Esters occur in most volatile oils and are frequently of essential import so far as the fragrance of these oils is concerned. On account of their mostly pleasant odor they play an important role in perfumery. As illustrations of oils rich in esters, the oils of bergamot, lavender, geranium, Siberian pine needle and Roman chamomille may be mentioned. It rarely happens that an oil consists almost exclusively of esters. Such is the case with the distillates of Gaultheria procumbens and Betula lenta, which consist to the extent of about 99 p. c. of methyl salicylate, and which are commonly known as wintergreen oil. With the exception of those esters which are solid at ordinary temperature and hence can be readily isolated, the identification of esters is not an easy matter. Characteristic derivatives, such as can be prepared of alcohols, ketones and hydrocarbons, are known only in exceptional cases. Hence one is dependant on fractional distillation and the characterization of the saponification products. To this difficulty is added the circumstance that the boiling points of esters of different composition are close to each other. As a result separation by fractionation is rendered impossible. A further difficulty arises from the decomposition of many esters when distilled under ordinary pressure. Some decompose even when distilled under diminished pressure.
As a rule, esters can be readily obtained synthetically. A common method consists in passing hydrogen chloride into a mixture of the components, which mixture is some times diluted with a suitable solvent. In many instances, however, this method cannot be applied because of the susceptibility of some of the acids and alcohols toward hydrogen chloride. In such cases the ester can be obtained by the action of the alkyl iodide on the silver salt of the acid, or by the action of acid anhydrides or acid chlorides on alcoholates. Not infrequently mere boiling of the alcohol with the acid anhydride suffices. Thus many alcohols can be converted quantitatively into their acetates by boiling them with acetic acid anhydride and some anhydrous sodium acetate.