Esters called functional derivatives of carboxylic acids of the general formula RC(O)OR" .
Esters of carboxylic acids (as well as sulfonic acids) are named similarly to salts, only instead of the name of the cation, the name of the corresponding alkyl or aryl is used, which is placed before the name of the anion and is written together with it. The presence of the -COOR ester group can also be expressed in a descriptive way, for example, "R-ester of (such and such) acid" (this method is less preferred due to its cumbersomeness):
Esters of lower alcohols and carboxylic acids are volatile liquids, with a pleasant smell, poorly soluble in water and well - in most organic solvents. The odors of esters are reminiscent of the smells of various fruits, which is why in the food industry, essences are prepared from them that mimic fruit odors. The increased volatility of esters is used for analytical purposes.
Hydrolysis. The most important of the acylation reactions is the hydrolysis of esters with the formation of an alcohol and a carboxylic acid:
The reaction is carried out in both acidic and alkaline environments. acid catalyzed ester hydrolysis - the reverse reaction of esterification, proceeds according to the same mechanism A AC 2:
The nucleophile in this reaction is water. The equilibrium shift towards the formation of alcohol and acid is provided by the addition of excess water.
Alkaline hydrolysis is irreversible; during the reaction, a mole of alkali is consumed per mole of ether, i.e., alkali in this reaction acts as a consumable reagent, and not a catalyst:
Hydrolysis of esters in alkaline environment proceeds via the bimolecular acyl mechanism B AC 2 through the stage of formation of the tetrahedral intermediate (I). The irreversibility of alkaline hydrolysis is provided by the practically irreversible acid-base interaction of the carboxylic acid (II) and the alkoxide ion (III). The resulting anion of carboxylic acid (IV) is itself a fairly strong nucleophile and therefore is not subjected to nucleophilic attack.
Ammonolysis of esters. Amides are obtained by ammonolysis of esters. For example, under the action of aqueous ammonia on diethyl fumarate, full fumaric acid amide is formed:
In the ammonolysis of esters with amines with low nucleophilicity, the latter are first converted into amides of alkali or alkaline earth metals:
Amides of carboxylic acids: nomenclature; the structure of the amide group; acid-base properties; acid and alkaline hydrolysis; splitting by hypobromites and nitrous acid; dehydration to nitriles; chemical identification.
Amides called functional derivatives of carboxylic acids of the general formula R-C (O) -NH 2- n R "n, where n = 0-2. In unsubstituted amides, the acyl residue is connected to an unsubstituted amino group, in N-substituted amides one of the hydrogen atoms is replaced by one alkyl or aryl radical, in N,N-substituted - by two.
Compounds containing one, two, or three acyl groups attached to the nitrogen atom are generically called amides (primary, secondary, and tertiary, respectively). The names of primary amides with an unsubstituted group - NH 2 are derived from the names of the corresponding acyl radicals by replacing the suffix -oil (or -yl) with -amide. Amides formed from acids with the suffix -carboxylic acid receive the suffix -carboxamide. Amides of sulfonic acids are also named after their respective acids, using the suffix -sulfonamide.
The names of the radicals RCO-NH- (as well as RSO 2 -NH-) are formed from the names of amides, changing the suffix -amide to -amido-. They are used if there is an older group in the rest of the molecule or the substitution occurs in a more complex structure than the radical R:
In the names of N-substituted primary amides RCO-NHR" and RCO-NR"R" (as well as similar sulfonamides), the names of the radicals R" and R" are indicated before the name of the amide with the symbol N-:
Amides of this type are often referred to as secondary and tertiary amides, which is not recommended by IUPAC.
N-Phenyl-substituted amides are given the suffix -anilide in their names. The position of the substituents in the aniline residue is indicated by numbers with strokes:
In addition, semi-systematic names have been preserved in which the suffix -amide is connected to the base of the Latin name of the carboxylic acid (formamide, acetamide), as well as some trivial names such as "anilides" (acylated anilines) or "toluidides" (acylated toluidines).
Amides are crystalline substances with relatively high and distinct melting points, which allows some of them to be used as derivatives for the identification of carboxylic acids. In rare cases, they are liquids, for example, formic acid amides - formamide and N,N-dimethylformamide - known dipolar aprotic solvents. The lower amides are highly soluble in water.
Amides are one of the most resistant to hydrolysis functional derivatives of carboxylic acids, due to which they are widely distributed in nature. Many amides are used as medicines. For about a century, paracetamol and phenacetin, which are substituted amides of acetic acid, have been used in medical practice.
The structure of amides. The electronic structure of the amide group is largely similar to the structure of the carboxyl group. The amide group is a p,π-conjugated system in which the lone pair of electrons of the nitrogen atom is conjugated with the electrons of the C=O π bond. Delocalization of the electron density in the amide group can be represented by two resonance structures:
Due to conjugation, the C-N bond in amides has partially doubly linked character, its length is significantly less than the length of a single bond in amines, while the C=O bond is somewhat longer than the C=O bond in aldehydes and ketones. Amide group due to conjugation has a flat design . Below are the geometric parameters of the N-substituted amide molecule, determined using X-ray diffraction analysis:
An important consequence of the partially doubly bonded nature of the C-N bond is a rather high energy barrier to rotation around this bond, for example, for dimethylformamide it is 88 kJ/mol. For this reason, amides having different substituents on the nitrogen atom can exist as π-diastereomers. N-substituted amides exist predominantly as Z-isomers:
In the case of N,N-disubstituted amides, the ratio of E- and Z-isomers depends on the volume of radicals connected to the nitrogen atom. The stereoisomers of amides are configurationally unstable, their existence has been proven mainly by physicochemical methods, and they have been isolated individually only in a few cases. This is due to the fact that the rotation barrier for amides is still not as high as for alkenes, for which it is 165 kJ/mol.
