Esters are organic compounds that result from a condensation reaction between an acid (usually a carboxylic acid) and an alcohol. (A condensation reaction is a chemical reaction in which two molecules link together, usually with the expulsion of a small molecule such as water or ammonia.) A condensation reaction involving esters, called esterification, is an important chemical reaction in organic synthesis, the laboratory production of chemical compounds.
An ester is named after the alcohol and acid from which it is derived. Ethanoic (acetic)
‘ acid gives ethanoates (acetates) such as ethyl ethanoate, while butanoic (butyric) acid gives butanoates (butyrates). Animal fats, which are esters of carboxylic acids with the alcohol glycerol, are called triglycerides.
Esters do not generally dissolve in water. They have boiling points slightly higher than hydrocarbons (compounds made up solely of hydrogen and carbon) of similar molecular weights. Low-molecular-weight esters are usually colorless liquids; high-molecular-weight esters are generally solids. Many have pleasant odors, lending fragrance and flavor to flowers and fruits. Cyclic esters are known as lactones, their carbon atoms are bonded in the shape of rings. Several found in plants are used as drugs.
Esterification (the making of an ester) generally takes place in a solvent other than water—usually an alcohol. The chemical reaction between the alcohol and an acid dissolved in it produces an ester and water. New esters can also be made by mixing different alcohols with an ester.
Esterification has been studied since the middle of the 1800 s because it involves an equilibrium (in other words, it is reversible). Hydrolysis (a chemical process that changes compounds by taking up the elements of water) is the reaction that splits the ester to give the parent acid and alcohol. It is usually catalyzed (speeded up) by dilute mineral acid or a base in the presence of water and at a high temperature.
Esters can also react with bases. Such a reaction is irreversible, causing the ester to break down into its two basic parts—an acid and an alcohol. Since bases react with acids to form salts, the reaction of an ester and a base produces an alcohol and the salt of the acid. This type of hydrolysis is historically called saponification—one of the oldest chemical reactions known. Soap is made by saponification of natural fats and oils (lard or tallow) with lye (sodium dissolved in water or potassium hydroxide).
Low-molecular-weight esters (such as ethyl ethanoate and butyl ethanoate) are used industrially as solvents for lacquers, resins, and varnish. High-molecular-weight esters may be used as softeners for plastics. The plastic called polymethyl methacrylate is a glass substitute more commonly known as Lucite or Plexiglas. It is made using the ester of an unsaturated acid. Polyethylene terephthalate, a complex ester, is the basis of textile fibers such as Terylene, Fortrel, and Dacron. It is also used in sheets as photographic film (MylerTM ).
Aspirin is an ester, the product of the reaction between salicylic acid and acetic anhydride. Cellulose is a substance that forms the walls of plant cells and the woody parts of trees and plants. Cellulose esters are used in photographic films, as a textile fiber (rayon), and in explosives. Natural oils and waxes are largely simple esters. Beeswax is mainly myri-cyl palmitate. Menthyl acetate is the ester found in peppermint oil.
Low-molecular-weight esters have characteristic fruitlike odors. The flavors and odors of many fruits and flowers are due to a complex mixture of minute traces of esters. Today, esters are widely used as synthetic flavors and in perfumes. Esters can duplicate the taste and smell of bananas (isoamyl ethanoate), rum (isobutyl propanoate), pineapple (butyl bu-tanoate), apple (isoamyl valerate), and wine/co-gnac (ethyl decanoate). Hundreds of esters in trace amounts contribute to the flavor of wine.