Hydrolysis of esters and all other acid derivatives requires acid or alkaline catalysis. Acid hydrolysis produces carboxylic acids and alcohols (reverse esterification reaction); alkaline hydrolysis produces salts of carboxylic acids and alcohols.
Acid hydrolysis of esters:
Mechanism S N , 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.
Mechanism S N, Nu = − OH
Formation of salt-like compounds Amides are neutral substances, since the basic properties of ammonia are weakened by the replacement of the hydrogen atom in it by an acidic residue. Therefore, the NH 2 group in amides, unlike amines, only forms an onium cation with difficulty. However, with strong acids, amides give salts, for example 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 amines. Acetamide, for example, readily dissolves mercuric oxide, forming the compound (CH 3 CONH) 2 Hg.
It is possible, however, that during the formation of metal derivatives, isomerization of the amide occurs and the resulting compound has the isomeric (tautomeric) structure of an imido acid salt
i.e., there is an analogy with salts of hydrocyanic acid.
2. Action of nitrous acid Amides react with nitrous acid, like primary amines, to form carboxylic acids and release nitrogen:
3. Saponification When boiling with mineral acids and alkalis, amides add water, forming carboxylic acid and ammonia:
4. Action of alkyl halides. The action of alkyl halides on amides or their metal derivatives produces N-substituted amides:
5. Effect of phosphorus pentachloride. The action of phosphorus pentachloride on amides produces chloramides
easily decomposing into hydrochloric acid and imide chlorides
The latter with ammonia can produce 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 reaction. When amides are exposed to hypohalogenite or bromine and alkali, 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 reaction course is less plausible, since the formation of a fragment
with a nitrogen atom carrying two free electron pairs is unlikely.
This mechanism is contradicted, 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 a 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 aromatic conn. as a result of the inductive and especially mesomeric effect, it affects the distribution of electron density: the nucleus becomes partially positive. charge, which is 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. Thus, the introduction of the NO 2 group sharply increases the reaction. ability to org. conn. in relation to nucleophilic reagents and makes it difficult to deal with electroph. 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. connection, carry out decomposition. ptions associated, as a rule, with a change in the carbon skeleton, and then transformed into another function or removed. In aromatic In some cases, a shorter scheme is often used: nitration-transformation of the NO 2 group.
Formation of nitron compounds in a series aromatic nitro compounds associated with the isomerization of the benzene ring into the quinoid form; for example, nitrobenzene forms with conc. H 2 SO 4 colored salt-like product of type I, o-nitrotoluene exhibits photochromism as a result of intramol. proton transfer to form a bright blue O derivative:
When bases act on primary and secondary nitro compounds, salts of nitro compounds are formed; ambident anions of salts in solutions with electrophiles are capable of producing both O- and C-derivatives. Thus, when the salts of nitro compounds are alkylated with alkyl halides, trialkylchlorosilanes or R 3 O + BF - 4, O-alkylation products are formed. Latest 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 nitronic acids are thermally unstable and disintegrate intramol. mechanism:
R-ts and s r a r s in about m with connections and S-N. Primary and secondary nitro compounds when heated. with mineral K-tami is present. alcohol or water solution alkalis form carbonyl compounds. (see Nave reaction). R-tion passes through the gaps. formation of nitron compounds:
As initial conn. Silyl nitrone ethers can be used. Action strong qualities on aliphatic nitro compounds can lead to hydroxamic compounds, for example:
There are many known methods for the reduction of nitro compounds to amines. Iron filings, Sn and Zn are widely used. kit; with catalytic in hydrogenation, Ni-Raney, Pd/C or Pd/PbCO 3 and others are used as catalysts. Aliphatic nitro compounds are easily reduced to the amines LiAlH 4 and NaBH 4 in the presence. Pd, Na and Al amalgams, at heating. with hydrazine over Pd/C; for aromatic nitro compounds, TlCl 3, CrCl 2 and SnCl 2 are sometimes used, aromatic. poly-nitro compounds are selectively reduced to nitramines by Na hydrosulfide in CH 3 OH. There are ways to choose. reduction of the NO 2 group in polyfunctional nitro compounds without affecting other functions.
