentration of the complex in the solution obtained after mixing a 0.05 M solution (22) with a 10 3 M solution 2 was calculated (3M). It has been established that the stability constant of the complex decreases with increasing temperature from 0.157 M 1 at 17.5 C to 0.065 M 1 at 25 C. Based on the values ​​of K f at different temperatures, the enthalpy of formation H = 2.95 kcal/mol and the entropy of formation S = 15.4 e.u. the particle in question. These values ​​are consistent with the results of quantum chemical calculations. It should also be noted that the thermodynamic and spectroscopic characteristics of the detected 2-alkyne π-complex are very similar to the characteristics of the corresponding alkene complexes. The energy of 1:1 π-complexes, along with the enthalpy of the reaction, suggests, by analogy with olefins, the formation of a second intermediate in the form of a 2:1 complex between bromine and acetylene. The reasons for the appearance of such a trimolecular complex during the bromination of triple bonds can be explained as follows. If we assume that electrophilic addition in solution proceeds by an ionic mechanism, including the formation of solvated bromyrenium ion [H H] +, then the energy of heterolytic dissociation of the 2 H H π-complex should be compensated by the energy of solvation of the resulting ions and [ H H] +. However, the energy of heterolytic bond cleavage is very high and in the gas phase is calculated to be 161.4 kcal/mol. At the same time, the enthalpy of formation of ion 3 from and 2 as a result of the decomposition of the trimolecular complex 2 2 H H lies in the region of 40 kcal/mol. Thus, the formation of a 2:1 complex allows

11 significantly reduce the energy barrier of the heterolytic dissociation process, leading to cationic reaction intermediates. Available information about the mechanism of bromination of alkynes allows us to depict the energy profile of the reaction as shown in Scheme 12. The reaction begins with the exothermic formation of a 1:1 reactive complex, which is lower in energy than the starting reagents. Interaction with the second bromine molecule leads to the formation of a 2:1 complex, from which subsequently, along with the trihalide anion, two different cationic intermediates can be formed, the β-bromovinyl cation, the energy of which is comparable to the energy of the starting compounds, or the cyclic bromine ion, which lies much higher in energy . The nature of the intermediate can be determined based on the stereochemical result of the reaction. The final attack of the nucleophile, which is apparently ion 3, leads to the formation of addition products. As already noted, the reaction path and stereochemistry of the addition products are determined primarily by the structure of the original acetylene. Bromination of acetylenes with copper (II) bromide. Divalent copper halides, in particular u2, are quite widely used to introduce 157 bromine atoms into the molecules of various compounds. The paper reports the results of a study of the interaction of a number of substituted acetylenes with copper (II) bromide in boiling methanol. Solutions of cuprous bromide in boiling solvents contain, in addition to the salt itself, another brominating agent. This conclusion was made based on the analysis of kinetic data for the process under consideration. The authors believe that under these conditions, partial reversible dissociation of u 2 can occur according to the scheme in which copper bromide acts as a source of free bromine of low concentration in the solution 2 u 2 2 u + 2. This assumption is consistent with the fact that bromine can be distilled from a boiling solution of u 2 in acetonitrile. In boiling methanol, due to relatively low temperatures (64 C), u 2 is not able to decompose according to the above scheme; It was found that a 0.1 M solution when boiled for 12 hours gives no more than 2.1% u(i). However, the presence in the solution of a substrate with a multiple bond in the molecule promotes the rapid consumption of trace amounts of bromine and thereby shifts the equilibrium of the reaction towards self-decomposition u 2. During the bromination of acetylenes with a non-terminal triple bond, the formation is observed in high yields of 1,2-dibromoalkenes having exclusively h e m a VMU, chemistry, 3

