The simplest organic compounds are hydrocarbons composed of carbon and hydrogen. Depending on the nature of the chemical bonds in hydrocarbons and the ratio between carbon and hydrogen, they are divided into saturated and unsaturated (alkenes, alkynes, etc.)
limiting Hydrocarbons (alkanes, hydrocarbons of the methane series) are compounds of carbon with hydrogen, in the molecules of which each carbon atom spends no more than one valence to connect with any other neighboring atom, and all the valences not spent on the connection with carbon are saturated with hydrogen. All carbon atoms in alkanes are in the sp 3 state. Limit hydrocarbons form a homologous series characterized by the general formula With n H 2n+2. The ancestor of this series is methane.
Isomerism. Nomenclature.
Alkanes with n=1,2,3 can only exist as one isomer
Starting from n=4, the phenomenon of structural isomerism appears.
The number of structural isomers of alkanes increases rapidly with an increase in the number of carbon atoms, for example, pentane has 3 isomers, heptane has 9, etc.
The number of alkane isomers also increases due to possible stereoisomers. Starting from C 7 H 16, the existence of chiral molecules is possible, which form two enantiomers.
Alkanes nomenclature.
The dominant nomenclature is the IUPAC nomenclature. At the same time, it contains elements of trivial names. Thus, the first four members of the homologous series of alkanes have trivial names.
CH 4 - methane
C 2 H 6 - ethane
C 3 H 8 - propane
C 4 H 10 - butane.
The names of the remaining homologues are derived from Greek Latin numerals. So, for the following members of a series of normal (unbranched) structure, the names are used:
C 5 H 12 - pentane, C 6 H 14 - hexane, C 7 H 18 - heptane,
C 14 H 30 - tetradecane, C 15 H 32 - pentadecane, etc.
Basic IUPAC rules for branched alkanes
a) choose the longest unbranched chain, the name of which is the basis (root). The suffix "an" is added to this stem.
b) number this chain according to the principle of least locants,
c) the substitute is indicated in the form of prefixes in alphabetical order, indicating the location. If there are several identical substituents in the parent structure, then their number is indicated by Greek numerals.
Depending on the number of other carbon atoms with which the considered carbon atom is directly connected, there are distinguished: primary, secondary, tertiary and quaternary carbon atoms.
As substituents in branched alkanes, alkyl groups or alkyl radicals appear, which are considered as the result of the elimination of one hydrogen atom from the alkane molecule.
The name of the alkyl groups is formed from the name of the corresponding alkanes by replacing the last suffix "an" with the suffix "il".
CH 3 - methyl
CH 3 CH 2 - ethyl
CH 3 CH 2 CH 2 - propyl
For the name of branched alkyl groups, chain numbering is also used:
Starting from ethane, alkanes are able to form conformers, which correspond to the hindered conformation. The possibility of transition from one hindered conformation to another through the eclipsed conformation is determined by the rotation barrier. Determining the structure, composition of conformers, and barriers to rotation are the tasks of conformational analysis. Methods for obtaining alkanes.
1. Fractional distillation of natural gas or gasoline fraction of oil. In this way, individual alkanes up to 11 carbon atoms can be isolated.
2. Hydrogenation of coal. The process is carried out in the presence of catalysts (oxides and sulfides of molybdenum, tungsten, nickel) at 450-470 about C and pressures up to 30 MPa. Coal and catalyst are ground into powder and hydrogenated in suspension by bubbling hydrogen through the suspension. The resulting mixtures of alkanes and cycloalkanes are used as motor fuels.
3. Hydrogenation of CO and CO 2 .
CO + H 2 alkanes
CO 2 + H 2 alkanes
Co, Fe, etc. are used as catalysts for these reactions. d - elements.
4.Hydrogenation of alkenes and alkynes.
5.organometallic synthesis.
a). Wurtz synthesis.
2RHal + 2Na R R + 2NaHal
This synthesis is of little use if two different haloalkanes are used as organic reagents.
b). Protolysis of Grignard reagents.
R Hal + Mg RMgHal
RMgHal + HOH RH + Mg(OH)Hal
in). Interaction of lithium dialkylcuprates (LiR 2 Cu) with alkyl halides
LiR 2 Cu + R X R R + RCu + LiX
Lithium dialkylcuprates themselves are obtained in a two-stage method
2R Li + CuI LiR 2 Cu + LiI
6. Electrolysis of salts of carboxylic acids (Kolbe synthesis).
2RCOONa + 2H 2 O R R + 2CO 2 + 2NaOH + H 2
7. Fusion of salts of carboxylic acids with alkalis.
The reaction is used to synthesize lower alkanes.
