SALICYLIC ACID

also called ortho-hydroxybenzoic acid a white, crystalline solid that is used chiefly in the preparation of aspirin and other pharmaceutical products. The free acid occurs naturally in small amounts in many plants, particularly the various species of Spiraea. The methyl ester also occurs widely in nature; it is the chief constituent of oil of wintergreen. Salicylic acid was first prepared by the Italian chemist R. Piria in 1838 from salicylaldehyde. In 1860 the German chemists H. Kolbe and E. Lautemann discovered a synthesis based on phenol and carbon dioxide. Today the compound is made from dry sodium phenolate and carbon dioxide. Most salicylic acid produced commercially is treated with acetic anhydride for the preparation of aspirin. Large quantities of salicylic acid are esterified with methyl alcohol to yield methyl salicylate, synthetic oil of wintergreen, for medicinal and flavouring use. A certain amount is used to prepare phenyl salicylate for sunburn creams and enteric-coated pills and to make salicylanilide for use as a fungicide and mildew preventive. Salicylic acid is a component of preparations used to combat warts, corns, calluses, and various skin diseases; the sodium salt is employed as an antipyretic and analgesic. Large amounts of the crude acid are used in the manufacture of certain classes of dyes. Pure salicylic acid crystallizes from hot water in the form of white needles, which sublime without decomposition at temperatures up to 155 C (311 F) and melt at 159 C (318 F). Above 200 C (392 F) the acid decomposes to phenol and carbon dioxide.