12 158 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY T SCHEMA u 2 MeH + 2 u = (81%); H (64%). strictly trans configuration (Scheme 13). From the above it follows that in this case it is impossible to unambiguously determine which compound (u 2, free bromine, or both of these brominating agents) is directly responsible for the formation of the adduct. Bromination of acetylenes with a terminal triple bond under the conditions under consideration leads to the formation of tribromo derivatives according to the equations given in Scheme 14. According to the authors, the trihalogenation of terminal alkynes cannot be carried out with free bromine. For this reaction, a mechanism was proposed that included the following sequence of transformations (Scheme 15). A possible mechanism for the initial stage of formation of 1,2-dibromoalkene involves the transfer of a halogen from a copper atom to a carbon atom, occurring within a 1:1 complex according to Scheme 16. Results slightly different from those described above were obtained when carrying out a similar reaction at room temperature. As shown in Scheme 17, the reaction of phenylacetylene with copper (II) bromide in methyl alcohol at 25 C leads to the formation of bromophenylacetylene (31) and 2-phenyl-1,1,2-tribromoethylene (32). As for product (31), one of the possible ways of its formation is the direct exchange of hydrogen with a bromine atom. Considering the high yield (68%) and low yield (14%) () = 2 under these conditions, the authors proposed an alternative route to the tribromo derivative, which consists of the initial formation followed by its dibromination under the influence of u 2. Experimental data support this mechanism data according to which it reacts with u 2 /MeH to form () = 2 (Scheme 18), and with increasing temperature up to the boiling point of the solvent, the yield of the tribromo derivative increases noticeably (from 11% at 25 C to 69% at the boiling point of methanol). Scheme 1 4 H 4 u 2 / MeH - 4 u, - H H H 4 u 2 / MeH - 4 u, - H 2 () = (67%), H 2 H (93%) 2 () H 57% 2 H Me 6 u 2 / MeH - 6 u, - H 2 () Me + 50% H 47% Me

13 159 Scheme 1 5 H u 2 slow. H u 2 H - H 2 () Scheme 1 6 u(ii) + H H X u L X H X u X H X ux ux + H 2 + X X ux Scheme 1 7 H u 2 / MeH + () C ch e m a 1 8 u 2 / MeH () When a number of alkyl- and phenyl-substituted acetylenes are brominated with copper(ii) bromide in acetonitrile at room temperature, only the corresponding dibromoalkenes are obtained, with the exception of propargyl alcohol (in which case, along with the expected dibromide, the formation of a tribromo derivative is observed ). A characteristic feature of the reaction with u 2 under these conditions is its very high stereospecificity. Thus, alkylacetylenes and methylphenylacetylene give only trans-dibromoalkene, and in the case of tert-butylphenylacetylene, as in the case of bromination with molecular bromine in chloroform, the cis isomer is the predominant reaction product. The E-isomer is formed as practically the only product when phenylacetylene reacts with 2 5 equivalents of u 2 even when the reaction is carried out for 48 hours. This means that bromide 8 VMU, chemistry, 3

