Ester is an organic functional group that forms many sweet-smelling compounds. The chemical structure of an ester is represented by the general formula, R-CO-OR', where a central carbon atom that has two bonds to an oxygen atom (the carbonyl group), C=O, a single bond to another carbon atom represented by R, and a single bond to an oxygen atom connected to a carbon atom represented by R'. The R and R' groups can be the same or different. If the R and R' groups are bonded to each other, they form a ring and constitute a cyclic ester or lactone.
If an ester group is treated with water (hydrolysis), it yields an organic acid and an alcohol. Esters are classified according to which acid and alcohol are produced by hydrolysis. For example, methyl acetate, CH3-COOCH3, forms methanol, CH3OH, and acetic acid or vinegar, HOCOCH3, when treated with water. It consists of a central carbon atom that has two bonds to an oxygen atom (the carbonyl group), C=O, a bond to a carbon atom connected to three hydrogen atoms (methyl group), -CH3, and a bond to an oxygen atom connected to a methyl group, -OCH3. Low molecular weight esters that produce vinegar or acetic acid when hydrolyzed are used as solvents in coatings or paints on automobiles and inks. Larger esters can be used as a plasticizer, that is, to soften materials that are normally hard or brittle. Esters are important chemical compounds for various pharmaceutical and agricultural applications.
Esters having an acetic acid or vinegar base are called acetates. They are used extensively as solvents, due to their ability to dissolve various greases. Methyl acetate is a solvent for many oils and resins and is important in the manufacture of artificial leather. In the pharmaceutical and food processing industries, ethyl acetate is used as an extraction solvent. Butyl acetate does not discolor readily and dries at a reasonable rate; therefore it is an ideal solvent for inks. It is also used as a cleaning liquid for silicon wafers. Butyl acetate is a good solvent for the manufacture of lacquers, because it is a good medium for less compatible ingredients. Amyl acetates are utilized as extracting liquids in the isolation of penicillin and in the manufacture of polishes for leather goods.
Of all the chemical compounds approved by the Food and Drug Administration (FDA) for use as flavorings in food, esters make up the largest single portion of this group. The flavoring esters are a major constituent of essential oils which are used in foods as a natural flavoring. (An essential oil is a solution obtained from liquefying the vapors formed from heating the organic matter of a flavorful plant.) The American Indians used the dried leaves of a fern that commonly grows in the eastern United States, especially in North Carolina and Pennsylvania, to make wintergreen tea. When the essential oil of this plant was analyzed by scientists, it was made up almost entirely of the ester, methyl salicylate. This ester contains a benzene ring or phenyl group in its molecular structure and will produce methanol when heated with water. Methyl salicylate, or "sarsaparilla" flavor is also the major ingredient in "birch beer" or root beer. It also gives that wintergreen taste to toothpaste, chewing gum, and candy. Methyl salicylate can also be used to relieve muscular aches and pains and rheumatic conditions. Other aromatic esters are used as flavors or perfumes. Chocolate flavoring uses isoamyl phenylacetate, and strawberry is a mix of ethylmethylphenylglycinate and methyl cinnamate. The aromatic ester, benzyl acetate, is used largely in jasmine and gardenia fragrances, and benzyl benzoate, an ester with practically no odor, is used as a base in many perfumes. Some aromatic esters, such as Bitrex, are very bitter tasting, but are added to household products such as cleaning fluids, nail polish removers, and detergents to keep children from wanting to drink these poisonous substances.
Salicylic acid acetate, or aspirin, is only one of many esters used as medicines. Phenyl salicylate, a similar aromatic ester, is used in the treatment of rheumatic arthritis. Methyl phenidate, an ester which produces methanol when it is heated with water, is used to stimulate the central nervous system. The pharmaceutical industry has discovered that certain undesirable properties of drugs, such as bad taste or swelling of the skin at the spot of an injection, can be avoided by converting the original drug into an ester. The antibiotic dalactine (clindamycin), a bitter tasting drug, was converted to its palmitate ester in order to make its flavor less harsh.
The macrolides are large ring lactones or cyclic esters. Most of these unusual esters are isolated from microorganisms that grow in the soil and are being used as antibiotics in human and veterinary medicine. The well known antibiotic erythromycin is an example of a macrocylic lactone consisting of 12 carbon atoms and one oxygen atom bonded to a carbonyl group and the more potent roxithromycin contains 14 carbon atoms, an oxygen atom and a carbonyl group bonded as a cyclic ester.
See also Esterification.
Kirk-Othmer Encyclopedia of Chemical Technology, Pigments to Powders, Handling. New York: Wiley, 1999.
Loudon,G. Mark. Organic Chemistry. Oxford: Oxford University Press, 2002.
Patai, S., ed. Synthesis of Carboxylic Acids, Esters and Their Derivatives. New York: Wiley, 1991.
Andrew J. Poss