Acid-base properties. Amides have weak both acidic and basic properties . The basicity of the amides lies within the range of Pk BH + from -0.3 to -3.5. The reason for the reduced basicity of the amino group in amides is the conjugation of the lone pair of electrons of the nitrogen atom with the carbonyl group. When interacting with strong acids, amides are protonated at the oxygen atom in both dilute and concentrated acid solutions. This kind of interaction underlies acid catalysis in amide hydrolysis reactions:
Unsubstituted and N-substituted amides exhibit weak NH-acid properties , comparable to the acidity of alcohols and remove a proton only in reactions with strong bases.
Acid-base interaction underlies the formation of amides intermolecular associates , the existence of which explains the high melting and boiling points of amides. The existence of two types of associates is possible: linear polymers and cyclic dimers. The predominance of one type or another is determined by the structure of the amide. For example, N-methylacetamide, for which the Z-configuration is preferred, forms a linear associate, and lactams, which have a rigidly fixed E-configuration, form dimers:
N, N-Disubstituted amides form dimers due to the dipole-dipole interaction of 2 polar molecules:
Acylation reactions. Due to the presence of a strong electron-donating amino group in the conjugated amide system, the electrophilicity of the carbonyl carbon atom, and hence the reactivity of amides in acylation reactions, is very low. Low acylating ability of amides is also explained by the fact that the amide ion NH 2 - is a bad leaving group. Of the acylation reactions, hydrolysis of amides is important, which can be carried out in acidic and alkaline media. Amides are much more difficult to hydrolyze than other functional derivatives of carboxylic acids. The hydrolysis of amides is carried out under more severe conditions compared to the hydrolysis of esters.
Acid hydrolysis amides - irreversible reaction leading to the formation of a carboxylic acid and an ammonium salt:
In most cases, the acid hydrolysis of amides proceeds according to the mechanism bimolecular acid acylation A AC 2 , i.e. similar to the mechanism of acid hydrolysis of esters. The irreversibility of the reaction is due to the fact that ammonia or amine in an acidic environment is converted into an ammonium ion that does not have nucleophilic properties:
Alkaline hydrolysis too irreversible reaction; as a result of it, a salt of a carboxylic acid and ammonia or an amine are formed:
Alkaline hydrolysis of amides, like the hydrolysis of esters, proceeds via tetrahedral mechanism AT AC 2 . The reaction begins with the addition of a hydroxide ion (nucleophile) to the electrophilic carbon atom of the amide group. The resulting anion (I) is protonated at the nitrogen atom, and then a good leaving group, an ammonia or amine molecule, is formed in the bipolar ion (II). It is believed that the slow stage is the decay of the tetrahedral intermediate (II).
For anilides and other amides with electron-withdrawing substituents at the nitrogen atom, the decomposition of the tetrahedral intermediate (I) can proceed through the formation of the dianion (II):
Cleavage with nitrous acid. When interacting with nitrous acid and other nitrosating agents, amides are converted into the corresponding carboxylic acids with yields up to 90%:
Dehydration. Unsubstituted amides under the action of phosphorus (V) oxide and some other reagents (POC1 3, PC1 5, SOCl 2) are converted into nitriles:
47. Carboxylic acids: halogenation according to Gell-Volhard-Zelinsky, using the reaction for synthesis a -hydroxy and a -amino acids.
Halogenation of aliphatic carboxylic acids.
Aliphatic carboxylic acids are halogenated at the α-position with chlorine or bromine in the presence of catalytic amounts red phosphorus or phosphorus halides (Gell-Volhard-Zelinsky reaction ). For example, when hexanoic acid is brominated in the presence of red phosphorus or phosphorus(III) chloride, 2-bromohexanoic acid is formed in high yield, for example:
It is not the carboxylic acid itself that undergoes bromination, but the acid chloride formed from it in situ. The acid chloride has stronger CH-acid properties than the carboxylic acid and more easily forms the enol form.
Enol (I) adds bromine to form a halogen derivative (II), which further abstracts a hydrogen halide and turns into an α-halogen-substituted acid halide (III). At the last stage, the unsubstituted carboxylic acid halide is regenerated.
Other heterofunctional acids are synthesized from the resulting α-halo-substituted acids using nucleophilic substitution reactions.
The hydrolysis of esters is catalyzed by both acids and bases. Acid hydrolysis of esters is usually carried out by heating with hydrochloric or sulfuric acid in an aqueous or aqueous-alcoholic medium. In organic synthesis, acid hydrolysis of esters is most often used for mono- and dialkyl-substituted malonic esters (Chapter 17). Mono- and disubstituted derivatives of malonic ester, when boiled with concentrated hydrochloric acid, undergo hydrolysis followed by decarboxylation.
For base-catalyzed hydrolysis, an aqueous or aqueous-alcoholic solution of NaOH or KOH is usually used. Best results are obtained using a thin suspension of potassium hydroxide in DMSO containing a small amount of water.
