Sodium ester. Esters Esters - Concept, Properties, Application

The compounds obtained by the esterification response from carboxylic acids are made to the ester. In this case, it is replaced by it from the carboxyl group on alkoxyradical. As a result, sophisticated esters are formed, the formula of which is generally written as R-SOO-R. "

The structure of the ester group

The polarity of chemical bonds in molecules of esters is similar to the polarity of bonds in carboxylic acids. The main difference is the absence of a moving hydrogen atom, on the site of which the hydrocarbon residue is placed. At the same time, the electrophilic center is located on the carbon atom of the ester group. But the carbon atom of an alkyl group connected to it is also positively polarized.

Electricity, and therefore, the chemical properties of esters are determined by the structure of the hydrocarbon residue that occupied the place of the N atom in the carboxyl group. If a hydrocarbon radical forms a conjugate system with an oxygen atom, then the reactivity increases significantly. This happens, for example, in acrylic and vinyl ether.

Physical properties

Most of the esters are liquids or crystalline substances with a pleasant aroma. The temperature of their boiling is usually lower than that of carboxylic acids close by the molecular weights. What confirms the decrease in intermolecular interactions, and this, in turn, is explained by the absence of hydrogen bonds between adjacent molecules.

However, as well as the chemical properties of esters, physical depend on the characteristics of the structure of the molecule. Or rather, on the type of alcohol and carboxylic acid, of which it is formed. On this basis, the esters are divided into three main groups:

1. Fruit esters. They are formed from lower carboxylic acids and the same single-cattle alcohols. Fluids with characteristic pleasant flower-fruit smells.
2. Waxes. Are the highest derivatives (the number of carbon atoms from 15 to 30) acids and alcohols having one functional group. These are plastic substances that are easily softened in their hands. The main component of the beeswax is minecyl palmitate with 15 H 31 coa of 31 H 63, and the Chinese - cerisolic ester with 25 H 51 SAO 26 H 53. They do not dissolve in water, but soluble in chloroform and benzene.
3. Fats. Formed from glycerin and medium and higher carboxylic acids. Animal fats are usually solid under normal conditions, but easily melt when the temperature is raised (butter, pork fat, etc.). For vegetable fats, a liquid state (linen, olive, soybean oil) is characteristic. The principal 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 - limit acids.

Chemical properties

Esters react with nucleophiles, which leads to the substitution of an alkoxy group and acylation (or alkylation) of a nucleophilic agent. If there is an α-hydrogen atom in the structural formula of the ester, then ester condensation is possible.

1. Hydrolysis. Acid and alkaline hydrolysis is possible, which is a reaction, inverse esterification. In the first case, hydrolysis is reversible, and the acid acts as a catalyst:

R-SOO-R "+ H 2 O<―> R-SOO-N + R "-OH

The main hydrolysis is irreversible and is usually called washed, and sodium and potassium salts of fatty carboxylic acids - soaps:

R-SOO-R "+ NaOH -\u003e R-SOO-Na + R" -Oη

2. Ammonolysis. The nucleophilic agent can act ammonia:

R-SOO-R "+ NH 3 -\u003e R-SO-NH 2 + R" -OH

3. Reatterification. This chemical property of esters can also be classified as methods of obtaining them. Under the action of alcohols in the presence of H + or it is possible to replace the hydrocarbon radical connected to oxygen:

R-SOO-R "+ R" "- OH -\u003e R-SOO-R" "+ R" -OH

4. Hydrogen reduction leads to the formation of two different alcohol molecules:

R-co-or "+ LiAlh 4 -\u003e R-Cη 2 -Oη + R" Oh

5. The burning is another reaction typical of esters:

2cη 3 -COO-Cη 3 + 7O 2 \u003d 6CO 2 + 6H 2 O

6. Hydrogenation. If there are multiple communication in the hydrocarbon chain of the ether molecule, then the hydrogen molecules can be additionally connected, which occurs in the presence of platinum or other catalysts. For example, solid hydrogenated fats (margarine) can be obtained from oils.