14 160 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY T copper(ii) does not dissociate into u and 2 under the conditions under consideration, otherwise the trans-dibromide would have to isomerize into cis-dibromide, as happens in the case of interaction with molecular bromine with increasing reaction time and increasing bromine concentration in solution . The reaction of acetylenes with u 2 is apparently ionic in nature. This is confirmed experimentally, since carrying out the reaction in the dark or in the light, bubbling through a solution of oxygen or nitrogen, or in the presence of radical scavengers such as m-dinitrobenzene, does not have a noticeable effect on the yield or ratio of isomeric products. The absence of propargyl bromide among the reaction products is also consistent with the reaction occurring via an ionic mechanism. Further, it should be noted that during bromination of u 2 the stereospecificity of the formation of the trans isomer for alkynes with =, alkyl and =H, primary or secondary alkyl is much higher than during bromination with bromine. In addition, under kinetic control conditions, the ratio of E/Z isomers in the reaction products of alkylphenylacetylenes noticeably decreases upon transition from the primary alkyl group to the secondary and then to the tertiary. These patterns can be explained by assuming that the reaction proceeds through the formation of an intermediate, which is an open vinyl cation in which u(i) is weakly coordinated both with the π-orbital of the double bond and with the lone pair of electrons on the bromine atom. In this case, the attacking particle is bromideione, coordinated with the copper atom (u 3). In the case when the radical is sterically heavily loaded (for example, = t-bu), it will prevent the attack of nucleophilic particles from its own side and promote cis-bromination of the triple bond Bromination of acetylenes with tetrabutylammonium tribromide (TBAT) The work proposes to use it as a bromination reagent for alkynes TBAT, which is a complex salt whose structure corresponds to the formula (4 H 9) 4 N + 3. This reagent is very stable, non-toxic and therefore easy to use. The bromination reaction with its participation proceeds according to the equation presented in Scheme 19. Bu 4 N " - Bu 4 N + - " =, (H 3) 2 (H); 33 " = H, H 3, H, H, H(2 H 5) 2 The yield of products (33) ranges from 84 to 96% depending on the nature of the starting acetylene. It has been established that, regardless of whether the reaction is carried out at low temperature and the stoichiometric ratio of the reactants or at a higher temperature and with a higher concentration of TBAT relative to the concentration of acetylene, in either case trans-1,2-dibromoalkene is the only reaction product. The presence of the cis isomer was not detected even chromatographically. In addition, whatever the temperature and ratio of the reagents, among the reaction products there are no tetrabromo derivatives or any other substances formed as a result of secondary reactions. An increase in the concentration of TBAT relative to the concentration of acetylene leads to a decrease in the yield of dibromoalkene due to the processes of resinization of the substance. Observation of the progress of the reaction in different solvents showed that the best results are obtained when the reaction is carried out in a medium of low polarity chloroform. Although ethanol and methanol are more polar solvents, the solubility of the reagents in them is much lower than in chloroform, so alcohols cannot be used as a reaction medium for the reaction in question. The same work notes that carrying out the reaction in the light or in the dark, in an atmosphere of an inert gas or in air, as well as in the presence of m-dinitrobenzene or oxygen (radical scavengers) does not have a noticeable effect on the results of the reaction; the latter always proceeds stereospecifically and gives high yields of the product. It can be assumed that the process of interaction of acetylenes with TBAT is ionic in nature. It is known that tribromide anion 3 has a linear structure in which the bonds between bromine atoms are weaker than similar bonds in molecule 2. It is believed that this anion can dissociate according to the equation: Scheme 1 9

15 161 Scheme 20 (" - ()) δ " δ = - - " In the case under consideration, the formation of molecular bromine as a result of the decomposition of the tribromide anion should lead to the production of a mixture of cis- and trans-isomers or due to the addition of free bromine at triple bond, or due to the subsequent isomerization of trans-dibromoalkene, which occurs with the participation of 2 as a catalyst. However, experimental data indicate the absence of a cis isomer among the reaction products. The interaction of Me with molecular bromine in acetic acid in the presence of bromide ions leads to the formation of trans -1,2-dibromo derivative as almost the only (99%) product. In the case of TBAT, the cis isomer was not obtained even when an equimolar mixture of this reagent with trans-1,2-dibromoalkene was kept under reaction conditions for 10 hours. These are the results allow us to assume the existence in solution of an undissociated ion 3, which can add to the alkyne according to the trimolecular mechanism Ad E 3. As shown in Scheme 20, this mechanism involves the attack of two tribromide anions at once at the triple bond of acetylene, which leads to a transition state in which both bonds are formed simultaneously (within the same transition state). The high stereospecificity of the formation of trans-1,2-dibromoalkene can just as successfully be explained by the interaction of the tribromide anion with an alkyne via the Ad E 2 mechanism, which proceeds through the formation of a cyclic bromine zwitterion ion as a reactive reaction intermediate (Scheme 21). Further addition of a bromide or tribromide ion results in the formation of an exclusively trans isomer of 1,2-dibromoalkene. A final choice between these two reaction mechanisms has not been made. Here it is necessary to mention the possibility of competition between bimolecular and trimolecular addition processes, as well as the influence of reaction conditions and the nature of acetylenes on the probability of the reaction proceeding along one or another path. It is assumed that the Ad E 3 mechanism should be more susceptible to the effects of steric hindrances that arise in the presence of bulky substituents in the molecule than the Ad E 2 mechanism, however, direct confirmation of this assumption does not yet exist Bromination of acetylenes with N-bromosuccinimide (NBS) in dimethyl sulfoxide (DMSO ) The reaction of diphenylacetylene with NBS/DMSO gives benzyl smoothly and in high yield (Scheme 22). In the case of unsymmetrical acetylenes, the reaction proceeds ambiguously, leading to a mixture of three products, in which, as shown in the example of the meth- Scheme 2 1 () - " = " - " 9 VMU, chemistry, 3