The latter method is preferred for saponification of esters of hindered acids, another modification of this method is alkaline hydrolysis of hindered esters in the presence of 18-crown-6-polyester:
For preparative purposes, base catalyzed hydrolysis has a number of clear advantages over acid hydrolysis. The rate of basic hydrolysis of esters is typically a thousand times faster than that of acid catalysis. Hydrolysis in an acidic medium is a reversible process, in contrast to hydrolysis in the presence of a base, which is irreversible.
18.8.2.A. Mechanisms of ester hydrolysis
Hydrolysis of esters with pure water is in most cases a reversible reaction, leading to an equilibrium mixture of carboxylic acid and starting ester:
This reaction in acidic and alkaline media is greatly accelerated, which is associated with acid-base catalysis (Chapter 3).
According to K. Ingold, the mechanisms of ester hydrolysis are classified according to the following criteria:
(1) Type of catalysis: acidic (symbol A) or basic (symbol B);
(2) Type of cleavage, showing which of the two -C-O bonds in the ester is cleaved as a result of the reaction: acyl oxygen (index AC) or alkyl oxygen (index AL):
(3) Molecularity of reaction (1 or 2).
From these three criteria, eight different combinations can be made, which are shown in Figure 18.1.
These are the most common mechanisms. Alkaline saponification is almost always of type B AC 2. Acid hydrolysis (as well as esterification) in most cases has an A AC 2 mechanism.
The AAC 1 mechanism is usually observed only in strongly acidic solutions (for example, in conc. H 2 SO 4), and is especially common for esters of sterically hindered aromatic acids.
The mechanism of BAC 1 is still unknown.
The B AL 2 mechanism was found only in the case of exceptionally strong spatially screened acyl groups and neutral hydrolysis of -lactones. The mechanism of A AL 2 is still unknown.
According to the mechanism And AL 1 usually react tertiary-alkyl esters in a neutral or acidic environment. The same substrates under similar conditions can react according to the B AL 1 mechanism, however, upon transition to a slightly more alkaline environment, the B AL 1 mechanism is immediately replaced by the B AC 2 mechanism.
As can be seen from Scheme 18.1, reactions catalyzed by acids are reversible, and from the principle of microscopic reversibility (Chapter 2) it follows that acid-catalyzed esterification also proceeds by similar mechanisms. However, with base catalysis, the equilibrium is shifted towards hydrolysis (saponification), since the equilibrium is shifted due to the ionization of the carboxylic acid. According to the above scheme, in the case of mechanism A AC 1, the COOR and COOH groups are protonated at the alkoxy or hydroxyl oxygen atom. Generally speaking, from the point of view of thermodynamics, the protonation of carbonyl oxygen, the C=O group, is more advantageous, because in this case, the positive charge can be delocalized between both oxygen atoms:
Nevertheless, the solution also contains a tautomeric cation, a necessary intermediate in the A AC 1 mechanism, in small amounts. Both B1 mechanisms (of which B AC 1 is unknown) are in fact not catalytic at all, because at the beginning the dissociation of the neutral ether occurs.
Of the eight Ingold mechanisms, only six have been experimentally proven.
Esters are called functional derivatives of carboxylic acids of the general formula RC(0)0R".
Ways to get. The most significant way to obtain esters is the acylation of alcohols and phenols with various acylating agents, for example, carboxylic acid, acid chlorides, anhydrides. They can also be obtained by the Tishchenko reaction.
Esters with high yields are obtained by alkylation of salts of carboxylic acids with alkyl halides:
Esters are formed by the electrophilic addition of carboxylic acids to alkenes and alkynes. The reaction is often used to obtain esters of tertiary alcohols, for example tert- butyl ethers:
The addition of acetic acid to acetylene produces an industrially important monomer vinyl acetate, zinc acetate on activated carbon is used as a catalyst:
Hydrolysis. The most important of the acylation reactions is the hydrolysis of esters with the formation of an alcohol and a carboxylic acid:
The reaction is carried out in both acidic and alkaline environments. Acid-catalyzed hydrolysis of esters - the reverse reaction of esterification, proceeds according to the same mechanism Als 2
Alkaline hydrolysis is irreversible; during the reaction, a mole of alkali is consumed per mole of ether, i.e., alkali in this reaction acts as a consumable reagent, and not a catalyst:
The hydrolysis of esters in an alkaline medium proceeds according to the bimolecular acyl mechanism BAC2 through the stage of formation of the tetrahedral intermediate (I). The irreversibility of alkaline hydrolysis is provided by the practically irreversible acid-base interaction of the carboxylic acid (II) and the alkoxide ion (III). The resulting anion of carboxylic acid (IV) is itself a fairly strong nucleophile and therefore is not subjected to nucleophilic attack.
