Esters are called compounds obtained by the reaction of esterification from carboxylic acids. In this case, OH- is replaced from the carboxyl group by an 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. However, 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, which takes the place of the H atom in the carboxyl group. If a hydrocarbon radical forms an conjugated system with an oxygen atom, then the reactivity increases markedly. This happens, for example, in acrylic and vinyl esters.
Physical properties
Most esters are liquids or crystalline substances with a pleasant aroma. Their boiling point is usually lower than that of similar molecular weights of carboxylic acids. This confirms a 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 specifically, 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 odors.
- Waxes. They are derivatives of higher (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 the hands. The main component of beeswax is myricyl palmitate 15 31 31 63 , and Chinese - cerotinic ester 25 51 26 53 . They are not soluble in water, but soluble in chloroform and benzene.
- Fats. Formed from glycerol and middle and higher carboxylic acids. Animal fats, as a rule, are solid under normal conditions, but they melt easily when the temperature rises (butter, pork fat, etc.). Vegetable fats are characterized by a liquid state (linseed, 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, which leads to substitution of the alkoxy group and acylation (or alkylation) of the nucleophilic agent. If the structural formula of the ester contains an α-hydrogen atom, then ester condensation is possible.
1. Hydrolysis. Acid and alkaline hydrolysis is possible, which is a reverse reaction of esterification. In the first case, hydrolysis is reversible, and the acid acts as a catalyst:
R-COO-R '+ H 2 O <―> R-COO-H + R'-OH
The main hydrolysis is irreversible and is usually called saponification, and the sodium and potassium salts of fatty carboxylic acids are called soaps:
R-COO-R '+ NaOH -> R-COO-Na + R'-OΗ
2. Ammonolysis. Ammonia may be the nucleophilic agent:
R-COO-R '+ NH 3 -> R-CO-NH 2 + R'-OH
3. Transesterification. This chemical property of esters can also be attributed to methods for their preparation. Under the influence of alcohols in the presence of H + or OH - it is possible to replace a hydrocarbon radical connected with oxygen:
R-COO-R '+ R' '- OH -> R-COO-R' '+ R'-OH
4. Hydrogen reduction leads to the formation of molecules of two different alcohols:
R-CO-OR '+ LiAlH 4 -> R-CΗ 2 -OΗ + R'OH
5. Combustion is another typical ester reaction:
2CΗ 3 –COO – CΗ 3 + 7O 2 = 6CO 2 + 6H 2 O
6. Hydrogenation. If in the hydrocarbon chain of the ether molecule there are multiple bonds, then it is possible to attach hydrogen molecules, which occurs in the presence of platinum or other catalysts. So, for example, from oils it is possible to obtain solid hydrogenated fats (margarine).
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 food industry, to obtain polymers and polyester fibers.
Ethyl acetate. It is used as a solvent for nitrocellulose, cellulose acetate and other polymers, for the manufacture and dissolution of varnishes. Due to its pleasant aroma it is used in food and perfume industries.
Butyl acetate. Also used as a solvent, but also polyester resins.
Vinyl acetate (CH 3 —COO — CH = CH 2 ). It is used as the basis of the polymer necessary in the preparation of glue, varnishes, synthetic fibers and films.
Malonic ether. Due to its special chemical properties, this ester is widely used in chemical synthesis to produce carboxylic acids, heterocyclic compounds, aminocarboxylic acids.
Phthalates. Phthalic acid esters are used as plasticizing additives for polymers and synthetic rubbers, and dioctyl phthalate is also used as a repellent.
Methyl acrylate and methyl methacrylate. Easily polymerize with the formation of organic glass sheets resistant to various influences.