8.Hydrogenolysis of carbonyl compounds and haloalkanes.
a). carbonyl compounds. Synthesis of Clemmens.
b). Halogenalkanes. catalytic hydrogenolysis.
Ni, Pt, Pd are used as catalysts.
c) Halogenalkanes. Reactive recovery.
RHal + 2HI RH + HHal + I 2
Chemical properties of alkanes.
All bonds in alkanes are of low polarity; therefore, they are characterized by radical reactions. The absence of pi bonds makes addition reactions impossible. Alkanes are characterized by substitution, elimination, and combustion reactions.
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Type and name of the reaction | |
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1. Substitution reactions | |
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A) with halogens(with chlorineCl 2 – in the light, Br 2 - when heated) the reaction obeys Markovnik's rule (Markovnikov's rules) - first of all, the halogen replaces the hydrogen at the least hydrogenated carbon atom. The reaction takes place in stages - no more than one hydrogen atom is replaced in one stage. Iodine reacts most difficultly, and moreover, the reaction does not go to the end, since, for example, when methane reacts with iodine, hydrogen iodide is formed, which reacts with methyl iodide to form methane and iodine (reversible reaction): |
CH 4 + Cl 2 → CH 3 Cl + HCl (chloromethane) CH 3 Cl + Cl 2 → CH 2 Cl 2 + HCl (dichloromethane) CH 2 Cl 2 + Cl 2 → CHCl 3 + HCl (trichloromethane) CHCl 3 + Cl 2 → CCl 4 + HCl (tetrachloromethane). |
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B) Nitration (Konovalov's reaction) Alkanes react with a 10% solution of nitric acid or nitrogen oxide N 2 O 4 in the gas phase at a temperature of 140 ° and low pressure to form nitro derivatives. The reaction also obeys Markovnikov's rule. One of the hydrogen atoms is replaced by a NO 2 residue (nitro group) and water is released |
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2. Elimination reactions | |
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A) dehydrogenation- removal of hydrogen. Reaction conditions catalyst-platinum and temperature. |
CH 3 - CH 3 → CH 2 \u003d CH 2 + H 2 |
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B) cracking the process of thermal decomposition of hydrocarbons, which is based on the reactions of splitting the carbon chain of large molecules with the formation of compounds with a shorter chain. At a temperature of 450–700 o C, alkanes decompose due to the breaking of C–C bonds (stronger C–H bonds are retained at this temperature) and alkanes and alkenes with a smaller number of carbon atoms are formed |
C 6 H 14 C 2 H 6 +C 4 H 8 |
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C) complete thermal decomposition |
CH 4 C + 2H 2 |
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3. Oxidation reactions | |
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A) combustion reaction When ignited (t = 600 o C), alkanes react with oxygen, while they are oxidized to carbon dioxide and water. |
С n Н 2n+2 + O 2 ––> CO 2 + H 2 O + Q CH 4 + 2O 2 ––> CO 2 + 2H 2 O + Q |
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B) Catalytic oxidation- at a relatively low temperature and with the use of catalysts, it is accompanied by the breaking of only a part of the C–C bonds, approximately in the middle of the molecule and C–H, and is used to obtain valuable products: carboxylic acids, ketones, aldehydes, alcohols. |
For example, with incomplete oxidation of butane (breaking the C 2 -C 3 bond), acetic acid is obtained |
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4. Isomerization reactions not typical for all alkanes. Attention is drawn to the possibility of converting some isomers into others, the presence of catalysts. |
C 4 H 10 C 4 H 10 |
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5.. Alkanes with 6 or more carbon backbones also react dehydrocyclization, but always form a 6-membered cycle (cyclohexane and its derivatives). Under the reaction conditions, this cycle undergoes further dehydrogenation and turns into an energetically more stable benzene cycle of an aromatic hydrocarbon (arene). |
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Mechanism of halogenation reaction:
Halogenation
Halogenation of alkanes proceeds by a radical mechanism. To initiate the reaction, a mixture of alkane and halogen must be irradiated with UV light or heated. Chlorination of methane does not stop at the stage of obtaining methyl chloride (if equimolar amounts of chlorine and methane are taken), but leads to the formation of all possible substitution products, from methyl chloride to carbon tetrachloride. Chlorination of other alkanes results in a mixture of hydrogen substitution products at different carbon atoms. The ratio of chlorination products depends on temperature. The rate of chlorination of primary, secondary, and tertiary atoms depends on temperature; at low temperatures, the rate decreases in the series: tertiary, secondary, primary. As the temperature rises, the difference between the speeds decreases until it becomes the same. In addition to the kinetic factor, the distribution of chlorination products is influenced by a statistical factor: the probability of an attack by chlorine on a tertiary carbon atom is 3 times less than the primary one and two times less than the secondary one. Thus, the chlorination of alkanes is a non-stereoselective reaction, except in cases where only one monochlorination product is possible.
