Sodium ester. Esters - concept, properties, application
Compounds obtained by the esterification reaction from carboxylic acids are commonly referred to as esters. In this case, OH- is replaced from the carboxyl group by the alkoxy radical. As a result, esters are formed, the formula of which is generally written as R-COO-R ".
The structure of the ester group
The polarity of chemical bonds in ester molecules is similar to the polarity of bonds in carboxylic acids. The main difference is the absence of a mobile hydrogen atom, in the place of which a hydrocarbon residue is located. At the same time, the electrophilic center is located on the carbon atom of the ester group. But the carbon atom of the alkyl group connected to it is also positively polarized.
Electrophilicity, and hence the chemical properties of esters, are determined by the structure of the hydrocarbon residue that took the place of the H atom in the carboxyl group. If the hydrocarbon radical forms a conjugated system with the oxygen atom, then the reactivity increases markedly. This happens, for example, in acrylic and vinyl esters.
Physical properties
Most esters are liquid or crystalline with a pleasant aroma. Their boiling point is usually lower than that of carboxylic acids with close molecular weights. This confirms the decrease in intermolecular interactions, and this, in turn, is explained by the absence of hydrogen bonds between neighboring molecules.
However, just like the chemical properties of esters, the physical ones depend on the structural features of the molecule. More precisely, on the type of alcohol and carboxylic acid from which it is formed. On this basis, esters are divided into three main groups:
- Fruit esters. They are formed from lower carboxylic acids and the same monohydric alcohols. Liquids with characteristic pleasant floral-fruity aromas.
- Waxes. They are derivatives of higher (the number of carbon atoms from 15 to 30) acids and alcohols, each having one functional group. These are plastic substances that soften easily in the hands. The main component of beeswax is myricyl palmitate С 15 Н 31 СООС 31 Н 63, and the Chinese one is ceryl ester of cerotinic acid С 25 Н 51 СООС 26 Н 53. They are insoluble in water, but soluble in chloroform and benzene.
- Fats. Formed from glycerin and medium and higher carboxylic acids. Animal fats, as a rule, are solid under normal conditions, but melt easily when the temperature rises (butter, pork fat, etc.). Vegetable fats are characterized by a liquid state (flaxseed, olive, soybean oil). The fundamental difference in the structure of these two groups, which affects the differences in the physical and chemical properties of esters, is the presence or absence of multiple bonds in the acid residue. Animal fats are glycerides of unsaturated carboxylic acids, and vegetable fats are saturated acids.
Chemical properties
Esters react with nucleophiles, resulting in alkoxy substitution and acylation (or alkylation) of the nucleophilic agent. If there is an α-hydrogen atom in the structural formula of an ester, then ester condensation is possible.
1. Hydrolysis. Acid and alkaline hydrolysis is possible, which is a reaction opposite to esterification. In the first case, hydrolysis is reversible, and the acid acts as a catalyst:
R-СОО-R "+ Н 2 О<―>R-COO-H + R "-OH
Basic hydrolysis is irreversible and is usually called saponification, and sodium and potassium salts of fatty carboxylic acids are called soaps:
R-COO-R "+ NaOH -> R-COO-Na + R" -OΗ
2. Ammonolysis. Ammonia can act as a nucleophilic agent:
R-СОО-R "+ NH 3 -> R-СО-NH 2 + R" -OH
3. Transesterification. This chemical property of esters can also be attributed to the methods of their preparation. Under the action of alcohols in the presence of H + or OH -, it is possible to replace the hydrocarbon radical combined with oxygen:
R-COO-R "+ R" "- OH -> R-COO-R" "+ R" -OH
4. Reduction with hydrogen leads to the formation of molecules of two different alcohols:
R-CO-OR "+ LiAlH 4 -> R-СΗ 2 -ОΗ + R" OH
5. Combustion is another reaction typical of esters:
2CΗ 3 -COO-CΗ 3 + 7O 2 = 6CO 2 + 6H 2 O
6. Hydrogenation. If there are multiple bonds in the hydrocarbon chain of the ether molecule, then hydrogen molecules can be added along them, which occurs in the presence of platinum or other catalysts. So, for example, it is possible to obtain solid hydrogenated fats (margarine) from oils.