Interesterification. With the help of this reaction, the interconversion of esters of the same acid is carried out according to the scheme:
Interesterification is a reversible process, catalyzed by both acids and bases, and proceeds by the same mechanisms as the reactions of esterification and hydrolysis of esters. The equilibrium is shifted by well-known methods, namely, the use of an excess alcohol-reagent (R "OH in the above diagram - to shift to the right) or distillation of one of the reaction products, if it is the lowest boiling component. For example, a known anesthetic is obtained by transesterification novocaine(base) from ethyl ester of p-aminobenzoic acid:
ester condensation. When two ester molecules are condensed in the presence of a basic catalyst, esters of β-oxo acids are formed:
The ethyl acetate molecule has weak CH-acid properties due to the inductive effect of the ester group and is able to interact with a strong base - the ethoxide ion:
Amides of carboxylic acids. Ways to get. The structure of the amide group. Acid-base properties of amides. Acid and alkaline hydrolysis. Cleavage of amides by halogens in an alkaline medium and nitrous acid. Dehydration to nitriles.
Amides are functional derivatives of carboxylic acids of the general formula R-C (O) -NH2_nR "n, where P = 0-2.
Ways to get. The most important method for the preparation of amides is the acylation of ammonia and amines with acid halides, anhydrides and esters.
Acylation of ammonia and amines with acid halides. The acylation reaction of ammonia and amines with acid halides is exothermic and is carried out with cooling:
Acylation of ammonia and amines with anhydrides. For the acetylation of amines, the most accessible of the anhydrides, acetic anhydride, is most often used:
Ammonolysis of esters. Amides are obtained by ammonolysis of esters. For example, under the action of aqueous ammonia on diethyl fumarate, full fumaric acid amide is formed:
The structure of amides. The electronic structure of the amide group is largely similar to the structure of the carboxyl group. The amide group is a p, n-conjugated system in which the lone pair of electrons of the nitrogen atom is conjugated with the electrons of the C=0 n-bond. Delocalization of the electron density in the amide group can be represented by two resonance structures:
Due to conjugation, the C-N bond in amides has a partially doubly bonded character, its length is significantly less than the length of a single bond in amines, while the C=0 bond is somewhat longer than the C=0 bond in aldehydes and ketones. The amide group, due to conjugation, has a planar configuration. Below are the geometric parameters of the iV-substituted amide molecule, determined using X-ray diffraction analysis:
Acid-base properties. Amides have both weak acidic and basic properties. The basicity of amides lies within the range of pA "ext + from -0.3 to -3.5. The reason for the reduced basicity of the amino group in amides is the conjugation of the lone pair of electrons of the nitrogen atom with the carbonyl group. When interacting with strong acids, amides are protonated at the oxygen atom as in dilute and in concentrated acid solutions.This kind of interaction underlies acid catalysis in the hydrolysis of amides:
Acylation reactions. Due to the presence of a strong electron-donating amino group in the conjugated amide system, the electrophilicity of the carbonyl carbon atom, and hence the reactivity of amides in acylation reactions, is very low. The low acylating ability of amides is also explained by the fact that the amide ion NH2- is a poor leaving group. Of the acylation reactions, the hydrolysis of amides, which can be carried out in acidic and alkaline media, is of practical importance. Amides are much more difficult to hydrolyze than other functional derivatives of carboxylic acids. The hydrolysis of amides is carried out under more severe conditions compared to the hydrolysis of esters.
Acid hydrolysis of amides is an irreversible reaction leading to the formation of a carboxylic acid and an ammonium salt:
Alkaline hydrolysis is also an irreversible reaction; as a result of it, a salt of a carboxylic acid and ammonia or an amine are formed:
Cleavage with nitrous acid. When interacting with nitrous acid and other nitrosating agents, amides are converted into the corresponding carboxylic acids with yields up to 90%:
Carbonic acid and its functional derivatives; phosgene, chlorocarbon ethers, carbamic acid and its esters (urethanes). Carbamide (urea), basic and nucleophilic properties. hydrolysis of urea. Acylureas (ureides), ureido acids. Interaction of urea with nitrous acid and hypobromites. Guanidine, basic properties.
Carbonic acid traditionally does not belong to organic compounds, but it and its functional derivatives have a certain similarity with carboxylic acids and their derivatives, and therefore are considered in this chapter.
Dibasic carbonic acid is an unstable compound that readily decomposes into carbon dioxide and water. In an aqueous solution of carbon dioxide, only 0.1% of it exists in the form of carbonic acid. Carbonic acid forms two series of functional derivatives - complete (medium) and incomplete (acid). Acid esters, amides and other derivatives are unstable and decompose with the release of carbon dioxide:
Full carbonic acid chloride - phosgene COC1 2 - a low-boiling liquid with the smell of rotten hay, very toxic, causes pulmonary edema, is formed as a harmful impurity during the photochemical oxidation of chloroform as a result of improper storage of the latter.
In industry, phosgene is obtained by radical chlorination of carbon monoxide (II) in a reactor filled with activated carbon:
Phosgene, like carboxylic acid chlorides, has a high acylating ability; many other functional derivatives of carbonic acid are obtained from it.
When phosgene interacts with alcohols, two types of esters are formed - complete (carbonates) and incomplete (chlorocarbon ethers, or chloroformates), the latter are both esters and acid chlorides. In this case, tertiary amines or pyridine are used as an acceptor of hydrogen chloride and a nucleophilic catalyst.
Carbamic acid- incomplete amide of carbonic acid - an unstable compound, decomposes with the formation of ammonia and carbon dioxide:
Esters of carbamic acid - carbamates, or urethanes, - stable compounds obtained by the addition of alcohols to isocyanates or by acylation of ammonia and amines with the corresponding chloroformate:
Urea(carbamide) - a complete amide of carbonic acid - was first isolated from urine by I. Ruel (1773). It is the most important end product of protein metabolism in mammals; an adult excretes 25-30 g of urea per day. Urea was first synthesized by F. Wöhler (1828) by heating ammonium cyanate:
This synthesis was the first example of obtaining an organic substance from an inorganic compound.