16 162 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY Scheme NBS / DMSO Scheme 2 3 NBS / DMSO Me Me + Me + 6: 3: 1 Methylphenylacetylene, dibromoketone content is negligible (Scheme 23) Bromination element (si, Ge, Sn )substituted acetylenes The bromination of organoelement acetylenes has been practically unstudied until recently. It has been shown that bromination of bis(trimethylsilyl)acetylene with bromine in l 4 leads to the formation of dibromoduct with a yield of 56%. The latter is the only product even when using excess bromine in combination with prolonged heating of the reaction mixture. Lower conversion temperatures and carrying out the reaction in pentane significantly increase the yield of 1,2-dibromo-1,2-bis(trimethylsilyl)ethene (82%). The authors attribute the trans configuration to the resulting dibromide, but no data on the basis of which such an assignment could be made are provided in the papers. (Trialkylsilyl)acetylenes 3 Si H (=Me, Et) are easily brominated in the absence of a solvent, and at C one molecule of bromine is added, and at C two. It was found that in the dark and in the presence of an inhibitor (hydroquinone), the reaction slows down somewhat and proceeds with less thermal effect, although the yield of products does not change significantly. The authors believe that, along with the electrophilic bromination process, free radical addition of bromine also takes place. The introduction of alkoxy groups to the silicon atom leads to a decrease in the activity of the triple bond in the bromination reaction. The stereochemistry of the products was not discussed by the authors. We found that 3 Si gives 1,2-dibromoducts in reactions with 2 and TBAT. In this case, the composition of the products significantly depends on the nature of the brominating reagent (Scheme 24). The assignment of cis-, trans-isomers was performed using NMR spectroscopy methods. The Z-structure of one of the isomers was supported by the presence of a strong Overhauser effect (NEs) between the protons of the Me 3 Si group and the ortho-protons of the aromatic system (Scheme 25). Scheme Z/E = 90/10 3 Si Z,E- 3 Si()=() TBAT 34, 35 36, 37 Z/E = 10/90 =Me (34, 36), Et (35 , 37)

17 163 Scheme 2 5 H o H o H 1 Si H o H 1 Si H o -H 1 NEs (Z-36) no H o -H 1 NEs (E-36) Scheme 2 6 (Me) 3 Si 2 (Me) 3 Si + 38 (Me) 3 Si Z-39 (85%) E-39 (15%) Scheme Si 2 3 Si 40 Z-41 Smoothly passed as shown in Scheme 26 interaction with acetylene bromine (38). The reaction of bromine with the more sterically loaded 3 Si (40) led to a dibromoduct, the Z-structure of which was confirmed by X-ray diffraction data. This acetylene did not react with TBAT (Scheme 27). In the case of Et 3 Ge, the reaction with both bromine and TBAT proceeds ambiguously, giving mixtures of products of addition at the triple bond and cleavage of the Ge bond. In contrast, (Et)3Ge (42) upon interaction with bromine smoothly gives dibromoduct (43) in the form of a mixture of Z,E-isomers (1H NMR spectroscopy data). No Ge bond cleavage products were found in this case (Scheme 28). Alk 3 Sn in an electrophilic substitution reaction with 2 in DMSO or in a DMF/l 4 mixture gives bromodestannylation products. We have shown that the softer brominating reagent, TBAT, also produces products of Sn bond cleavage (Scheme 29). Reactions of 1-(phenylacetylenyl)germatranes (44, 45) with both 2 and TBAT lead only to Z-isomers, the structures of which are confirmed by X-ray diffraction data. As shown in Scheme 30, it behaves similarly in the reaction with 2 germatrane (46). The presence of a noticeable amount of trans-isomer (E-43) in the mixture obtained by reacting (Et)3Ge (42) with 2 allowed us to synthesize the E-isomer of compound (47) (Scheme 31). The structure of compound (E-47), obtained according to scheme 31, was also confirmed by X-ray diffraction data. This is the only case when both geometric 10 VMU, chemistry, 3