Halogenation is one of the substitution reactions. Halogenation of alkanes obeys the Markovnik rule (Markovnikov's Rules) - the least hydrogenated carbon atom is halogenated first. Halogenation of alkanes takes place in stages - no more than one hydrogen atom is halogenated in one stage.
CH 4 + Cl 2 → CH 3 Cl + HCl (chloromethane)
CH 3 Cl + Cl 2 → CH 2 Cl 2 + HCl (dichloromethane)
CH 2 Cl 2 + Cl 2 → CHCl 3 + HCl (trichloromethane)
CHCl 3 + Cl 2 → CCl 4 + HCl (tetrachloromethane).
Under the action of light, the chlorine molecule decomposes into atoms, then they attack the methane molecules, tearing off their hydrogen atom, as a result of which methyl radicals CH 3 are formed, which collide with chlorine molecules, destroying them and forming new radicals.
Nitration (Konovalov's reaction)
Alkanes react with a 10% solution of nitric acid or nitrogen oxide N 2 O 4 in the gas phase at a temperature of 140 ° and low pressure to form nitro derivatives. The reaction also obeys Markovnikov's rule.
RH + HNO 3 \u003d RNO 2 + H 2 O
i.e., one of the hydrogen atoms is replaced by a NO 2 residue (nitro group) and water is released.
The structural features of the isomers strongly affect the course of this reaction, since it most easily leads to the substitution of the nitro group of the hydrogen atom in the SI residue (available only in some isomers), the hydrogen in the CH 2 group is less easily replaced and even more difficult in the CH 3 residue.
Paraffins are fairly easily nitrated in the gas phase at 150-475°C with nitrogen dioxide or nitric acid vapor; at the same time occurs partially and. oxidation. Nitration of methane produces almost exclusively nitromethane:
All available data point to a free radical mechanism. As a result of the reaction, mixtures of products are formed. Nitric acid at ordinary temperature has almost no effect on paraffinic hydrocarbons. When heated, it acts mainly as an oxidizing agent. However, as M. I. Konovalov (1889) found, when heated, nitric acid acts in part in a “nitrating” way; the nitration reaction with weak nitric acid proceeds especially well when heated and at elevated pressure. The nitration reaction is expressed by the equation.
The homologues following methane give a mixture of different nitroparaffins due to the accompanying splitting. When ethane is nitrated, nitroethane CH 3 -CH 2 -NO 2 and nitromethane CH 3 -NO 2 are obtained. From propane, a mixture of nitroparaffins is formed:
Nitration of paraffins in the gas phase is now carried out on an industrial scale.
Sulfachlorination:
A practically important reaction is the sulfochlorination of alkanes. When an alkane interacts with chlorine and sulfur dioxide during irradiation, hydrogen is replaced by a chlorosulfonyl group:
The steps for this reaction are:
Cl+R:H→R+HCl
R + SO 2 → RSO 2
RSO 2 + Cl:Cl→RSO 2 Cl+Cl
Alkanesulfonic chlorides are easily hydrolyzed to alkanesulfoxylates (RSO 2 OH), whose sodium salts (RSO 3 ¯ Na + - sodium alkane sulfonate) exhibit properties similar to soaps and are used as detergents.
DEFINITION
Alkanes- saturated (aliphatic) hydrocarbons, the composition of which is expressed by the formula C n H 2 n +2.
Alkanes form a homologous series, each chemical compound of which differs in composition from the next and the previous one by the same number of carbon and hydrogen atoms - CH 2, and the substances included in the homologous series are called homologues.
Under normal conditions, C 1 -C 4 - gases, C 5 -C 17 - liquids, starting with C 18 - solids. Alkanes are practically insoluble in water, but highly soluble in non-polar solvents, such as benzene.
