Esters

Esters, in organic chemistry, compounds formed, along with water, by the reaction of acids and alcohols. Because this process is analogous to the neutralization of an acid by a base in salt formation, esters were formerly called ethereal salts. This term is misleading because esters, unlike salts, are not ionized in solution.

Esters can be formed from both organic and inorganic acids. For example, the simple ester ethyl nitrate may be obtained from ethyl alcohol and nitric acid (an inorganic acid), and the ester ethyl ethanoate may be obtained from ethyl alcohol and ethanoic acid (an organic acid). Another method of preparing esters is to employ not the acid itself but its chloride. For example, ethyl ethanoate may be prepared by the action of alcohol upon ethanoyl chloride, the chloride of ethanoic acid. Another important method of preparation is by the reaction of the silver salts of acids with an alkyl halide (usually an iodine compound). For example, ethyl ethanoate may be prepared from silver ethanoate and ethyl iodide.

Esters are broken up by the action of water into their component acids and alcohols, a reaction greatly speeded by the presence of acids. For example, ethyl ethanoate is broken up into ethanoic acid and ethyl alcohol. The conversion of an acid into an ester is termed esterification. The reaction between an ester and a metallic base is known as saponification (see Soap). When the decomposition of an ester occurs upon its reaction with water, the ester is said to be hydrolysed.

The esters of organic acids are usually colourless, neutral liquids, pleasant-smelling and generally insoluble in water but readily soluble in organic solvents. Many esters have a fruity odour and are prepared synthetically in large quantities for commercial use as artificial fruit essences and other flavourings and as components of perfumes (see Essential Oils).

All natural fats and oils (other than mineral oils) and most waxes are mixtures of esters. For example, esters are the principal constituents of beef fat (tallow), pig fat (lard), fish oils (including cod-liver oil), and flaxseed oil (linseed oil). Esters of cetyl alcohol are found in the head oil of the sperm whale, and esters of myricyl alcohol in beeswax. Nitroglycerin, an important explosive, is an ester.

Esters such as amyl ethanoate (banana oil), ethyl ethanoate, and cyclohexanol ethanoate are the principal solvents for lacquer preparations. Other esters, such as dibutyl phthalate and tricresyl phosphate, are used as plasticizers in lacquers. Amyl ethanoate is employed as odour bait in grasshopper poisons, and several of the methanoates are good fumigants. Esters also have an important function in organic synthesis.

Esters have important medical uses. Ethyl nitrite is a diuretic and an antipyretic. Amyl nitrite is used in the treatment of asthma and epileptic convulsions as an antispasmodic. Nitroglycerine and amyl nitrite both cause blood-vessel dilation thereby lowering blood pressure. Ethyl chaulmoograte has been used in the treatment of leprosy. Dimethyl sulphate (often used in organic synthesis as a methylating agent) and diethyl sulphate are extremely dangerous in vapour form and must be handled cautiously.