18 164 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY T Scheme 2 8 (Et) 3 Ge 2 (Et) 3 Ge + 42 (Et) 3 Ge Z-43 (75%) E-43 (25%) Scheme 2 9 TBAT Bu 3 Sn - Bu 3 Sn Scheme N Ge TBAT 1 2 N Ge 44, 45, 48, 49 1 = 2 = H (44, 47); 1 = 2 = Me (45, 48); 1 = H, 2 = (46, 49) Scheme 3 1 (Et) 3 Ge Z,E-43 TEA / 6 H 6-3 EtH N Ge + N Ge Z-47 E-47

19,165 isomers of 1,2-dibromides were characterized by X-ray diffraction (data from the Cambridge Structural Data Bank). Fundamentally different results were obtained in the case of reactions of 2 and TBAT with 1-(phenylacetylenyl)silatrane (50). When interacting (50) with 2, the main direction of the process is the splitting of the Si bond. However, Z N(H 2 H 2) 3 Si ()=() (52) is also formed in small quantities. In the case of the reaction with TBAT, the amount of 1,2-dibromoduct was 30% (Scheme 32). The different behavior of compound (50) in these reactions can be explained by the fact that bromine is a stronger electrophile compared to TBAT; this results in a more favorable electrophilic substitution reaction when treated (50) with molecular bromine. The interaction of Alk 3 M (M = Si, Ge, Sn) with NBS/DMSO leads to complex mixtures of difficult-to-identify products. In contrast, 1-(phenylacetylenyl)germatranes (44, 45) upon treatment with NBS/DMSO give dibromoketones (53, 54), for the latter X-ray diffraction data were obtained (Scheme 33). The reaction of trwith NBS or N-chlorosuccinimide (NS) in the absence of DMSO proceeds with cleavage of the Ge bond (Scheme 34). 2. IODOCHLORINATION REACTIONS The iodochlorination reagent can be either iodine monochloride (ICl) itself or various systems based on molecular or polyvalent iodine, and in some cases the formation of ICl occurs in situ as the reaction proceeds. As a rule, most methods lead to fairly high yields of the desired iodochloro derivatives, despite the possible formation of by-products. Differ- Scheme 3 2 N Si TBAT - N Si 51 + N Si 52 Scheme 3 3 N N 2 NBS / DMSO Ge Ge 44, (= H), 54 (= Me) 11 VMU, chemistry , 3