Electronic structure of alkanes and their features
In alkane molecules, primary (i.e., linked by one bond), secondary (i.e., bonded by two bonds), tertiary (i.e., bonded by three bonds) and quaternary (i.e., bonded by four bonds) carbon atoms are distinguished.
C 1 H3 - C 2 H 2 - C 1 H 3 (1 - primary, 2 - secondary carbon atoms);
CH 3 -C 3 H(CH 3) -CH 3 (3-tertiary carbon atom);
CH 3 - C 4 (CH 3) 3 - CH 3 (4-quaternary carbon atom).
Carbon atoms in saturated hydrocarbons are in sp 3 hybridization. Consider this on the example of methane - CH 4 . The methane molecule generally corresponds to the formula AB 4 . The central atom is a carbon atom, hydrogen atoms are ligands. Let's write down the electronic configuration of the carbon atom in the ground state and draw its electron-graphic formula:
6 C 1s 2 2s 2 2p 2 .
To accept four hydrogen atoms, the carbon atom must go into an excited state:
We perform similar operations for the hydrogen atom:
All valence electrons of carbon enter into hybridization, therefore, the carbon atom is in sp 3 hybridization. The angles between bonds in alkane molecules are 109.5 o (Fig. 1).
Rice. 1. The structure of the methane molecule.
Structural isomerism (isomerism of the carbon skeleton) is characteristic of saturated hydrocarbons. So, pentane has the following isomers:
CH 3 -CH 2 -CH 2 -CH 2 -CH 3 (pentane);
CH 3 -CH(CH 3) -CH 2 -CH 3 (2-methylbutane);
CH 3 -C (CH 3) 2 -CH 3 (2,2 - dimethylpropane).
For alkanes, starting with heptane, optical isomerism is characteristic.
Examples of problem solving
EXAMPLE 1
Alkanes are saturated hydrocarbons. In their molecules, atoms have single bonds. The structure is determined by the formula CnH2n+2. Consider alkanes: chemical properties, types, applications.
In the structure of carbon, there are four orbits along which atoms rotate. Orbitals have the same shape, energy.
Note! The angles between them are 109 degrees and 28 minutes, they are directed to the vertices of the tetrahedron.
A simple carbon bond allows alkane molecules to rotate freely, as a result of which the structures take on various forms, forming vertices at carbon atoms.
All alkane compounds are divided into two main groups:
- Hydrocarbons of an aliphatic compound. Such structures have a linear connection. The general formula looks like this: CnH2n+2. The value of n is equal to or greater than one, means the number of carbon atoms.
- Cycloalkanes of cyclic structure. The chemical properties of cyclic alkanes differ significantly from those of linear compounds. The formula of cycloalkanes to some extent makes them similar to hydrocarbons that have a triple atomic bond, that is, to alkynes.
Types of alkanes
There are several types of alkane compounds, each of which has its own formula, structure, chemical properties and alkyl substituent. The table contains the homologous series
Name of alkanes
The general formula for saturated hydrocarbons is CnH2n+2. By changing the value of n, a compound with a simple interatomic bond is obtained.
Useful video: alkanes - molecular structure, physical properties
Varieties of alkanes, reaction options
Under natural conditions, alkanes are chemically inert compounds. Hydrocarbons do not react to contact with a concentrate of nitric and sulfuric acid, alkali and potassium permanganate.
Single molecular bonds determine the reactions characteristic of alkanes. Alkane chains are characterized by a non-polar and weakly polarizable bond. It is somewhat longer than S-N.
General formula of alkanes
substitution reaction
Paraffin substances differ in insignificant chemical activity. This is explained by the increased strength of the chain bond, which is not easy to break. For destruction, a homological mechanism is used, in which free radicals take part.
For alkanes, substitution reactions are more natural. They do not react to water molecules and charged ions. During substitution, hydrogen particles are replaced by halogen and other active elements. Among these processes are halogenation, nitration and sulfochlorination. Such reactions are used to form alkane derivatives.
Free radical substitution occurs in three main steps:
- The appearance of a chain on the basis of which free radicals are created. Heating and ultraviolet light are used as catalysts.
- The development of a chain in the structure of which interactions of active and inactive particles take place. This is how molecules and radical particles are formed.
- At the end, the chain is terminated. Active elements create new combinations or disappear altogether. The chain reaction ends.
Halogenation
The process is radical. Halogenation occurs under the influence of ultraviolet radiation and thermal heating of the hydrocarbon and halogen mixture.