In industry, urea is obtained from ammonia and carbon dioxide at elevated pressure and temperature (180-230 ° C, 150-200 atm):
Urea has weak basic properties (p. uHin + 0.1), forms salts with strong acids. Salts of nitric and oxalic acids are insoluble in water.
Urea is protonated at the oxygen atom, not nitrogen. This is probably due to the delocalization of lone pairs of electrons of nitrogen atoms due to p, π conjugation.
In boiling water, urea hydrolyzes to form ammonia and carbon dioxide; acids and bases catalyze this reaction:
The primary products formed when urea is heated are ammonia and isocyanic acid. Isocyanic acid can trimerize to cyanuric acid or condense with a second urea molecule to form a biuret. Depending on the heating rate, one or another pathway of urea decomposition dominates:
The action of hypohalogenites also leads to the decomposition of urea. Depending on conditions, nitrogen or hydrazine may be formed; the latter is obtained in this way in industry:
Urea also exhibits nucleophilic properties in alkylation and acylation reactions. Alkylation of urea, depending on the alkylating agent, can lead to O- and TV-alkyl derivatives:
Guanidine, or iminourea (H 2 N) 2 C \u003d NH, is industrially obtained by fusing urea with ammonium nitrate or by heating orthocarbonic acid esters with ammonia:
Guanidine is a colorless crystalline substance with strong basic properties. The high basicity at the level of alkali metal hydroxides is due to the complete delocalization of the positive charge in the symmetrical guanidinium cation:
Remains of guanidine and biguanidine are found in some natural compounds and medicinal substances.
The hydrolysis of esters and all other acid derivatives requires acidic or alkaline catalysis. With acid hydrolysis, carboxylic acids and alcohols are obtained (reverse esterification reaction), with alkaline hydrolysis, salts of carboxylic acids and alcohols are formed.
Acid hydrolysis of esters:
S N mechanism, nucleophile - H 2 O, the alkoxy group is replaced by hydroxyl.
Alkaline hydrolysis of esters: the reaction proceeds in two stages with 2 moles of base, the resulting acid is converted into a salt.
S N mechanism, Nu = -OH
Formation of salt compounds Amides are neutral substances, since the basic properties of ammonia are weakened by the substitution of a hydrogen atom in it with an acidic residue. Therefore, the NH 2 group in amides, unlike amines, forms an onium cation only with difficulty. However, with strong acids, amides give salts, such as Cl, which are easily decomposed by water. On the other hand, the hydrogen of the NH 2 group in amides is more easily replaced by metals than in ammonia and in amines. Acetamide, for example, easily dissolves mercury oxide, forming the compound (CH 3 CONH) 2 Hg.
It is possible, however, that during the formation of metal derivatives, amide isomerization occurs and the resulting compound has an isomeric (tautomeric) structure of an imidic acid salt
i.e., there is an analogy with hydrocyanic acid salts.
2. Action of nitrous acid Amides react with nitrous acid, like primary amines, to form carboxylic acids and release nitrogen:
3. Saponification When boiled with mineral acids and alkalis, amides add water, forming carboxylic acid and ammonia:
4. Action of halide alkyls. Under the action of alkyl halides on amides or their metal derivatives, N-substituted amides are obtained:
5. Action of phosphorus pentachloride. Under the action of phosphorus pentachloride on amides, chloramides
easily decomposed into hydrochloric acid and imide chlorides
The latter with ammonia can give salts amidines;
6. Conversion to amines. By vigorous reduction of amides, primary amines with the same number of carbon atoms can be obtained:
7. Hoffmann's reaction. Under the action of hypohalogenite or bromine and alkali on amides, amines are formed, and the carbon atom of the carbonyl group is cleaved off in the form of CO 2 (A. Hoffman). The course of the reaction can be represented as follows:
In educational manuals, another interpretation of the mechanism of this reaction is still often found:
However, this course of the reaction is less plausible, since the formation of a fragment
with a nitrogen atom carrying two free electron pairs is unlikely.
This mechanism is opposed, in particular, by the fact that if the radical R is optically active, then it does not racemize as a result of the reaction. Meanwhile, even the fleeting existence of the free radical R - : would lead to the loss of optical activity.
Chemical properties. The nitro group is one of the most strong electron-withdrawing groups and is able to effectively delocalize negative. charge. In the aromatic conn. as a result of induction and especially mesomeric effects, it affects the distribution of electron density: the nucleus acquires a partial positive. charge, to-ry localized Ch. arr. in ortho and para positions; Hammett constants for the NO 2 group s m 0.71, s n 0.778, s + n 0.740, s - n 1.25. So arr., the introduction of the NO 2 group dramatically increases the reaction. ability org. conn. in relation to nukleof.reagents and complicates p-tion with elektrof. reagents. This determines the widespread use of nitro compounds in org. synthesis: the NO 2 group is introduced into the desired position of the org molecule. Comm., carry out decomp. p-tion associated, as a rule, with a change in the carbon skeleton, and then transformed into another function or removed. In the aromatic In a row, a shorter scheme is often used: nitration-transformation of the NO 2 group.