20 166 VESTN. ISCO. UN-TA. SER. 2. CHEMISTRY T Scheme 3 4 N N Ge NBS or NS Ge Hal SiMe 3 Scheme 3 5 " Il / H 3 N I + l " l 55 E-(56) Z-(56) " I = alk or; "= H, alk or which in the choice of a particular reagent is determined by the ease of use, availability, toxicity, as well as regio- and stereoselectivity of electrophilic iodochlorination. The peculiarities of the behavior of each of the reagents described in the literature in reactions with alkynes are discussed in detail below. Iodochlorination with Il Iodochlorination of ordinary acetylenes with Il is described in only one work. Boiling the reagents in acetonitrile leads to the formation of iodochloroalkenes (yield 15–85%) in the form of mixtures of Z- and E-isomers with a predominant content of the latter (Scheme 35). This method has a number of significant disadvantages. In the absence of commercially available iodine monochloride, it must be obtained from halogens. Inconvenience in handling Il is associated with its viscosity and toxicity. The tendency of this reagent to disproportionate often leads to high yields of by-products, in particular unstable diiodides. This in turn requires additional purification steps, which reduce the yields of the desired products. In order to avoid the above disadvantages of working with Il, a large number of alternative iodochlorination reagents were developed. Generation of iodine monochloride in situ At the beginning of the 20th century. Publications have appeared that describe the formation of iodine monochloride during the reaction. In these works, mixtures of iodine with chlorides of mercury (II), copper (I), silver (I) and gold (I) were used as reagents. Similar reactions were later described in an aqueous environment. The degree of conversion in terms of iodine in this case is 30–60%, which also indicates the loss of most of the halogen, most likely due to hydrolysis of alkyl iodides or transition to an inert metal iodide. Another source of electrophilic iodine is a mixture of Sbl 5 with I Iodochlorination of multiple bonds using the Sbl 5 /I 2 system Treatment of phenyl-substituted acetylenes (57) with a mixture of Sbl 5 /I 2 smoothly leads to the production of chloriodoalkenes (58), with the E-isomer being predominant. Reactions, as a rule, are accompanied by the formation of small amounts of dichloro- and diiodoadducts (59; X = Cl, I) (Scheme 36).

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Sections: Chemistry

The set of tasks for conducting a written examination of knowledge for students is composed of five questions.

1. The task is to establish correspondence between a concept and a definition. A list of 5 concepts and their definitions is compiled. In the compiled list, concepts are numbered by numbers, and definitions are numbered by letters. The student needs to correlate each of the given concepts with the definition given to him, i.e. in a series of definitions, find the only one that reveals a specific concept.
2. The task is in the form of a test of five questions with four possible answers, of which only one is correct.
3. The task is to exclude an unnecessary concept from a logical series of concepts.
4. A task to complete a chain of transformations.
5. Solving problems of different types.

Option I

1st task. Establish a correspondence between the concept and definition:

Definition:

1. The process of aligning electron orbitals in shape and energy;
2. Hydrocarbons, in the molecules of which carbon atoms are connected to each other by a single bond;
3. Substances that are similar in structure and properties, but differ from each other by one or more groups - CH2;
4. Hydrocarbons of a closed structure having a benzene ring.
5. A reaction in which one new substance is formed from two or more molecules;

a) arenas;
b) homologues;
c) hybridization;
d) alkanes;
d) accession.

2nd task. Take a test with four possible answers, of which only one is correct.

1. Penten-2 can be obtained by dehydration of alcohol:

a) 2-ethylpentine-3;
b) 3-ethylpentine-2;
c) 3-methylhexine-4;
d) 4-methylhexine-2.

3. Angle between axes sp-hybrid orbital of the carbon atom is equal to:

a) 90°; b) 109 ° 28’; c) 120° d) 180°.

4. What is the name of the product of complete bromination of acetylene:

a) 1,1,2,2-tetrabromoethane;
b) 1,2-dibromoethene;
c) 1,2-dibromoethane;
d) 1,1 –dibromoethane.

5. The sum of the coefficients in the equation for the combustion reaction of butene is equal to:

a) 14; b) 21; at 12; d) 30.

3rd task

Eliminate the unnecessary concept:

Alkenes, alkanes, aldehydes, alkadienes, alkynes.

4th task

Carry out transformations:

5th task

Solve the problem: Find the molecular formula of a hydrocarbon whose mass fraction of carbon is 83.3%. The relative density of the substance with respect to hydrogen is 36.

Option II

1st task

Definition:

1. A chemical bond formed by overlapping electron orbitals along a bond line;
2. Hydrocarbons, in the molecules of which carbon atoms are connected to each other by a double bond;
3. A reaction that results in the replacement of one atom or group of atoms in the original molecule with other atoms or groups of atoms.
4. Substances that are similar in quantitative and qualitative composition, but differ from each other in structure;
5. Hydrogen addition reaction.

a) replacement;
b) σ-bond;
c) isomers;
d) hydrogenation;
e) alkenes.