Application of esters

Esters and their derivatives are used in various industries. Many of them dissolve various organic compounds well used in perfumery and food industry, to obtain polymers and polyester fibers.

Ethyl acetate. It is used as a solvent for nitrocellulose, acetylcellulose and other polymers, for the manufacture and dissolving varnishes. Thanks to a pleasant aroma, it is used in food and perfume industries.

Butyl acetate. Also used as a solvent, but already polyester resins.

Vinyl acetate (CH 3 -COO-CH \u003d CH 2). Used as the basis of the polymer needed in the preparation of glue, varnishes, synthetic fibers and films.

Malonic ether. Due to its special chemical properties, this ester is widely used in a chemical synthesis for obtaining carboxylic acids, heterocyclic compounds, aminocarboxylic acids.

Phthalates. Flothic acid esters are used as plasticizing additives to polymers and synthetic rubbers, and dioctyl phthalate - also as a repellent.

Methyl acrylate and methyl methacrylate. Easily polymerized with the formation of organic glass sheets resistant to various exposure.

Fats and oils are natural esters, which are formed by trucatomic alcohol - glycerin and higher fatty acidular fatty acids containing an even number of carbon atoms. In turn, sodium or potassium salts of higher fatty acids are called soaps.

In the interaction of carboxylic acids with alcohols ( esterification reaction) Esters are formed:

This reaction is reversible. Reaction products can interact with each other with the formation of source substances - alcohol and acid. Thus, the reaction of esters with water is hydrolysis of the ester - the inverse response of esterification. Chemical equilibrium, installed in equality of speeds of straight (esterification) and reverse (hydrolysis) reactions, can be shifted towards the formation of water by the presence of water-based funds.

Esters in Nature and Technology

The esters are widespread in nature, they are used in the technique and various industries. They are good solventsorganic substances, their density is less than the density of water, and they practically do not dissolve in it. Thus, esters with a relatively small molecular weight are easily flammable liquids with low boiling temperatures, there are smells of various fruits. They are used as solvents of varnishes and paints, flavors of food industry products. For example, methyl ester of oil acid has a smell of apples, an ethyl ester of this acid - the smell of pineapples, acetic acid isobutyl ester - the smell of bananas:

The esters of higher carboxylic acids and higher mono-block alcohols are called waxes. So, the bee wax consists of the main thing about
one of the palmitic acid ester and myricyl alcohol C 15 H 31 COOC 31 H 63; Couscallo wax - spermacet - ester of the same palmitic acid and cetyl alcohol C 15 H 31 COOC 16 H 33.

Fat.

The most important representatives of esters are fats.

Fat.- Natural compounds that are glycerol esters and higher carboxylic acids.

The composition and structure of fats can be reflected by the general formula:

Most fat are formed by three carboxylic acids: oleinova, palmitic and stearin. Obviously, two of them are limiting (saturated), and oleic acid contains a double bond between carbon atoms in the molecule. Thus, the composition of fats may include remnants of both limiting and non-safe carboxylic acids in various combinations.

Under normal conditions, fats containing in its composition residual acids are most often fluid. They are called oils. These are mainly fats of plant origin - linen, hemp, sunflower and other oils. Less frequent fats of animal origin, such as fish oil. Most of the natural fats of animal origin under normal conditions - solid (low-melting) substances and contain mainly the remnants of limit carboxylic acids, for example, bars fat. So, palm oil is solid under normal fat conditions.

The composition of fats determines their physical and chemical properties. It is clear that for fat containing the remains of unsaturated carboxylic acids, all the reactions of unsaturated compounds are characteristic. They discolor bromine water, enter other reactions of attachment. The most important reaction in practical terms is hydrogenation of fats. Hydrogenation of liquid fat is obtained solid esters. It is this reaction that underlies the preparation of margarine - solid fat from vegetable oils. Conditionally, this process can be described by the reaction equation:

hydrolysis:

Soap

All fats, like other sophisticated esters, are exposed hydrolysis. Hydrolysis of esters is a reversible reaction. To shift the balance 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 fat proceeds irreversibly and leads to the formation of carboxylic acid salts, which are called soaps. Hydrolysis of fats in the alkaline medium is called fats.