The whole process occurs according to Markovnikov's rule. Its essence lies in the fact that the hydrogen atom belonging to hydrogenated carbon is the first to be halogenated. The process starts with a tertiary atom and ends with primary carbon.
Sulfochlorination
Another name is the Reed reaction. It is carried out by the method of free radical substitution. Thus, alkanes react to the action of a combination of sulfur dioxide and chlorine under the influence of ultraviolet radiation.
The reaction begins with the activation of the chain mechanism. At this time, two radicals are released from chlorine. The action of one is directed to the alkane, resulting in the formation of a molecule of hydrogen chloride and an alkyl element. Another radical combines with sulfur dioxide, creating a complex combination. For equilibrium, one chlorine atom is taken from another molecule. The result is an alkane sulfonyl chloride. This substance is used to produce surface-active components.
Sulfochlorination
Nitration
The nitration process involves the combination of saturated carbons with gaseous tetravalent nitrogen oxide and nitric acid, brought to a 10% solution. The reaction will require a low level of pressure and a high temperature, approximately 104 degrees. As a result of nitration, nitroalkanes are obtained.
splitting off
By separating the atoms, dehydrogenation reactions are carried out. The molecular particle of methane completely decomposes under the influence of temperature.
Dehydrogenation
If a hydrogen atom is separated from the carbon lattice of paraffin (except methane), unsaturated compounds are formed. These reactions are carried out under conditions of significant temperature conditions (400-600 degrees). Various metal catalysts are also used.
Obtaining alkanes occurs by carrying out the hydrogenation of unsaturated hydrocarbons.
decomposition process
Under the influence of temperatures during alkane reactions, ruptures of molecular bonds and the release of active radicals can occur. These processes are known as pyrolysis and cracking.
When the reaction component is heated to 500 degrees, the molecules begin to decompose, and complex radical alkyl mixtures are formed in their place. In this way, alkanes and alkenes are obtained in industry.
Oxidation
These are chemical reactions based on the donation of electrons. Paraffins are characterized by autoxidation. The process uses the oxidation of saturated hydrocarbons by free radicals. Alkane compounds in the liquid state are converted to hydroperoxide. First, the paraffin reacts with oxygen. Active radicals are formed. Then the alkyl particle reacts with a second oxygen molecule. A peroxide radical is formed, which subsequently interacts with the alkane molecule. As a result of the process, hydroperoxide is released.
Alkane oxidation reaction
Application of alkanes
Carbon compounds are widely used in almost all major areas of human life. Some of the types of compounds are indispensable for certain industries and the comfortable existence of modern man.
Gaseous alkanes are the basis of valuable fuel. The main component of most gases is methane.
Methane has the ability to create and release large amounts of heat. Therefore, it is used in significant volumes in industry, for consumption at home. When mixing butane and propane, a good household fuel is obtained.
Methane is used in the production of such products:
- methanol;
- solvents;
- freon;
- ink;
- fuel;
- synthesis gas;
- acetylene;
- formaldehyde;
- formic acid;
- plastic.
Methane application
Liquid hydrocarbons are designed to create fuel for engines and rockets, solvents.
Higher hydrocarbons, where the number of carbon atoms exceeds 20, are involved in the production of lubricants, paints and varnishes, soaps and detergents.
A combination of fatty hydrocarbons with less than 15 H atoms is paraffin oil. This tasteless transparent liquid is used in cosmetics, in the creation of perfumes, and for medical purposes.
Vaseline is the result of the combination of solid and fatty alkanes with less than 25 carbon atoms. The substance is involved in the creation of medical ointments.
Paraffin, obtained by combining solid alkanes, is a solid, tasteless mass, white in color and odorless. The substance is used to produce candles, an impregnating substance for wrapping paper and matches. Paraffin is also popular in the implementation of thermal procedures in cosmetology and medicine.
Note! Synthetic fibers, plastics, detergent chemicals and rubber are also made from alkane mixtures.
Halogenated alkane compounds act as solvents, refrigerants, and also as the main substance for further synthesis.
Useful video: alkanes - chemical properties
Conclusion
Alkanes are acyclic hydrocarbon compounds with a linear or branched structure. A single bond is established between the atoms, which is indestructible. Reactions of alkanes based on the substitution of molecules, characteristic of this type of compounds. The homologous series has the general structural formula CnH2n+2. Hydrocarbons belong to the saturated class because they contain the maximum allowable number of hydrogen atoms.