2nd task

1. Alkanes are characterized by isomerism:

a) the provisions of the multiple connection;
b) carbon skeleton;

d) geometric.

2. What is the name of the hydrocarbon

a) 2-methylbutene-3;
b) 3-methylbutene-1;
c) penten-1;
d) 2-methylbutene-1.

3. Angle between axes sp The 3-hybrid orbital of the carbon atom is equal to:

4. Acetylene can be obtained by hydrolysis:

a) aluminum carbide;
b) calcium carbide;
c) calcium carbonate;
d) calcium hydroxide.

5. The sum of the coefficients in the propane combustion reaction equation is equal to:

a) 11; b) 12; c) 13; d) 14.

3rd task

Eliminate the unnecessary concept:

Alcohols, alkanes, acids, ethers, ketones.

4th task

Carry out transformations:

5th task

Solve the problem:

What volume of air will be required for complete combustion of 5 liters. ethylene. The volume fraction of oxygen in the air is 21%.

Option III

1st task

Establish a correspondence between the concept and definition:

Definition:

1. The reaction of combining many identical molecules of a low molecular weight substance (monomers) into large molecules (macromolecules) of a polymer;
2. Hydrocarbons, in the molecules of which carbon atoms are connected to each other by a triple bond;
3. A bond formed as a result of overlapping electron orbitals outside the communication line, i.e. in two areas;
4. Halogen elimination reaction;
5. The hydration reaction of acetylene to form ethanal.

a) halogenation;
b) polymerization;
c) Kucherova;
d) alkynes;
e) π-bond.

2nd task

Take a test with four possible answers, of which only one is correct.

1. Specify the formula of 4-methylpentine-1:

2. In the bromination reaction of propene, the following is formed:

a) 1,3-dibromopropane;
b) 2-bromopropane;
c) 1-bromopropane;
d) 1,2-dibromopropane.

3. Angle between axes sp The 2-hybrid orbital of the carbon atom is equal to:

a) 90°; b) 109°28’; c) 120° d) 180°.

4. What type of isomerism is characteristic of alkenes:

a) carbon skeleton;
b) the position of the multiple connection;
c) geometric;
d) all previous answers are correct.

5. The sum of the coefficients in the equation for the combustion reaction of acetylene is equal to:

a) 13; b) 15; c) 14; d) 12.

3rd task

Eliminate the unnecessary concept:

Hydrogenation, hydration, hydrohalogenation, oxidation, halogenation.

4th task

Carry out transformations:

5th task

Solve the problem: Find the molecular formula of a hydrocarbon whose mass fraction of hydrogen is 11.1%. The relative density of the substance in air is 1.863.

IV option

1st task

Establish a correspondence between the concept and definition:

Definition:

1. Hydrocarbons, in the molecules of which the carbon atoms are connected to each other by two double bonds;
2. The reaction of producing high-molecular substances (polymers) with the release of a by-product (H 2 O, NH 3);
3. Isomerism, in which substances have a different order of bonding of atoms in the molecule;
4. A reaction as a result of which several products are formed from a molecule of the original substance;
5. Water addition reaction.

Concept:

a) structural;
b) hydration;
c) alkadienes;
d) polycondensation;
d) decomposition.

2nd task

Take a test with four possible answers, of which only one is correct.

1. Indicate the type of isomerism for a pair of substances:

a) the provisions of the multiple connection;
b) carbon skeleton;
c) positions of the functional group;
d) geometric.

2. Benzene is obtained from acetylene by the reaction:

a) dimerization;
b) oxidation;
c) trimerization;
d) hydration.

3. Alkanes are characterized by reactions:

a) accession;
b) substitution;
c) polymerization;
d) oxidation.