When P(III) acts on aromatic nitro compounds, a sequence occurs. deoxygenation of the NO 2 group with the formation of highly reactive nitrenes. The solution is used for the synthesis of condenser. heterocycles, for example:
R-ts and preservation of the NO 2 group. Aliphatic nitro compounds containing an a-H atom are easily alkylated and acylated, usually forming O-derivatives. However, mutual mod. dilithium salts of primary nitro compounds with alkyl halides, anhydrides or acid halides carbon kits leads to C-alkylation or C-acylation products, e.g.:
There are known examples of intramol. C-alkylation, e.g.:
Primary and secondary nitro compounds react with aliphatic compounds. amines and CH 2 O with the formation of p-amino derivatives (Mannich solution); in the solution you can use previously prepared 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, the solution can lead to heterocyclic. connection, for example: when interacting primary nitroalkane with two equivalents of primary amine and excess formaldehyde are formed. Forms V, if the reagents are taken in the ratio 1:1:3-comm. Forms VI.
Aromatic nitro compounds easily enter into nucleophilic solutions. substitution and much more difficult - in the district of electroph. substitution; in this case, the nucleophile is directed to the ortho- and po-positions, and the electrophile is directed to the meta-position to the NO 2 group. Electrof speed constant nitration of nitrobenzene is 5-7 orders of magnitude less than that of benzene; this produces m-dinitrobenzene.
When carboxylation of primary nitroalkanes by the action of CH 3 OMgOCOOCH 3 a-nitrocarbon compounds 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 environment (Ter Meer's solution), heme-dinitro compounds are formed. Electrolysis of a-halo-nitroalkanes in aprotic solutions, as well as the treatment of nitro compounds Cl 2 in an alkaline medium or the electrooxidation of salts of nitro compounds lead to vic-dinitro compounds:
The nitro group does not render creatures. influence on free radical alkylation or arylation of aromatic. conn.; r-tion leads to the basis. to ortho- and para-substituted products.
To reduce nitro compounds without affecting the NO 2 group, NaBH 4, LiAlH 4 are used with low t-rah or solution of diboron in THF, for example:
Aromatic di- and tri-nitro compounds, in particular 1,3,5-trinitroben-zene, form stable, brightly colored crystalline compounds. they say complexes with aromatic electron donor compounds (amines, phenols, etc.). Complexes with picric acid are used for the isolation and purification of aromatic compounds. hydrocarbons. Interaction di- and trinitrobenzenes with strong bases (HO - , RO - , N - 3 , RSO - 2 , CN - , aliphatic amines) leads to the formation of Meisen-Haimer complexes, which are isolated in the form of 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, the alcohol nucleophilically attaches to chromic acid, during which 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. section E, 7.1.5.1). In the second stage, probably going 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 one:
| n-CH 3 O > 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, multiples carbon-carbon bonds are almost not affected.
Aldehydes are also formed when alcohols are heated with an aqueous neutral solution of dichromate, but only benzyl alcohols give good yields.
Higher yields of aldehydes can be obtained by oxidation of primary alcohols rubs-butyl chromate (in petroleum ether, benzene or carbon tetrachloride) or manganese dioxide (in acetone, petroleum ether, carbon tetrachloride or dilute sulfuric acid). These reagents allow you to good exits also obtain unsaturated and aromatic aldehydes.
The oxidation of secondary alcohols to ketones is even easier than the oxidation of primary alcohols. The yields here are higher because, firstly, the reactivity of secondary alcohols is higher than that of primary alcohols, and secondly, the resulting ketones are much more resistant to oxidation compared to aldehydes. Among steroids and terpenes, the oxidation of secondary alcohols with a complex of chromic acid and pyridine, as well as chromic anhydride in dimethylformamide, has proven itself. Chromic anhydride in acetone is also a good oxidizing agent; it can be used to oxidize unsaturated secondary alcohols without affecting the multiple carbon-carbon bond.
A new method, also suitable for sterically hindered alcohols, is oxidation with dimethyl sulfoxide in acetic anhydride.
According to the procedure given below, the reaction is carried out in a two-phase system. The resulting ketones are extracted with an organic solvent and are thus protected from further oxidation.
Disaccharides– carbohydrates, the molecules of which consist of two monosaccharide residues, which are connected to each other through the interaction of two hydroxyl groups.