In contact with
The structure of alkanes
The chemical structure (order of connection of atoms in molecules) of the simplest alkanes - methane, ethane and propane - is shown by their structural formulas given in Section 2. From these formulas it can be seen that there are two types of chemical bonds in alkanes:
S-S and S-N.
The C–C bond is covalent nonpolar. The C–H bond is covalent, weakly polar, because carbon and hydrogen are close in electronegativity (2.5 for carbon and 2.1 for hydrogen). The formation of covalent bonds in alkanes due to the common electron pairs of carbon and hydrogen atoms can be shown using electronic formulas:
Electronic and structural formulas reflect the chemical structure, but do not give an idea of the spatial structure of molecules, which significantly affects the properties of a substance.
Spatial structure, i.e. the mutual arrangement of the atoms of a molecule in space depends on the direction of the atomic orbitals (AO) of these atoms. In hydrocarbons, the main role is played by the spatial orientation of the atomic orbitals of carbon, since the spherical 1s-AO of the hydrogen atom is devoid of a definite orientation.
The spatial arrangement of carbon AOs, in turn, depends on the type of its hybridization (Part I, Section 4.3). The saturated carbon atom in alkanes is bonded to four other atoms. Therefore, its state corresponds to sp3 hybridization (Part I, Section 4.3.1). In this case, each of the four sp3-hybrid carbon AOs participates in axial (σ-) overlap with the s-AO of hydrogen or with the sp3-AO of another carbon atom, forming C-H or C-C σ-bonds.
Four σ-bonds of carbon are directed in space at an angle of 109o28 ", which corresponds to the smallest repulsion of electrons. Therefore, the molecule of the simplest representative of alkanes - methane CH4 - has the shape of a tetrahedron, in the center of which there is a carbon atom, and at the vertices - hydrogen atoms:
The H-C-H bond angle is 109o28". The spatial structure of methane can be shown using volumetric (scale) and ball-and-stick models.
For recording, it is convenient to use the spatial (stereochemical) formula.
In the molecule of the next homologue, C2H6 ethane, two tetrahedral sp3 carbon atoms form a more complex spatial structure:
Alkanes containing more than 2 carbon atoms are characterized by curved shapes. This can be shown using the example of n-butane (VRML model) or n-pentane:
Isomerism of alkanes
Isomerism is the phenomenon of the existence of compounds that have the same composition (the same molecular formula), but a different structure. Such connections are called isomers.
Differences in the order of connection of atoms in molecules (i.e. in the chemical structure) lead to structural isomerism. The structure of structural isomers is reflected by structural formulas. In the alkanes series, structural isomerism manifests itself when there are 4 or more carbon atoms in the chain, i.e. starting with butane C 4 H 10 . If in molecules of the same composition and the same chemical structure, a different mutual arrangement of atoms in space is possible, then spatial isomerism (stereoisomerism). In this case, the use of structural formulas is not enough, and molecular models or special formulas - stereochemical (spatial) or projection - should be used.
Alkanes, starting from ethane H 3 C–CH 3, exist in various spatial forms ( conformations) caused by intramolecular rotation along the C–C σ-bonds and exhibit the so-called rotational (conformational) isomerism.
In addition, if there is a carbon atom in the molecule associated with 4 different substituents, another type of spatial isomerism is possible, when two stereoisomers relate to each other as an object and its mirror image (just as the left hand relates to the right). Such differences in the structure of molecules are called optical isomerism.
. Structural isomerism of alkanes
Structural isomers - compounds of the same composition, differing in the order of binding atoms, i.e. the chemical structure of the molecules.
The reason for the manifestation of structural isomerism in the alkane series is the ability of carbon atoms to form chains of various structures. This type of structural isomerism is called isomerism of the carbon skeleton.
For example, an alkane of composition C 4 H 10 can exist in the form two structural isomers:
and alkane C 5 H 12 - in the form three structural isomers that differ in the structure of the carbon chain:
With an increase in the number of carbon atoms in the composition of molecules, the possibilities for chain branching increase, i.e. the number of isomers increases with the number of carbon atoms.
Structural isomers differ in physical properties. Alkanes with a branched structure, due to a less dense packing of molecules and, accordingly, smaller intermolecular interactions, boil at a lower temperature than their unbranched isomers.
Techniques for constructing structural formulas of isomers
Consider the example of an alkane With 6 H 14 .
1. First, we depict the linear isomer molecule (its carbon skeleton)
2. Then we shorten the chain by 1 carbon atom and attach this atom to any carbon atom of the chain as a branch from it, excluding extreme positions:
(2) or (3)
If you attach a carbon atom to one of the extreme positions, then the chemical structure of the chain will not change.