4. What is the name of a hydrocarbon with the formula

a) 4-ethylpentadiene-1,4;
b) 2-methylhexadiene-1,4;
c) 4-methylhexadiene-1,5;
d) 2-ethylpentadiene-1,4.

5. The sum of the coefficients in the equation for the combustion reaction of methane is equal to:

a) 7; b) 8; at 4; d) 6.

3rd task

Eliminate the unnecessary concept:

Ethane, ethanol, ethene, ethylene, ethyne.

4th task

Carry out transformations:

5th task

Solve the problem: What volume of air is required for complete combustion of 3 liters. methane The volume fraction of oxygen in the air is 21%.

As you already know, acetylene is a product of incomplete decomposition of methane. This process is called pyrolysis (from the Greek feast - fire, lysis - decomposition). Theoretically, acetylene can be represented as a product of the dehydrogenation of ethylene:

In practice, acetylene, in addition to the pyrolysis method, is very often obtained from calcium carbide:

The peculiarity of the structure of the acetylene molecule (Fig. 21) is that there is a triple bond between the carbon atoms, i.e. it is an even more unsaturated compound than ethylene, the molecule of which contains a double carbon-carbon bond.

Rice. 21.
Models of the acetylene molecule: 1 - ball-and-stick; 2 - scale

Acetylene is the founder of the homologous series of alkynes, or acetylene hydrocarbons.

Acetylene is a colorless, odorless gas, slightly soluble in water.

Let's consider the chemical properties of acetylene, which underlie its use.

Acetylene burns with a smoky flame in air due to the high carbon content in its molecule, so oxygen is used to burn acetylene:

The temperature of the oxygen-acetylene flame reaches 3200 °C. This flame can be used to cut and weld metals (Fig. 22).

Rice. 22.
Oxy-acetylene flame is used for cutting and welding metal

Like all unsaturated compounds, acetylene actively participates in addition reactions. 1) halogens (halogenation), 2) hydrogen (hydrogenation), 3) hydrogen halides (hydrohalogenation), 4) water (hydration).

Consider, for example, the hydrochlorination reaction - the addition of hydrogen chloride:

You understand why the product of acetylene hydrochlorination is called chloroethene. Why vinyl chloride? Because the monovalent ethylene radical CH 2 =CH- is called vinyl. Vinyl chloride is the starting compound for producing the polymer - polyvinyl chloride, which is widely used (Fig. 23). Currently, vinyl chloride is produced not by hydrochlorination of acetylene, but by other methods.

Rice. 23.
Application of polyvinyl chloride:
1 - artificial leather; 2 - electrical tape; 3 - wire insulation; 4 - pipes; 5 - linoleum; 6 - oilcloth

Polyvinyl chloride is produced using the polymerization reaction already familiar to you. The polymerization of vinyl chloride into polyvinyl chloride can be described using the following scheme:

or reaction equations:

The hydration reaction, which occurs in the presence of mercury salts containing the Hg 2+ cation as a catalyst, bears the name of the outstanding Russian organic chemist M. G. Kucherov and was previously widely used to obtain a very important organic compound - acetaldehyde:

The reaction of bromine addition - bromination - is used as a qualitative reaction to a multiple (double or triple) bond. When acetylene (or ethylene, or most other unsaturated organic compounds) is passed through bromine water, its discoloration can be observed. In this case, the following chemical transformations occur:

Another qualitative reaction to acetylene and unsaturated organic compounds is the discoloration of the potassium permanganate solution.

Acetylene is the most important product of the chemical industry, which is widely used (Fig. 24).

Rice. 24.
Application of acetylene:
1 - cutting and welding of metals; 2-4 - production of organic compounds (solvents 2, polyvinyl chloride 3, glue 4)

New words and concepts

1. Alkynes.
2. Acetylene.
3. Chemical properties of acetylene: combustion, addition of hydrogen halides, water (Kucherov reaction), halogens.
4. Polyvinyl chloride.
5. Qualitative reactions to multiple bonds: discoloration of bromine water and potassium permanganate solution.