During the formation of a disaccharide molecule, one water molecule is eliminated:
or for sucrose:
That's why molecular formula disaccharides 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 carry out many reactions that are part of the processes that occur in living nature. In 1953, the French chemist R. Lemieux first carried out the synthesis of sucrose, which contemporaries called “the conquest of the Everest of organic chemistry.”
In industry, sucrose is obtained from juice sugar cane(content 14-16%), sugar beet (16-21%), as well as some other plants, such as Canadian maple or earthen pear.
Everyone knows that sucrose is a crystalline substance that has a sweet taste and is highly soluble in water.
Sugarcane juice contains the carbohydrate sucrose, commonly called 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 back in 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 weight carbohydrates, the molecules of which consist of many monosaccharides.
In simplified form general scheme can be represented like this:
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, is 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 have vegetable origin. They are formed from glucose molecules as a result of polycondensation.
The equation for the polycondensation reaction, as well as its inverse hydrolysis process 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 structure.
When interacting with iodine, starch, unlike cellulose, gives a blue color.
Various functions These polysaccharides are also found in plant cells. Starch serves as a reserve nutrient, cellulose performs a structural, construction function. Walls plant cells constructed from cellulose.
CANNIZZAROREACTION, oxidation-reduction disproportionation of aldehydes under the influence of alkali with the formation of primary alcohols and carbonic acids, for example:
Aldehyde is treated with conc. aqueous or aqueous-alcoholic solution of alkali when cooled or slightly heated. Catalysts - decomp. metals (eg, Ag, Ni, Co, Cu) and their oxides. Aldehydes that do not contain an H atom in the a-position to the carbonyl group enter the solution. Otherwise, it is not the Cannizzaro reaction that is preferable, but rather the aldol condensation. Electron-withdrawing substituents in the aromatic ring. aldehydes speed up the process, and electron-donating ones slow it down. Benzaldehydes with substituents in ortho positions do not react in Cannizzaro; o- and p-hydroxybenzaldehydes react only in the presence. Ag. A reaction using two different aldehydes (the so-called cross Cannizzaro reaction) is used in Chapter. arr. for obtaining primary alcohols from aromatics in high yield. aldehydes. Formaldehyde is usually used as a reducing agent:
ArCHO + CH 2 O: ArCH 2 OH + HCOOH
During the synthesis of polyhydroxymethylated compounds. formaldehyde at the first stage participates in aldol condensation, and then as a reducing agent in the Cannizzaro cross reaction:
Cannizzaro's proposed mechanism for the homogeneous reaction. environment includes a hydride transfer stage
For aromatic aldehydes, the possibility of participation in the Cannizzaro reaction of radical anions formed as a result of one-electron transfer cannot be excluded. A reaction similar to the Cannizzaro reaction occurs with intramol. disproportionation of a-ketoaldehydes in the presence. alkalis (Cannizzaro rearrangement):
The Cannizzaro reaction is used for industrial purposes. synthesis of pentaerythritol, preparative production of alcohols, carbon compounds, etc. The process was discovered by S. Cannizzaro in 1853.
Pyrrole, furan and thiophene are five-membered heterocyclic compounds with one heteroatom.
The numbering of atoms in the heterocycle starts with the heteroatom and goes counterclockwise. Positions 2 and 5 are called a-positions, 3 and 4 are called b-positions.
According to formal characteristics, these compounds are classified as aromatic, since they are conjugated cyclic p-systems, which include 6p electrons - 4 electrons of the diene system - and a pair of heteroatom electrons. The cycle is almost flat, which means that the hybridization state of the heteroatom is close to sp 2.
Below are resonance structures illustrating the delocalization of electrons of a heteroatom along a heterocyclic ring using furan as an example.
The above resonance structures show that a heteroatom (in this case, an oxygen atom), as a result of mesomeric interaction with the diene π-system, transfers electron density to the ring, as a result of which a certain negative charge appears on the carbon atoms in the heterocycle, and, accordingly, a positive charge on the oxygen atom 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 classified as p-excess aromatic heterocyclic compounds. Resonance leads to some equalization of bond lengths in the heterocycle, which also indicates a certain aromaticity of the system.
Esters are called functional derivatives of carboxylic acids of the general formula RC(O)OR" .