The use of esters
Esters and their derivatives are used in various industries. Many of them dissolve various organic compounds well, are used in perfumery and the food industry, to obtain polymers and polyester fibers.
Ethyl acetate. It is used as a solvent for nitrocellulose, cellulose acetate and other polymers, for making and dissolving varnishes. Due to its pleasant aroma, it is used in the food and perfume industries.
Butyl acetate. Also used as a solvent, but already for polyester resins.
Vinyl acetate (CH 3 -COO-CH = CH 2). It is used as a base for the polymer required in the preparation of adhesives, varnishes, synthetic fibers and films.
Malonic ether. Due to its special chemical properties, this ester is widely used in chemical synthesis to obtain carboxylic acids, heterocyclic compounds, aminocarboxylic acids.
Phthalates. Phthalic acid esters are used as plasticizers for polymers and synthetic rubbers, and dioctyl phthalate is also used as a repellent.
Methyl acrylate and methyl methacrylate. They easily polymerize to form sheets of organic glass resistant to various influences.
Fats and oils are natural esters that are formed by a trihydric alcohol - glycerol and higher fatty acids with an unbranched carbon chain containing an even number of carbon atoms. In turn, sodium or potassium salts of higher fatty acids are called soaps.
When carboxylic acids react with alcohols ( esterification reaction) esters are formed:
This reaction is reversible. The reaction products can interact with each other to form the initial substances - alcohol and acid. Thus, the reaction of esters with water — ester hydrolysis — is the reverse of the esterification reaction. Chemical equilibrium, which is established when the rates of direct (esterification) and reverse (hydrolysis) reactions are equal, can be shifted towards the formation of ether by the presence of dehydrating agents.
Esters in nature and technology
Esters are widespread in nature, find application in technology and various industries. They are good solvents organic substances, their density is less than the density of water, and they practically do not dissolve in it. Thus, esters with a relatively low molecular weight are flammable liquids with low boiling points and smells of various fruits. They are used as solvents for varnishes and paints, flavorings for food products. For example, methyl ester of butyric acid has the smell of apples, ethyl ester of this acid has the smell of pineapples, isobutyl ester of acetic acid has the smell of bananas:
Esters of higher carboxylic acids and higher monobasic alcohols are called waxes... So, beeswax is mainly about
at once from an ester of palmitic acid and myricyl alcohol C 15 H 31 COOC 31 H 63; sperm whale wax - spermaceti - an ester of the same palmitic acid and cetyl alcohol C 15 H 31 COOC 16 H 33.
Fats
The most important representatives of esters are fats.
Fats- natural compounds, which are esters of glycerol and higher carboxylic acids.
The composition and structure of fats can be reflected by the general formula:
Most fats are formed by three carboxylic acids: oleic, palmitic, and stearic. Obviously, two of them are saturated (saturated), and oleic acid contains a double bond between carbon atoms in the molecule. Thus, the composition of fats can include residues of both saturated and unsaturated carboxylic acids in various combinations.
Under normal conditions, fats containing residues of unsaturated acids are most often liquid. They are called oils. These are mainly vegetable fats - flaxseed, hemp, sunflower and other oils. Less common are liquid animal fats such as fish oil. Most natural fats of animal origin under normal conditions are solid (low-melting) substances and contain mainly residues of saturated carboxylic acids, for example, mutton fat. So, palm oil is a solid fat under normal conditions.
The composition of fats determines their physical and chemical properties. It is clear that all reactions of unsaturated compounds are characteristic of fats containing residues of unsaturated carboxylic acids. They discolor bromine water and enter into other addition reactions. The most important reaction in practical terms is the hydrogenation of fats. Solid esters are obtained by hydrogenation of liquid fats. It is this reaction that underlies the production of margarine - solid fat from vegetable oils. Conventionally, this process can be described by the reaction equation:
hydrolysis:
Soap
All fats, like other esters, are exposed to hydrolysis... Ester hydrolysis is a reversible reaction. To shift the equilibrium towards the formation of hydrolysis products, it is carried out in an alkaline medium (in the presence of alkalis or Na 2 CO 3). Under these conditions, the hydrolysis of fats is irreversible and leads to the formation of carboxylic acid salts, which are called soaps. Hydrolysis of fats in an alkaline medium is called fat saponification.