When washing fat, glycerin and soap are formed - sodium or potassium salts of higher carboxylic acids:

Crib

The esters are most often obtained by acylation by hydroxy derivatives by carboxylic acids, their chloranhydrides and anhydrides, as well as keten (see below); The interaction of carboxylic acid salts with halides and tosylats according to B ^ -Mukhanism (p. 112) is largely widely used. From other methods, the attachment of carboxylic acids to acetylene can be noted (p. 142, part 1), rearrangement of Bayer Villager (p. 35), the Tishchenko reaction (p. 41). To obtain methyl esters, carboxylic acid reactions with diazomethane are used (later reviewed).

The names of the esters of R-CO-OR 1 usually consolidate from the name of the R 1 radical and the name of the acid with adding the end aM: acetic ethyl ester - ethyl acetate; Genzoic acid propyl ether - propylbenzoate; SHAVEL ORVERY DIRECTION Ether - dimethyl oxalate.

Chemical properties

The properties of esters are detected, on the one hand, a certain similarity with the properties of previously considered derivatives - chloride and anhydrides, on the other - noticeable originality; In particular, new types of reactions appear, such as acyloin condensation, pyrolysis and others.

Chemical reactions of esters can be divided into the following groups: I. Nucleophilic reactions of carbonyl group; II. O-alkyl splitting responses; III. Recovery reactions; IV. Pyrolytic cleavage reactions. Very important A-position reactions are not separately considered separately; Part of the material (ester condensation) will be considered in the section "Nucleophilic reactions of the carbonyl group", and a part in a special section dedicated to methylene prizes.

I. Nucleophilic reactions of carbonyl group.

The most characteristic reactions of this group are the interaction of esters with o- and n-n-nucleophiles and metal compounds, as well as condensation reactions with carbanions.

Esters, as well as previous types of derivatives, are subjected to hydrolysis and acylate o- and n-n-nucleophiles in general scheme:

For group -OR "* donor + M-effect noticeably exceeds acceptor- low-active with respect to nucleophiles (approximately at the level of activity of carbonyl group of carboxylic acids themselves). Esters are not related to active acylating reagents; when interacting with weak nucleophiles (water, alcohols ) Catalysis required.

The hydrolysis of esters is under the influence of aqueous solutions of acids or bases (usually alkalis). Hydrolysis S. acid catalysis leads to the formation of appropriate carboxylic acid and alcohol; Mechanisms of hydrolysis are inverse to the mechanisms of escort and catalyzed esterification; Depending on the structure of ethers and conditions, this may be the ADE mechanisms or A AC 1 (See page but, 111). Hydrolysis under the action of alkalisis, of course, leads to the formation of carboxylic salts: R "CO-OR 2 + NA + OH -\u003e R" -CO-CTNA + R 2 -OH mechanism here is different: this is a typical mechanism for the interaction of derivative carboxylic acids with anionic nucleophiles (it was discussed above on the example of Halogenide acyl). In this case, it looks like this:

Initially, an anionic nucleophile - hydroxide anion is additionally occurred, then the ejecting of an alkoxide anion, which, naturally, deposits the acid that occurs with the formation of alcohol and the more stable carboxylate anion. Since the speed definition stage is here bimolecular, the mechanism is indicated as VDS2, i.e. Bimolecular reaction of acyl derivatives catalyzed by bases (B - BASE). In contrast to acid hydrolysis, alkaline hydrolysis practically irreversible., Because Carboxylic acid salts are passive towards nucleophiles.

Ridrolysis of cyclic esters - lactones - leads to the formation of hydroxycyclot (with acid hydrolysis) or their salts (with alkaline hydrolysis):

The acylation of alcohols with esters by esters leads to the formation of new esters with alcohols-reagents with the displacement of "source" alcohols:

This reaction is different called perethetsrificzei (sometimes, especially in biochemistry, the term "trans passerification" is used) or alcoholism esters (by analogy with hydrolysis). The reaction is usually in acid catalysis by the mechanism A L C2:

The mechanism is completely similar to the esterification mechanism (p. 110). The reaction of microscopically reversible and can be shifted both in one and in other

the goy side using excess alcohol is used as a solvent.

r 2 -OH

either R it: usually excess

The deadterification occurs under action on the esters of the alknames of other alcohols:

The reaction is based on the mechanism of VDS2, similar to alkaline hydrolysis, with the difference that the acid is not formed here, and the reaction is reversible.