The formation of nitrone to-t in a series of aromatic nitro compounds is associated with the isomerization of the benzene ring into the quinoid form; for example, nitrobenzene forms with conc. H 2 SO 4 colored salt product f-ly I, o-nitrotoluene exhibits photochromism as a result vnutrimol. proton transfer to form a bright blue O-derivative:
Under the action of bases on primary and secondary nitro compounds, salts of nitro compounds are formed; ambident anions of salts in p-tions with electrophiles are able to give both O- and C-derivatives. So, alkylation of salts of nitro compounds with alkyl halides, trialkylchlorosilanes or R 3 O + BF - 4 gives O-alkylation products. Recent m.b. also obtained by the action of diazomethane or N,O-bis-(trimethylsilyl)acetamide on nitroalkanes with pK a< 3 или нитроновые к-ты, напр.:
Acyclic alkyl esters of nitrone to-t are thermally unstable and decompose according to intramol. mechanism:
R-ts and and with r and ry v o m s vyaz z and C-N. Primary and secondary nitro compounds at loading. with a miner. to-tami in the presence. alcohol or aqueous solution of alkali form carbonyl Comm. (see Neph reaction). R-tion passes through the interval. the formation of nitrone to-t:
As a source Comm. silyl nitrone ethers can be used. The action of strong to-t on aliphatic nitro compounds can lead to hydroxamic to-there, for example:
There are many methods for the reduction of nitro compounds to amines. Widely used iron filings, Sn and Zn in the presence. to-t; with catalytic hydrogenation as catalysts use Ni-Raney, Pd / C or Pd / PbCO 3, etc. Aliphatic nitro compounds are easily reduced to amines LiAlH 4 and NaBH 4 in the presence. Pd, Na and Al amalgams, when heated. with hydrazine over Pd/C; for aromatic nitro compounds, TlCl 3, CrCl 2 and SnCl 2 are sometimes used, aromatic. polynitro compounds are selectively reduced to nitramines with Na hydrosulfide in CH 3 OH. There are ways to choose. recovery of the NO 2 group in polyfunctional nitro compounds without affecting other f-tions.
Under the action of P(III) on aromatic nitro compounds, a succession occurs. deoxygenation of the NO 2 group with the formation of highly reactive nitrenes. R-tion is used for the synthesis of condenser. heterocycles, for example:
R-ts and with the preservation of the NO 2 group. Aliphatic nitro compounds containing an a-H-atom are easily alkylated and acylated to form, as a rule, O-derivatives. However, mutually mod. dilithium salts of primary nitro compounds with alkyl halides, anhydrides or carboxylic acid halides leads to products of C-alkylation or C-acylation, for example:
Known examples vnutrimol. C-alkylations, e.g.:
Primary and secondary nitro compounds react with aliphatic. amines and CH 2 O with the formation of p-amino derivatives (p-tion Mannich); in the district, you can use pre-obtained methylol derivatives of nitro compounds or amino compounds:
Nitromethane and nitroethane can condense with two molecules of methylolamine, and higher nitroalkanes with only one. At certain ratios of reagents p-tion can lead to heterocyclic. connection, for example: with interaction. primary nitroalkane with two equivalents of a primary amine and an excess of formaldehyde form Comm. f-ly V, if the reagents are taken in a ratio of 1:1:3-comm. forms VI.
Aromatic nitro compounds easily enter into p-tion nucleof. substitution and much more difficult, in the district of the electroph. substitution; in this case, the nucleophile is directed to the ortho and pore positions, and the electrophile is directed to the meta position to the NO 2 group. Velocity constant nitration of nitrobenzene is 5-7 orders of magnitude less than that of benzene; this produces m-dinitrobenzene.
During the carboxylation of primary nitroalkanes by the action of CH 3 OMgOCOOCH 3 a-nitrocarboxylic acids or their esters are formed.
When salts of mono-nitro compounds C (NO 2) 4 are treated with Ag or alkali metal nitrites, or when nitrites act on a-halo-nitroalkanes in an alkaline medium (Ter Meer district), gem-dinitro compounds are formed. Electrolysis of a-halo-nitroalkanes in aprotic p-solvents, as well as the treatment of Cl 2 nitro compounds in an alkaline medium or the electrooxidation of salts of nitro compounds lead to vic-dinitro compounds:
The nitro group does not render beings. influence on free-radical alkylation or aromatic arylation. conn.; p-tion leads to the main. to ortho- and para-substituted products.
To restore nitro compounds without affecting the NO 2 group, NaBH 4, LiAlH 4 are used at low temperatures or diborane solution in THF, for example:
Aromatic di- and tri-nitro compounds, in particular 1,3,5-trinitrobenzene, form stable brightly colored crystals. they say complexes with aromatic Comm.-donors of electrons (amines, phenols, etc.). Complexes with picric to-one is used to isolate and purify aromatic. hydrocarbons. Intermod. di- and trinitrobenzenes with strong bases (HO - , RO - , N - 3 , RSO - 2 , CN - , aliphatic amines) leads to the formation of Meisen-heimer complexes, which are isolated as colored alkali metal salts.
Suitable oxidizing agents for these reactions are chromic or nitric acid, chromium mixture, manganese dioxide or selenium dioxide.