DEFINITION
Alkanes saturated hydrocarbons are called, the molecules of which consist of carbon and hydrogen atoms, linked to each other only by σ-bonds.
Under normal conditions (at 25 o C and atmospheric pressure), the first four members of the homologous series of alkanes (C 1 - C 4) are gases. Normal alkanes from pentane to heptadecane (C 5 - C 17) are liquids, starting from C 18 and above are solids. As the relative molecular weight increases, the boiling and melting points of alkanes increase. With the same number of carbon atoms in a molecule, branched alkanes have lower boiling points than normal alkanes. The structure of the alkanes molecule using methane as an example is shown in fig. one.
Rice. 1. The structure of the methane molecule.
Alkanes are practically insoluble in water, since their molecules are of low polarity and do not interact with water molecules. Liquid alkanes mix easily with each other. They dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride, diethyl ether, etc.
Obtaining alkanes
The main sources of various saturated hydrocarbons containing up to 40 carbon atoms are oil and natural gas. Alkanes with a small number of carbon atoms (1 - 10) can be isolated by fractional distillation of natural gas or gasoline fraction of oil.
There are industrial (I) and laboratory (II) methods for obtaining alkanes.
C + H 2 → CH 4 (kat = Ni, t 0);
CO + 3H 2 → CH 4 + H 2 O (kat \u003d Ni, t 0 \u003d 200 - 300);
CO 2 + 4H 2 → CH 4 + 2H 2 O (kat, t 0).
— hydrogenation of unsaturated hydrocarbons
CH 3 -CH \u003d CH 2 + H 2 →CH 3 -CH 2 -CH 3 (kat \u003d Ni, t 0);
— reduction of haloalkanes
C 2 H 5 I + HI → C 2 H 6 + I 2 (t 0);
- alkaline melting reactions of salts of monobasic organic acids
C 2 H 5 -COONa + NaOH → C 2 H 6 + Na 2 CO 3 (t 0);
- interaction of haloalkanes with metallic sodium (Wurtz reaction)
2C 2 H 5 Br + 2Na → CH 3 -CH 2 -CH 2 -CH 3 + 2NaBr;
– electrolysis of salts of monobasic organic acids
2C 2 H 5 COONa + 2H 2 O → H 2 + 2NaOH + C 4 H 10 + 2CO 2;
K (-): 2H 2 O + 2e → H 2 + 2OH -;
A (+): 2C 2 H 5 COO - -2e → 2C 2 H 5 COO + → 2C 2 H 5 + + 2CO 2.
Chemical properties of alkanes
Alkanes are among the least reactive organic compounds, which is explained by their structure.
Alkanes under normal conditions do not react with concentrated acids, molten and concentrated alkalis, alkali metals, halogens (except fluorine), potassium permanganate and potassium dichromate in an acidic environment.
For alkanes, reactions proceeding according to the radical mechanism are most characteristic. The homolytic cleavage of C-H and C-C bonds is energetically more favorable than their heterolytic cleavage.
Radical substitution reactions proceed most easily at the tertiary carbon atom, more easily at the secondary carbon atom, and lastly at the primary carbon atom.
All chemical transformations of alkanes proceed with splitting:
1) C-H bonds
- halogenation (S R)
CH 4 + Cl 2 → CH 3 Cl + HCl ( hv);
CH 3 -CH 2 -CH 3 + Br 2 → CH 3 -CHBr-CH 3 + HBr ( hv).
- nitration (S R)
CH 3 -C (CH 3) H-CH 3 + HONO 2 (dilute) → CH 3 -C (NO 2) H-CH 3 + H 2 O (t 0).
– sulfochlorination (S R)
R-H + SO 2 + Cl 2 → RSO 2 Cl + HCl ( hv).
– dehydrogenation
CH 3 -CH 3 → CH 2 \u003d CH 2 + H 2 (kat \u003d Ni, t 0).
— dehydrocyclization
CH 3 (CH 2) 4 CH 3 → C 6 H 6 + 4H 2 (kat = Cr 2 O 3, t 0).
2) C-H and C-C bonds
- isomerization (intramolecular rearrangement)
CH 3 -CH 2 -CH 2 -CH 3 →CH 3 -C (CH 3) H-CH 3 (kat \u003d AlCl 3, t 0).