The transesterification reactions are used both for synthesis and for splitting esters. In particular, the methyl esters of natural fatty acids (comfortable forms for chromato-mass spectrometric analysis) can be obtained from natural esters of these acids when processing excess methanol in the presence of H2SO4. Alcoholiz is used in polyester synthesis (will be examined later). Some biochemical reactions also refer to the transesterification; In particular, cholesterol esters are formed in the body.

Lacton alcoholys leads to hydroxyc acid esters:

In addition to Alcoholysis, there is another version of the deadterness - acidoliz; This is an exchange reaction with a carboxylic acid molecule, and the ester of this new acid is formed, and the "old" acid is supplanted:

The acylation of N-nucleophiles with esters with esters leads to education amidov (with the acylation of ammonia, primary and secondary amines), hydrazid (when acylation of hydrazine and its substituted), hydraulic acids (with hydroxylamine acylation):

N-nucleophiles used (especially hydrazine and hydroxylamine) are more active than o-nucleophiles, so their interaction with esters can pass without catalysis, although in some cases the main or acid catalysis is used. The mechanism of noncatalithic interaction is a special case of the mechanisms of reactions of derivative acids with H-Y reagents:

To obtain Alche *) 0V_Acillation with esters, it is used less frequently than acylation by chloranhydrides and anhydrides, but still quite a few examples of such synthesis are known. To receive hydrazid and hydroxamic acids acylation esters is best methodbecause Hydrazine and hydroxylamine - strong nucleophiles, and when they interact with energetic acylating reagents - acylgalloids and anhydrides - reactions can proceed too violently and lead to diacylation products, and for hydrazine - also three and tetracylation.

The interaction of esters with metallorganic compounds, as well as for acyl halides, can lead to ketones or go further - before the formation of tertiary alcohols. When interacting with liquidalkyls, the reaction can be stopped at certain conditions at the ketone formation stage:

When interacting with the reactivities of Grignar, the reaction, as a rule, does not stop at the ketone formation stage and goes further before the formation of tertiary alcohol:

Condensation reactions involving a carbonyl group of esters have a great preparative value. One of them - condensation of esters with ketones (protruding in the role of methylene components):

The reaction was reviewed earlier (p. 27); As a result, 1,3-diketones are formed, widely used in organic synthesis.

Another extremely important reaction - condensation of two sophisticated ether molecules in the presence of a strong base ( esphine condensationor condensation of Claiseen):

The reaction is similar to the previous one, with the difference that in the role of the methylene component is not ketone, but the second molecule of the ester. Reaction products are p-oxocarboxylic acid esters. The condensation option is widely used. same Molecules of ester (R I \u003d CH2R R 2 \u003d R 4), i.e. Self-condensation of esters under the action of severe grounds. The simplest and most famous example is the condensation of two ethyl acetate molecules to form acetoxus ether (622) - one of the most widely used in organic synthesis substances:

In some cases use condensation different ethers (cross condensation); In these cases, it is necessary that one of the esters (carbonyl component) does not contain a-methylene group and at the same time its carbonyl group had increased activity (in order to suppress the samocondensation of the methylene component). Such ether, in particular, is the diethyl ether of oxalic acid (diethyloxalate) (623), one of the typical partners in cross-condensation reactions:

An important private case of compound ester condensation is intramolecular condensation of dicarboxylic acid esters; At the same time, the carbocyclic structure is closed; A 2-alkoxycarbonyl cyclic ketone derivative is formed:

This option is often called condensation of Dikman It is most successful in the formation of 5- and 6-membered cycles (n \u003d 3, 4). Dicman condensation is one of the classical carbocyclization methods.