When fats are saponified, glycerin and soaps are formed - sodium or potassium salts of higher carboxylic acids:
Crib
Esters are most often obtained by acylation of hydroxy derivatives with carboxylic acids, their acid chlorides and anhydrides, as well as ketenes (see below); the interaction of salts of carboxylic acids with halides and tosylates according to the D ^ -mechanism is widely used (p. 112). Other methods include the addition of carboxylic acids to acetylene (p. 142, part 1), the Bayer-Villiger rearrangement (p. 35), and the Tishchenko reaction (p. 41). To obtain methyl esters, the reaction of carboxylic acids with diazomethane is used (to be discussed later).
The names of esters R-CO-OR 1 usually consist of the name of the radical R 1 and the name of the acid with the addition of the ending am: ethyl acetate - ethyl acetate; benzoic acid propyl ester - propyl benzoate; oxalic acid dimethyl ester - dimethyl oxalate.
Chemical properties
The properties of esters show, on the one hand, a certain similarity with the properties of the previously considered derivatives - acid chlorides and anhydrides, on the other - a noticeable originality; in particular, new types of reactions appear, such as acyloin condensation, pyrolysis, and others.
The chemical reactions of esters can be divided into the following groups: I. Nucleophilic reactions of the carbonyl group; II. O-alkyl bond cleavage reactions; III. Recovery reactions; IV. Pyrolytic cleavage reactions. The very important reactions of the a-position are not considered separately; part of the material (ester condensation) will be considered in the section "Nucleophilic reactions of the carbonyl group", and part - in a special section devoted to methylene-active compounds.
I. Nucleophilic reactions of the carbonyl group.
The most characteristic reactions of this group are the interaction of esters with O- and N-nucleophiles and organometallic compounds, as well as condensation reactions with carbanions.
Esters, like the previous types of derivatives, undergo hydrolysis and acylate O- and N-nucleophiles according to the general scheme:
For the -OR "* group, the donor + M-effect is noticeably superior to the acceptor; it is inactive with respect to nucleophiles (approximately at the level of activity of the carbonyl group of the carboxylic acids themselves). Esters do not belong to active acylating reagents; when interacting with weak nucleophiles (water, alcohols ) requires catalysis.
The hydrolysis of esters takes place under the action of aqueous solutions of acids or bases (usually alkalis). Hydrolysis with acidic catalysis leads to the formation of the corresponding carboxylic acid and alcohol; hydrolysis mechanisms are opposite to those of acid-catalyzed esterification; depending on the structure of the ethers and conditions, these can be the mechanisms of Ade2 or A ac 1 (see p. HO, 111). Hydrolysis under the action of alkalis naturally leads to the formation of carboxylic acid salts: R "CO-OR 2 + Na + OH -> R" -CO-CTNa + + R 2 -OH The mechanism is different here: this is a typical mechanism of interaction of carboxylic acid derivatives with anionic nucleophiles (discussed above for the example of acyl halides). In this case, it looks like this:
First, the anionic nucleophile, the hydroxide anion, is attached, then the alkoxide anion is pushed out, which naturally deprotonates the resulting acid with the formation of an alcohol and a more stable carboxylate anion. Since the speed-determining stage is here bimolecular, the mechanism is designated as Вдс2, i.e. bimolecular reaction of acyl derivatives, catalyzed by bases (B - Base). Unlike acid hydrolysis, alkaline hydrolysis practically irreversible., since carboxylic acid salts are passive towards nucleophiles.
Hydrolysis of cyclic esters - lactones - leads to the formation of hydroxy acids (with acid hydrolysis) or their salts (with alkaline hydrolysis):
Acylation of alcohols with esters leads to the formation of new esters with reagent alcohols displacing the "original" alcohols:
This reaction is otherwise called re-eternalization(sometimes, especially in biochemistry, the term "transesterification" is used) or alcoholysis esters (by analogy with hydrolysis). The reaction usually proceeds with acid catalysis according to the AL c2 mechanism:
The mechanism is completely similar to that of esterification (p. 110). The reaction is microscopically reversible and can be shifted in one or the other
On the other hand, using an excess of alcohol is used as a solvent.
r 2 -oh
or R OH: usually excessive
Transesterification also occurs when the alcohol esters of other alcohols are acted upon:
The reaction proceeds according to the Bdc2 mechanism, similar to alkaline hydrolysis, with the difference that an acid salt is not formed here, and the reaction is reversible.