Esters 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 written together with it. The presence of an ester group -COOR can also be reflected in a descriptive way, for example, “R-ester of (such and such) acid” (this method is less preferable due to its cumbersomeness):
Esters of lower alcohols and carboxylic acids are volatile liquids with a pleasant odor, poorly soluble in water and well soluble in most organic solvents. The odors of esters resemble the odors of various fruits, so in the food industry they are used to prepare essences that imitate 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 alcohol and carboxylic acid:
The reaction takes place in both acidic and alkaline environments. Acid-catalyzed hydrolysis of esters - the reverse reaction to esterification, proceeds according to the same mechanism A AC 2:
The nucleophile in this reaction is water. A shift in equilibrium towards the formation of alcohol and acid is ensured 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 into alkaline environment proceeds via a bimolecular acyl mechanism B AC 2 through the stage of formation of tetrahedral intermediate (I). The irreversibility of alkaline hydrolysis is ensured by the practically irreversible acid-base interaction of carboxylic acid (II) and alkoxide ion (III). The resulting carboxylic acid anion (IV) is itself a fairly strong nucleophile and therefore is not subject to nucleophilic attack.
Ammonolysis of esters. Amides are obtained by ammonolysis of esters. For example, when aqueous ammonia reacts with diethyl fumarate, complete fumaric acid amide is formed:
During 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; structure of the amide group; acid-base properties; acid and alkaline hydrolysis; splitting with hypobromites and nitrous acid; dehydration to nitriles; chemical identification.
amides are 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 amides, by two.
Compounds containing one, two or three acyl groups attached to a 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. Sulfonic acid amides are also named after their corresponding acids, using the suffix -sulfonamide.
The names of the radicals RCO-NH- (like RSO 2 -NH-) are formed from the names of amides, changing the suffix -amide to -amido-. They are used if the rest of the molecule contains more senior group or the substitution occurs in a more complex structure than the R radical:
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-:
These types of amides are often referred to as secondary and tertiary amides, which are not recommended by IUPAC.
N-Phenyl-substituted amides are given the suffix -anilide in their names. The position of substituents in the aniline residue is indicated by numbers with primes:
In addition, semi-systematic names have been preserved, in which the suffix -amide is combined with the base of the Latin name for 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, allowing 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. 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, have been used in medical practice. acetic acid.
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 electron density in the amide group can be represented by two resonant structures:
Due to conjugation, the C-N bond in amides has partially bilinked character, its length is significantly less than the length of a single bond in amines, while the C=O bond is slightly longer than the C=O bond in aldehydes and ketones. Amide group due to conjugation has a flat configuration . Below are geometric parameters N-substituted amide molecules identified by X-ray diffraction analysis:
An important consequence of the partially bicoupled nature C-N connections The energy barrier for rotation around this bond is quite high; for example, for dimethylformamide it is 88 kJ/mol. For this reason, amides having different substituents on the nitrogen atom can exist in the form of 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. Stereoisomers of amides are configurationally unstable, their existence has been proven mainly by physicochemical methods, in individual form they stood out only in isolated 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 amides lies within the range of Pk BH + values 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 interactions underlie the formation of amides intermolecular associates , the existence of which explains the high melting and boiling temperatures 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 with 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 therefore 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, the 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 stringent conditions compared to the hydrolysis of esters.
Acid hydrolysis amides - irreversible reaction leading to the formation of carboxylic acid and ammonium salt:
In most cases, 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 ammonium ion, which does not have nucleophilic properties:
Alkaline hydrolysis Same irreversible reaction; as a result, a carboxylic acid salt and ammonia or amine are formed:
Alkaline hydrolysis of amides, like the hydrolysis of esters, proceeds according to tetrahedral mechanism IN 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 is formed in the bipolar ion (II) - an ammonia or amine molecule. It is believed that the slow stage is the decomposition 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 a dianion (II):
Nitrous acid digestion. When interacting with nitrous acid and other nitrosating agents, amides are converted into the corresponding carboxylic acids with yields of up to 90%:
Dehydration. Unsubstituted amides under the influence 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, use of the reaction for synthesis a -hydroxy and a -amino acids.
Halogenation of aliphatic carboxylic acids.