During oxidation with chromic acid, alcohol nucleophilically adds to chromic acid, while water is split off and an ester of chromic acid is formed (this is the first stage of the reaction, it is similar to the formation of esters of carboxylic acids, cf. Sec. E, 7.1.5.1). In the second stage, which probably goes through a cyclic transition state, the a-hydrogen of the alcohol passes to the chromate residue, and the metal passes from the hexavalent state to the tetravalent state:
| n-CH3O> P-tert-C 4 H 9 > P-CH 3 > P-Cl> P-NO 2 | (G.6.20) |
When primary alcohols are oxidized, the resulting aldehyde must be protected from further oxidation to carboxylic acid. It is possible, for example, to constantly distill off the aldehyde from the reaction mixture: this is quite feasible, since the boiling point of the aldehyde is usually lower than the boiling point of the corresponding alcohol. Nevertheless, the yield of aldehydes during oxidation with dichromate rarely exceeds 60%. It is noteworthy that when the reaction is carried out properly, multiple carbon-carbon bonds are almost not affected.
Aldehydes are also formed by heating alcohols with an aqueous neutral dichromate solution, but only benzyl alcohols give good yields.
Higher yields of aldehydes can be obtained by oxidizing primary alcohols tert-butyl chromate (in petroleum ether, benzene or carbon tetrachloride) or manganese dioxide (in acetone, petroleum ether, carbon tetrachloride or dilute sulfuric acid). These reagents also make it possible to obtain unsaturated and aromatic aldehydes in good yields.
The oxidation of secondary alcohols to ketones is even easier than the oxidation of primary alcohols. The yields here are higher, since, firstly, the reactivity of secondary alcohols is higher than that of primary ones, and secondly, the resulting ketones are much more resistant to oxidation compared to aldehydes. In a series of steroids and terpenes, the oxidation of secondary alcohols with a complex of chromic acid with pyridine, as well as chromic anhydride in dimethylformamide, has proven itself well. A good oxidizing agent is also chromic anhydride in acetone; it can be used to oxidize unsaturated secondary alcohols without affecting the multiple carbon-carbon bond.
A new method, also suitable for hindered alcohols, is oxidation with dimethyl sulfoxide in acetic anhydride.
According to the method below, the reaction is carried out in a two-phase system. The formed ketones are extracted with an organic solvent and thus protected from further oxidation.
disaccharides- carbohydrates, the molecules of which consist of two monosaccharide residues, which are connected to each other due to the interaction of two hydroxyl groups.
In the process of formation of a disaccharide molecule, one molecule of water is split off:
or for sucrose:
Therefore, the molecular formula of disaccharides is C 12 H 22 O 11.
The formation of sucrose occurs in plant cells under the influence of enzymes. But chemists have found a way to implement many of the reactions that are part of the processes that occur in wildlife. In 1953, the French chemist R. Lemieux for the first time carried out the synthesis of sucrose, which was called by his contemporaries "the conquest of the Everest of organic chemistry."
In industry, sucrose is obtained from sugar cane juice (content 14-16%), sugar beet (16-21%), as well as some other plants, such as Canadian maple or pear.
Everyone knows that sucrose is a crystalline substance that has a sweet taste and is highly soluble in water.
Sugar cane juice contains the carbohydrate sucrose, commonly referred to as sugar.
The name of the German chemist and metallurgist A. Marggraf is closely associated with the production of sugar from beets. He was one of the first researchers to use a microscope in his chemical studies, with which he discovered sugar crystals in beet juice in 1747.
Lactose - crystalline milk sugar, was obtained from the milk of mammals as early as the 17th century. Lactose is a less sweet disaccharide than sucrose.
Now let's get acquainted with carbohydrates that have a more complex structure - polysaccharides.
Polysaccharides- high-molecular carbohydrates, the molecules of which consist of many monosaccharides.
In a simplified form, the general scheme can be represented as follows:
Now let's compare the structure and properties of starch and cellulose - the most important representatives of polysaccharides.
The structural unit of the polymer chains of these polysaccharides, the formula of which is (C 6 H 10 O 5) n, are glucose residues. In order to write down the composition of the structural unit (C 6 H 10 O 5), you need to subtract a water molecule from the glucose formula.
Cellulose and starch are of vegetable origin. They are formed from glucose molecules as a result of polycondensation.
The equation for the polycondensation reaction, as well as the inverse process of hydrolysis for polysaccharides, can be conditionally written as follows:
Starch molecules can have both a linear and branched type of structure, cellulose molecules can only have a linear one.
When interacting with iodine, starch, unlike cellulose, gives a blue color.
These polysaccharides also have various functions in the plant cell. Starch serves as a reserve nutrient, cellulose performs a structural, building function. Plant cell walls are made up of cellulose.