To carry out ester condensation, it is necessary to use strong baseFor it, it is possible to generate a carbanion from the A-position of the ester (A-position of the esters of the esters, it has a smaller sn-acidity than the A-position of carbonyl compounds, since COOR group is less electronized than carbonyl group of aldehydes and ketones ). Most often used as a base alcoholate of that alcohol, which forms the original ester[if you use alcoholates other alcohol, the reaction will be complicated by transesterification (see above)]. Sometimes the amides of metals are used, and in some cases such a sufficient basis, like phenalline. The mechanism of ester condensation is completely similar to those considered earlier the condensation mechanism of complex esters with ketones:

Here the mechanism is combined, similar to the aldol condensation (the formation of the carbanion and its attack on the carbonyl group), and the mechanism of type in AC -2 (intramolecular displacement of alkoxide anion).

II. Reactions of splitting of communication O-alkyl.

In the reactions described in the previous section, the connection is split O-acyl. At the same time, a number of reactions leading to similar results proceeds with communication splitting O-alkyl. This is reactions nucleophilic substitution with an alkyl carbon atom, where nucleophurg is displaced in the form of carboxylic acid or carboxylate anion.

A characteristic example of such reactions is the acidic hydrolysis of esters of tertiary, benzyl and allyl alcohols:

The key stage of the reaction is the dissociation of protonated ether (624) containing a "good" outgoing group; the dissociation is facilitated by the resistance of tertiary, allyl and benzyl cations (625). This is a typical S N 1 - the reaction indicated here as a Al 1; It is inverse esterification by the mechanism A Al 1 (p. 111).

A peculiar option for splitting the O-alkyl bond is the conversion of phenolphthalene (620) in an alkaline environment:

Under the action of alkali, phenolic-dyanion (626) is formed; Next occurs intramolecular 8 ^ -reaction with the displacement of carboxylateanion and to form a compound (627) containing a quinoid structure and therefore intensively painted. When acidifying the lactone cycle closes, and is regenerated colorful Compound (620). And the direct and reverse reaction is very quickly leaked at room temperature, which allows the use of phenolphthalein as an acid-base indicator.

III. Restoration of esters.

The most common reactions of this group are the restoration of esters to primary alcohols and aldehydes, as well as their reductive combination, called acylinic condensation.

Essentials - Liquids with pleasant fruit smells. In water, they dissolve very little, but are well soluble in alcohols. The esters are very common in nature. Their presence is caused by pleasant smells of flowers and fruits. They can even be in the crust of some trees.

Look at the screen and consider the composition of the esters that give the smell of colors. Slides are demonstrated: the smell of jasmine - benzylpropanoate, chrysanthemums - phenylethyl alcohol ester and formic acid. As we see the esters that have floral odors, this is most often derivatives of aromatic acids or aromatic alcohols. But the esters that are part of the fruit known to you have a rather simple composition.

The esters of higher mono large acids and higher monatomic alcohols are the basis of natural waxes. Waxes do not dissolve in water. They can be molded in the heated state. Examples of animal waxes can serve as beeswax, as well as burst (spermacet), contained in the cranium cranium cranium box (couch wax). The bee wax contains palmitic acid ester and myricyl alcohol (minecylpalmitat): CH 3 (CH 2) 14 -CO-O- (CH 2) 29 CH 3.

Reverse process- splitting of ester under the action of water to form carboxylic acid and alcohol - is called the hydrolysis of the ester.

The hydrolysis in the presence of an alkali proceeds irreversibly (because the formed negatively charged carboxylate - an anion RCOO - does not react with the nucleophilic reagent - alcohol).

This reaction is called washedester.

Application esters are very diverse (message).

They are used in industry as solvents and intermediate products in the synthesis of various organic compounds. Esters with a pleasant smell used in perfumery and food industry. Esters often serve as starting materials in the production of many pharmaceutical preparations.

Fats like esters. Classification of fats.

The most important representatives of esters are fats.

When heated fats with water in an alkaline environment, the French scientist E. Chevrol found that the fats are split and glycerol and various carboxylic acids are formed. The French scientist M.Brthlo in 1854 carried out the return process: when heated glycerol with higher carboxylic acids, it got fats and water.