Aliphatic carboxylic acids are halogenated to the α-position by chlorine or bromine in the presence of catalytic quantities 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 forms the enol form more easily.
Enol (I) adds bromine to form a halogen derivative (II), which subsequently eliminates hydrogen halide and turns into an α-halogenated acid halide (III). At the last stage, the acid halide of the unsubstituted carboxylic acid is regenerated.
From the resulting α-halogen-substituted acids, other heterofunctional acids are synthesized using nucleophilic substitution reactions.
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 undergo nucleophilic substitution reactions.
1. Hydrolysis of esters occurs 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 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 acidic, alkaline hydrolysis is irreversible:
Alkali does not act as a catalyst, but as a reagent. Hydrolysis begins with a nucleophilic attack by 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 the acid molecule and turns into an alcohol molecule:
Alkaline hydrolysis is irreversible because the carboxylate anion has a highly delocalized negative charge and is not susceptible to attack by the alcohol hydroxyl.
Alkaline hydrolysis of esters is often called saponification. The term comes from the name of the products of alkaline hydrolysis of fats - soap.
2. Interaction with ammonia (immonolysis) and its derivatives proceeds by a mechanism similar to alkaline hydrolysis:
3. The transesterification reaction (alcoholysis of esters) is catalyzed by both mineral acids and shells:
To change the equilibrium, the more volatile alcohol is distilled to the right.
4. Claisen ester condensation is characteristic of esters of carboxylic acids containing hydrogen atoms in the α-position. The reaction occurs in the presence strong reasons:
The alkoxide ion abstracts 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 ether molecule. The addition product (II) is formed. It splits off the alkoxide ion and turns into the final product (III). Thus, the entire scheme of the reaction mechanism can be divided into three stages:
If two esters containing α-hydrogen atoms react, a mixture of four possible products is formed. The reaction is used for the industrial production of acetoacetic ester.
5. Reduction 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.
DEFINITION
Compounds of organic nature, which are derivatives of carboxylic acids, formed during the interaction of the latter with alcohols:
General structural formula of esters:
where R and R’ are hydrocarbon radicals.
Hydrolysis of esters
One of the most characteristic abilities of esters (besides esterification) is their hydrolysis - splitting under the influence of water. In another way, the hydrolysis of esters is called saponification. Unlike the hydrolysis of salts, in this case it is practically irreversible. A distinction is made between alkaline and acid hydrolysis of esters. In both cases, alcohol and acid are formed:
a) acid hydrolysis
b) alkaline hydrolysis
Examples of problem solving
EXAMPLE 1
| Exercise | Determine the mass of acetic acid that can be obtained during the saponification reaction of ethyl acetate weighing 180 g. |
| Solution | Let us write the reaction equation for the hydrolysis of acetic acid ethyl ester using the gross formula: C 4 H 8 O 2 + H 2 O ↔ CH 3 COOH + C 2 H 5 OH. Let's calculate the amount of ethyl acetate ( molar mass- 88 g/mol), using the mass value from the problem conditions: υ (C 4 H 8 O 2) = m (C 4 H 8 O 2)/M (C 4 H 8 O 2) = 180/88 = 2 mol. According to the reaction equation, the number of moles of ethyl acetate and acetic acid are equal: υ (C 4 H 8 O 2) = υ (CH 3 COOH) = 2 mol. Then, you can determine the mass of acetic acid (molar mass - 60 g/mol): m(CH 3 COOH) = υ (CH 3 COOH) × M (CH 3 COOH) = 2 × 60 = 120g. |
| Answer | The mass of acetic acid is 120 grams. |
Esters are derivatives of acids in which the acidic hydrogen is replaced by alkyl (or generally hydrocarbon) radicals.
Esters are divided depending on which acid they are derived from (inorganic or carboxylic).
Among esters, a special place is occupied by natural esters - fats and oils, which are formed by the triatomic alcohol glycerol and higher fatty acids containing an even number carbon atoms. Fats are part of plant and animal organisms and serve as one of the sources of energy of living organisms, which is released during the oxidation of fats.
General formula of carboxylic acid esters:
where R and R" are hydrocarbon radicals (in formic acid esters R is a hydrogen atom).
General formula of fats:
where R", R", R"" are carbon radicals.