Transesterification reactions are used both for synthesis and for the cleavage of esters. In particular, methyl esters of natural fatty acids (convenient forms for gas chromatography-mass spectrometric analysis) can be obtained from natural esters of these acids by treatment with excess methanol in the presence of H2SO4. Alcoholysis is used in the synthesis of polyesters (discussed later). Some biochemical reactions also refer to transesterification; in particular, this is how cholesterol esters are formed in the body.
Alcoholysis of lactones leads to hydroxy acid esters:
In addition to alcoholysis, there is another variant of transesterification - acidolysis; this is an exchange reaction with a carboxylic acid molecule, and an ester of this new acid is formed, and the "old" acid is displaced:
Acylation of N-nucleophiles with esters leads to the formation amides(in the acylation of ammonia, primary and secondary amines), hydrazides(in the acylation of hydrazine and its substituted), hydroxamic acids(with acylation of hydroxylamine):
The N-nucleophiles used (especially hydrazine and hydroxylamine) are more active than O-nucleophiles; therefore, their interaction with esters can proceed without catalysis, although in some cases basic or acid catalysis is used. The mechanism of non-catalytic interaction is a special case of the mechanisms of reactions of acid derivatives with reagents of the H-Y type:
To obtain alsh *) 0b_acylation with esters is used less often than acylation with acid chlorides and anhydrides, but still there are many examples of such syntheses. To receive hydrazides and hydroxamic acids acylation with esters is the best method since hydrazine and hydroxylamine are strong nucleophiles, and when they interact with energetic acylating reagents - acyl halides and anhydrides - the reactions can proceed too violently and lead to diacylation products, and for hydrazine - also tri- and tetraacylation.
The interaction of esters with organometallic compounds, as for acyl halides, can lead to ketones or go further - to the formation of tertiary alcohols. When interacting with lithium alkyls, the reaction can, under certain conditions, be stopped at the stage of ketone formation:
When interacting with Grignard reagents, the reaction, as a rule, does not stop at the stage of ketone formation and proceeds further, until the formation of a tertiary alcohol:
Condensation reactions involving the carbonyl group of esters are of great preparative importance. One of them - condensation of esters with ketones(acting as a methylene component):
Reaction was discussed earlier (p. 27); as a result, 1,3-diketones are formed, which are widely used in organic synthesis.
Another extremely important reaction is condensation of two ester molecules in the presence of a strong base ( ester condensation or Claisen condensation):
The reaction is similar to the previous one, with the difference that the role of the methylene component is not the ketone, but the second ester molecule. The reaction products are esters of p-oxocarboxylic acids. The condensation option of two the same ester molecules (R i = CH2R R 2 = R 4), i.e. self-condensation of esters under the action of strong bases. The simplest and most famous example is the condensation of two ethyl acetate molecules to form acetoacetic ether(622) - one of the most widely used substances in organic synthesis:
In some cases, condensation is used different broadcasts(cross condensation); in these cases, it is necessary that one of the esters (the carbonyl component) does not contain an a-methylene group and, at the same time, its carbonyl group has an increased activity (in order to suppress the self-condensation of the methylene component). Such an ester, in particular, is oxalic acid diethyl ester (diethyl oxalate) (623), one of the typical partners in cross-condensation reactions:
An important special case of ester condensation is intramolecular condensation of esters of dicarboxylic acids; this closes the carbocyclic structure; a 2-alkoxycarbonyl derivative of the cyclic ketone is formed:
This option is often referred to as Dieckmann condensation it proceeds most successfully with the formation of 5- and 6-membered rings (n = 3, 4). Dieckmann condensation is one of the classical methods of carbocyclization.