CANNICEROREACTION, oxidizing-reducing disproportionation of aldehydes under the action of alkali with the formation of primary alcohols and carboxylic acids, for example:
The aldehyde is treated with conc. aqueous or water-alcohol solution of alkali during cooling or slight heating. Catalysts - decomp. metals (eg Ag, Ni, Co, Cu) and their oxides. Aldehydes that do not contain atomH in the a-position to the carbonyl group enter the p-tion. Otherwise, it is not the Cannizzaro reaction that is preferable, but the aaldol condensation. Electron-withdrawing substituents in the aromatic ring. aldehydes speed up the process, while electron donors slow it down. Benzaldehydes with substituents in the ortho positions do not react in Cannizzaro; o- and p-hydroxybenzaldehydes react only in the presence. Ag. R-tion with the use of two razl.aldehydes (the so-called cross Cannizzaro reaction) is used by Ch. arr. to obtain a high yield of primary alcohols from aromatic. aldehydes. In this case, formaldehyde usually acts as a reducing agent:
ArCHO + CH 2 O: ArCH 2 OH + HCOOH
In the synthesis of polyhydroxymethylated Comm. formaldehyde participates in the first stage in the aldol condensation, and then as a reducing agent in the cross Cannizzaro reaction:
The proposed mechanism of the Cannizzaro reaction in Homog. environment includes the stage of hydride transfer
For aromatic aldehydes, the possibility of participation in the Cannizzaro reaction of radical anions formed as a result of one-electron transfer cannot be ruled out. R-tion, similar to the Cannizzaro reaction, is carried out with intramol. disproportionation of a-ketoaldehydes in the presence. alkalis (Cannizzaro rearrangement):
Cannizzaro reaction is used for prom. synthesis of pentaerythritol, preparative production of alcohols, carboxylic acids, etc. R-tion was discovered by S. Cannizzaro in 1853.
Pyrrole, furan and thiophene are five-membered heterocyclic compounds with one heteroatom.
The numbering of atoms in a heterocycle begins with a heteroatom and proceeds counterclockwise. Positions 2- and 5-are called a-positions, 3- and 4- are called b-positions.
According to formal features, these compounds are aromatic, since they are conjugated cyclic p-systems, which include 6p electrons - 4 electrons of the diene system - and a pair of electrons of the heteroatom. The cycle is practically planar, which means that the hybridization state of the heteroatom is close to sp 2 .
Resonance structures are presented below, illustrating the delocalization of electrons of a heteroatom along a heterocyclic ring using furan as an example.
The above resonance structures show that the heteroatom (in this case, the oxygen atom), as a result of mesomeric interaction with the diene π-system, transfers the electron density to the ring, as a result of which a certain negative charge arises on the carbon atoms in the heterocycle, and on the oxygen atom, respectively, positive charge. The oxygen atom, of course, in addition to the positive mesomeric effect, also exhibits a negative inductive effect. However, its manifestation in the properties of the compounds under consideration is less pronounced, and therefore five-membered heterocycles with one heteroatom are referred to p-excess aromatic heterocyclic compounds. The resonance leads to some evenness of the bond lengths in the heterocycle, which also indicates a certain aromaticity of the system.
Esters are typical electrophiles. Due to the +M effect of the oxygen atom associated with the hydrocarbon radical, they exhibit a less pronounced electrophilic character compared to acid halides and acid anhydrides:
The electrophilicity of ethers increases if the hydrocarbon radical forms a conjugated system with the oxygen atom, the so-called. activated esters:
Esters enter into nucleophilic substitution reactions.
1. Hydrolysis of esters takes place in both acidic and alkaline environments.
Acid hydrolysis of esters is a sequence of reversible transformations opposite to the esterification reaction:
The mechanism of this reaction involves the protonation of the oxygen atom of the carbonyl group to form a carbocation, which reacts with a water molecule:
Alkaline hydrolysis. Hydrolysis in the presence of aqueous solutions of alkalis is easier than acidic because the hydroxide anion is a more active and less bulky nucleophile than water. Unlike acid hydrolysis, alkaline hydrolysis is irreversible:
Alkali acts not as a catalyst, but as a reactant. Hydrolysis begins with the nucleophilic attack of the hydroxide ion on the carbon atom of the carbonyl group. An intermediate anion is formed, which splits off the alkoxide ion and turns into a carboxylic acid molecule. The alkoxide ion, as a stronger base, abstracts a proton from an acid molecule and turns into an alcohol molecule:
Alkaline hydrolysis is irreversible because the carboxylate anion has a high negative charge delocalization and is not susceptible to attack by the alcohol hydroxyl.
Often the alkaline hydrolysis of esters is called saponification. The term comes from the name of the products of alkaline hydrolysis of fats - soap.
2. The interaction with ammonia (immonolysis) and its derivatives proceeds according to a mechanism similar to alkaline hydrolysis:
3. The reaction of interesterification (alcoholysis of esters) is catalyzed by both mineral acids and shells:
To shift the equilibrium to the right, the more volatile alcohol is distilled off.
4. Claisen ester condensation is typical for esters of carboxylic acids containing hydrogen atoms in the α-position. The reaction proceeds in the presence of strong bases:
The alkoxide ion splits off a proton from the α-carbon atom of the ether molecule. A mesomerically stabilized carbanion (I) is formed, which, acting as a nucleophile, attacks the carbon atom of the carbonyl group of the second ester molecule. The addition product (II) is formed. It splits off the alkoxide ion and turns into the final product (III). Thus, the whole scheme of the reaction mechanism can be divided into three stages:
If two esters containing α-hydrogen atoms react, then a mixture of four possible products is formed. The reaction is used for the industrial production of acetoacetic ester.
5. Recovery of esters:
Primary alcohols are formed by the action of hydrogen gas in the presence of a skeletal nickel catalyst (Raney nickel).
6. The action of organomagnesium compounds followed by hydrolysis leads to the formation of tertiary alcohols.