Fats are either “simple” or “mixed”. Simple fats contain residues of the same acids (i.e. R’ = R" = R""), while mixed fats contain different ones.
The most common fatty acids found in fats are:
1. Butyric acid CH 3 - (CH 2) 2 - COOH
3. Palmitic acid CH 3 - (CH 2) 14 - COOH
4. Stearic acid CH 3 - (CH 2) 16 - COOH
5. Oleic acid C 17 H 33 COOH
CH 3 -(CH 2) 7 -CH === CH-(CH 2) 7 -COOH
6. Linoleic acid C 17 H 31 COOH
CH 3 -(CH 2) 4 -CH = CH-CH 2 -CH = CH-COOH
7. Linolenic acid C 17 H 29 COOH
CH 3 CH 2 CH = CHCH 2 CH == CHCH 2 CH = CH(CH 2) 4 COOH
The following types of isomerism are characteristic of esters:
1. Isomerism of the carbon chain begins at the acid residue with butanoic acid, at the alcohol residue with propyl alcohol, for example, ethyl isobutyrate, propyl acetate and isopropyl acetate are isomeric to ethyl butyrate.
2. Isomerism of the position of the ester group -CO-O-. This type of isomerism begins with esters whose molecules contain at least 4 carbon atoms, such as ethyl acetate and methyl propionate.
3. Interclass isomerism, for example, propanoic acid is isomeric to methyl acetate.
For esters containing an unsaturated acid or an unsaturated alcohol, two more types of isomerism are possible: isomerism of the position of the multiple bond and cis-, trans-isomerism.
Esters of lower carboxylic acids and alcohols are volatile, water-insoluble liquids. Many of them have a pleasant smell. For example, butyl butyrate smells like pineapple, isoamyl acetate smells like pear, etc.
Esters of higher fatty acids and alcohols are waxy substances, odorless, and insoluble in water.
The pleasant aroma of flowers, fruits, and berries is largely due to the presence of certain esters in them.
Fats are widely distributed in nature. Along with hydrocarbons and proteins, they are part of all plant and animal organisms and constitute one of the main parts of our food.
According to their state of aggregation at room temperature, fats are divided into liquid and solid. Solid fats, as a rule, are formed by saturated acids, while liquid fats (often called oils) are formed by unsaturated acids. Fats are soluble in organic solvents and insoluble in water.
1. Hydrolysis or saponification reaction. Since the esterification reaction is reversible, therefore, in the presence of acids, the reverse hydrolysis reaction occurs:
The hydrolysis reaction is also catalyzed by alkalis; in this case, hydrolysis is irreversible, since the resulting acid and alkali form a salt:
2. Addition reaction. Esters containing an unsaturated acid or alcohol are capable of addition reactions.
3. Recovery reaction. Reduction of esters with hydrogen results in the formation of two alcohols:
4. Reaction of formation of amides. Under the influence of ammonia, esters are converted into acid amides and alcohols:
Receipt. 1. Esterification reaction:
Alcohols react with mineral and organic acids, forming esters. The reaction is reversible (the reverse process is hydrolysis of esters).
Reactivity monohydric alcohols in these reactions it decreases from primary to tertiary.
2. Interaction of acid anhydrides with alcohols:
3. Interaction of acid halides with alcohols:
Hydrolysis mechanism:
Liquid fats are converted to solid fats through a hydrogenation reaction. Hydrogen joins at the site of double bond cleavage in hydrocarbon radicals of fat molecules:
The reaction occurs when heated under pressure and in the presence of a catalyst - finely crushed nickel. The product of hydrogenation - solid fat (artificial lard), called lard, is used for the production of soap, stearin and glycerin. Margarine is an edible fat that consists of a mixture of hydrogenated oils (sunflower, cottonseed, etc.), animal fats, milk and some other substances (salt, sugar, vitamins, etc.).
An important chemical property of fats, like all esters, is the ability to undergo hydrolysis (saponification). Hydrolysis occurs easily when heated in the presence of catalysts - acids, alkalis, oxides of magnesium, calcium, zinc:
The hydrolysis reaction of fats is reversible. However, with the participation of alkalis, it reaches almost the end - alkalis convert the resulting acids into salts and thereby eliminate the possibility of interaction of acids with glycerin (reverse reaction).
| " |