- oxidation
2CH 3 -CH 2 -CH 2 -CH 3 + 5O 2 → 4CH 3 COOH + 2H 2 O (t 0, p);
C n H 2n + 2 + (1.5n + 0.5) O 2 → nCO 2 + (n + 1) H 2 O (t 0).
Application of alkanes
Alkanes have found application in various industries. Let us consider in more detail, using the example of some representatives of the homologous series, as well as mixtures of alkanes.
Methane is the raw material basis of the most important chemical industrial processes for producing carbon and hydrogen, acetylene, oxygen-containing organic compounds - alcohols, aldehydes, acids. Propane is used as an automotive fuel. Butane is used to produce butadiene, which is a raw material for the production of synthetic rubber.
A mixture of liquid and solid alkanes up to C 25, called vaseline, is used in medicine as the basis for ointments. A mixture of solid alkanes C 18 - C 25 (paraffin) is used to impregnate various materials (paper, fabrics, wood) to give them hydrophobic properties, i.e. water impermeability. In medicine, it is used for physiotherapeutic procedures (paraffin treatment).
Examples of problem solving
EXAMPLE 1
| Exercise | When chlorinating methane, 1.54 g of the compound was obtained, the vapor density in air of which is 5.31. Calculate the mass of manganese dioxide MnO 2 that will be required to produce chlorine if the ratio of the volumes of methane and chlorine introduced into the reaction is 1:2. |
| Decision | The ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure, is called the relative density of the first gas over the second. This value shows how many times the first gas is heavier or lighter than the second gas. The relative molecular weight of air is taken equal to 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of "relative molecular weight of air" is used conditionally, since air is a mixture of gases. Let's find the molar mass of the gas formed during the chlorination of methane: M gas \u003d 29 × D air (gas) \u003d 29 × 5.31 \u003d 154 g / mol. This is carbon tetrachloride - CCl 4 . We write the reaction equation and arrange the stoichiometric coefficients: CH 4 + 4Cl 2 \u003d CCl 4 + 4HCl. Calculate the amount of carbon tetrachloride substance: n(CCl 4) = m(CCl 4) / M(CCl 4); n (CCl 4) \u003d 1.54 / 154 \u003d 0.01 mol. According to the reaction equation n (CCl 4) : n (CH 4) = 1: 1, then n (CH 4) \u003d n (CCl 4) \u003d 0.01 mol. Then, the amount of chlorine substance should be equal to n(Cl 2) = 2 × 4 n(CH 4), i.e. n(Cl 2) \u003d 8 × 0.01 \u003d 0.08 mol. We write the reaction equation for the production of chlorine: MnO 2 + 4HCl \u003d MnCl 2 + Cl 2 + 2H 2 O. The number of moles of manganese dioxide is 0.08 moles, because n (Cl 2) : n (MnO 2) = 1: 1. Find the mass of manganese dioxide: m (MnO 2) \u003d n (MnO 2) × M (MnO 2); M (MnO 2) \u003d Ar (Mn) + 2 × Ar (O) \u003d 55 + 2 × 16 \u003d 87 g / mol; m (MnO 2) \u003d 0.08 × 87 \u003d 10.4 g. |
| Answer | The mass of manganese dioxide is 10.4 g. |
EXAMPLE 2
| Exercise | Set the molecular formula of trichloroalkane, the mass fraction of chlorine in which is 72.20%. Make up the structural formulas of all possible isomers and give the names of substances according to the substitutional IUPAC nomenclature. |
| Answer | Let's write the general formula of trichloroalkene: C n H 2 n -1 Cl 3 . According to the formula ω(Cl) = 3×Ar(Cl) / Mr(C n H 2 n -1 Cl 3) × 100% calculate the molecular weight of trichloroalkane: Mr(C n H 2 n -1 Cl 3) = 3 × 35.5 / 72.20 × 100% = 147.5. Let's find the value of n: 12n + 2n - 1 + 35.5x3 = 147.5; Therefore, the formula of trichloroalkane is C 3 H 5 Cl 3. Let us compose the structural formulas of the isomers: 1,2,3-trichloropropane (1), 1,1,2-trichloropropane (2), 1,1,3-trichloropropane (3), 1,1,1-trichloropropane (4) and 1 ,2,2-trichloropropane (5). CH 2 Cl-CHCl-CH 2 Cl (1); CHCl 2 -CHCl-CH 3 (2); CHCl 2 -CH 2 -CH 2 Cl (3); CCl 3 -CH 2 -CH 3 (4);  
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