ESTERS, in organic chemistry, compounds formed by the condensation of an alcohol and an acid, with elimination of water. Since this process is analogous to the neutralization of an acid by a basic hydroxide in salt formation these esters were formerly called ethereal salts. But this older term is misleading, since esters, unlike salts, are not conductors of electricity in solution. In the case of the polybasic acids, all the hydrogen atoms can be replaced in this way, and the compounds formed are known as neutral esters. If, however, some of the hydrogen of the acid remains undisplaced, then acid esters result. These acid esters retain some of the characteristic properties of the acids, forming, for example, salts, with basic oxides. Esters may be prepared by heating the silver salt of an acid with an alkyl iodide ; by heating the alcohols or alcoholates with an acid chloride ; by distilling the anhydrous sodium salt of an acid with a mixture of the alcohol and concentrated sulphuric acid ; or by heating for some hours on the water bath a mixture of an acid and an alcohol with a small quantity of hydrochloric or sulphuric acids (E. Fischer and A. Speier, 1896).
The esters of the aliphatic and aromatic acids are colourless neutral liquids, which are generally insoluble in water, but readily dissolve in alcohol and ether. Methyl oxalate, is a solid (m.p. 54° C). Many possess a fragrant odour and are pre pared in large quantities for use as artificial fruit essences; for example, ethyl acetate (acetic ether) butyl acetate, ethyl butyrate, (b.p. 121° C, odour of pine apple), isoamyl isovalerate (b.p. 196°, odour of apples) . They hydrolyse readily when boiled with solutions of caustic alkalies or mineral acids, yielding the constituent acid and alcohol. When heated with ammonia, they yield acid amides (q.v.). With Grignard reagents (q.v.) they furnish tertiary alco hols, except in the case of ethyl formate H•COZ•CZH6 (b.p. C), where secondary alcohols are obtained.
N. Menschutkin (1879-82) examined the rate of esterification of many acids with alcohols and found that the normal primary alcohols were all esterified at about the same rate, the secondary alcohols more slowly than the primary, and the tertiary alcohols still more slowly. The nature of the acid used affected the result, for in an homologous series of acids it was found that as the mole cule of the acid became more complex, the rate of esterification became less. The formation of an ester by the interaction of an acid with an alcohol is a "reversible" or "balanced" action, for as M. Berthelot and L. Pean de St. Gilles (1862) have shown in the case of the formation of ethyl acetate (b.p. 75° C) from ethyl alcohol and acetic acid, a point of equilibrium is reached beyond which the reacting system cannot pass, unless the system be dis turbed in some way by the removal of one of the products of the reaction. V. Meyer (1894) showed that in benzenoid compounds ortho-substituents hindered esterification of alcohols by acids in the presence of hydrochloric acid, this hindrance being particularly marked when two substituents are present in the ortho positions to the carboxyl group. In such a case the ester is best prepared by the action of an alkyl halide on the silver salt of the acid, and when once prepared can only be hydrolysed with great difficulty. The oils, fats and waxes (q.v.) are largely the esters of the higher fatty acids and glycerol. The esters of the higher fatty acids when distilled under atmospheric pressure are decomposed into an olefine and a fatty acid.
Esters of the mineral acids may be prepared by the general methods described above. The neutral esters are as a rule insolu ble in water and distil unchanged ; on the other hand, the acid esters are generally soluble in water, are non-volatile, and form salts with bases. Ethyl hydrogen sulphate (sulphovinic acid), obtained by the action of concentrated sulphuric acid on alcohol, is a colourless oily liquid with acid reaction, slowly decomposing in aqueous solution. On heating it forms diethyl sulphate and sulphuric acid. Dimethyl sulphate, a colourless liquid which boils at 187°-188° C, with partial de composition, is used as a methylating agent. Great care should be taken in using dimethyl and diethyl sulphates, as the respiratory organs are seriously affected by the vapours. Ethyl nitrate, a colourless liquid which boils at 86.3° C, is prepared by the action of nitric acid on ethyl alcohol. Ethyl nitrite, is a liquid which boils at 18° C; the crude product ob tained by distilling a mixture of alcohol, sulphuric and nitric acids and copper turnings is used in medicine under the name of "sweet spirits of nitre." Amyl nitrite, C5H11.ONO, boils at 96° C, and is employed in the preparation of anhydrous diazonium salts (see DIAZO-COMPOUNDS). It is also used in medicine.