To carry out the ester condensation, it is necessary to use strong base, because only with its help it is possible to generate a carbanion from the a-position of the ester (the a-position of esters has a lower CH-acidity than the a-position of carbonyl compounds, since the COOR group is less electron-withdrawing than the carbonyl group of aldehydes and ketones ). Most often used as a base alcoholate of the alcohol that forms the original ester[if you use alcoholic another alcohol, the reaction will be complicated by transesterification (see above)]. Sometimes metal amides are used, and in some cases such a superbase as phenyllithium. The mechanism of ester condensation is quite similar to the previously considered mechanism of condensation of esters with ketones:
This combines a mechanism similar to aldol condensation (the formation of a carbanion and its attack on the carbonyl group) and a mechanism of the Bac-2 type (intramolecular displacement of the alkoxide anion).
II. O-alkyl bond cleavage reactions.
In the reactions described in the previous section, the bond is cleaved O-acyl. At the same time, a number of reactions leading to similar results proceed with the cleavage of the bond O-alkyl. These are reactions nucleophilic substitution at the alkyl carbon atom, where the nucleofuge is displaced as a carboxylic acid or carboxylate anion.
A typical example of such reactions is acid hydrolysis of esters of tertiary, benzyl and allyl alcohols:
The key stage of the reaction is the dissociation of the protonated ether (624) containing the “good” leaving group; dissociation is facilitated by the stability of tertiary, allyl, and benzyl cations (625). This is a typical S N 1 reaction, referred to here as A al 1; it is the reverse of esterification by the A al 1 mechanism (p. 111).
A peculiar variant of the O-alkyl bond cleavage is the transformation of phenolphthalein (620) in an alkaline medium:
Under the action of alkali, phenolate dianion (626) is formed first; what happens next intramolecular 8S-reaction with displacement of the carboxylate anion and with the formation of compound (627), which contains a quinoid structure and therefore intensely colored. Upon acidification, the lactone cycle is closed and regenerated colorless compound (620). Both direct and reverse reactions proceed very quickly at room temperature, which makes it possible to use phenolphthalein as an acid-base indicator.
III. Recovery of esters.
The most common reactions in this group are the reduction of esters to primary alcohols and aldehydes, as well as their reductive combination called acyloin condensation.
Esters- liquids with pleasant fruity odors. They dissolve very little in water, but they are highly soluble in alcohols. Esters are very common in nature. Their presence is due to the pleasant smells of flowers and fruits. They can even be found in the bark of some trees.
Look at the screen and observe the composition of the esters that give the flowers a scent. Slides are shown: the smell of jasmine - benzylpropanoate, chrysanthemum - an ester of phenylethyl alcohol and formic acid. As we can see, esters that have floral odors are most often derived from aromatic acids or aromatic alcohols. But the esters that are part of the fruits you know have a fairly simple composition.
Esters of higher monobasic acids and higher monohydric alcohols are the basis of natural waxes. The waxes do not dissolve in water. They can be molded while hot. Examples of animal waxes include beeswax and blubber (spermaceti) in the skull of a sperm whale (sperm whale wax). Beeswax contains an ester of palmitic acid and myricyl alcohol (myricyl palmitate): CH 3 (CH 2) 14 –CO – O– (CH 2) 29 CH 3.
Reverse process- the cleavage of an ester by the action of water to form a carboxylic acid and an alcohol is called ester hydrolysis.
Hydrolysis in the presence of alkali is irreversible (since the formed negatively charged carboxylate - the RCOO anion - does not react with the nucleophilic reagent - alcohol).
This reaction is called saponification complex ether.
Application esters are very diverse (Message).
They are used in industry as solvents and intermediates in the synthesis of various organic compounds. Esters with a pleasant smell are used in perfumery and the food industry. Esters are often used as starting materials in the manufacture of many pharmaceuticals.
Fats like esters. Classification of fats.
The most important representatives of esters are fats.
When heating fats with water in an alkaline environment, the French scientist E. Chevreul found that fats are broken down and glycerol and various carboxylic acids are formed. The French scientist M. Berthelot in 1854 carried out the opposite process: when glycerin was heated with higher carboxylic acids, he obtained fats and water.
