Salicylic Acid

SALICYLIC ACID, employed in many medicinal prepara tions and in food preservation on account of its powerful antiseptic properties, is one of three hydroxybenzoic acids, It also finds considerable use in the manufacture of dyestuffs. (See DYES, SYNTHETIC.) It crystallises from hot water in prismatic needles melting at 156-159° C, and when care fully heated it sublimes without decomposition, this sublimation being sometimes applied as a method of purification. Salicylic acid and salicylaldehyde occur in the oils from various species of Spiraea, whereas methyl salicylate, a colourless oil boiling at 222° C, is the main constituent of oil of wintergreen from Gaul theria procumbens and G. fragrantissima. Salicylic acid was first obtained by R. Piria (1838) on treating salicylaldehyde with caustic potash, and until 1874 was mainly obtained by the action of potash on oil of wintergreen. It is now manufactured from phenol (see CARBOLIC ACID) by processes based on H. Kolbe's celebrated synthesis of the acid discovered in 1859 (Germ. Pat., and on R. Schmidt's improvement whereby sodium phen oxide (or phenolate), is converted into sodium phenylcarbonate, C6H50.CO2Na, and the latter salt heated under pressure until transformed completely into sodium salicylate, (American Patent 334290, English Patent 10167/1884).

Preparation.

Pure phenol is converted by concentrated aqueous caustic soda into sodium phenoxide and this salt is dried carefully and pulverised in a strong steel vessel (autoclave) cap able of withstanding considerable pressure. The anhydrous sodium phenoxide powder is then saturated under pressure (100 lb. per sq.in.) with carbon dioxide and heated in the autoclave at 120 140° C. The resulting sodium salicylate is dissolved in water, decolorised with stannous salts or with zinc and charcoal and then acidified with mineral acid, when salicylic acid is precipitated, col lected and washed with cold water. For colour making, the pre cipitated acid is sufficiently pure, but for preservative and medici nal purposes further purification is effected either by distillation in superheated steam or by sublimation. As salicylic acid is 35 times more soluble in hot than in cold water (Ioo parts of water at 15° C dissolve 0.225 part of salicylic acid), it may be readily crystallised from this solvent or from alcohol in which it dissolves more freely (I part in 2.5 parts of 90% alcohol).

The following methods of preparing salicylic acid are of scien tific interest : (I) from phenol, sodium carbonate and carbonyl chloride; (2) by the action of sodium on phenol and ethyl chloro carbonate, Cl.0O2C2H5; (3) by the process of F. Tiemann and K. Reimer who heated together at 100° C phenol, carbon tetra chloride and alcoholic potash.

Salicylic acid in aqueous solution develops a violet coloration with ferric chloride, and with bromine water it gives tribromo phenol bromide, Sodium in boiling amyl alcohol reduces it to n-pimelic acid, CO2H.[CH2b.0O2H. When heated

at 195-22o° C, it decomposes, yielding phenol and carbon dioxide, but also giving phenyl salicylate, (salol). This drug, also prepared by the action at 125° C of phosphorus oxychloride or carbonyl chloride on a mixture of sodium phenoxide and salicylate, crystallises from alcohol in rhombic plates melting at 42° C and boiling at 172° C,/I2 mm. Acetylsalicylic acid (see ASPIRIN), prepared by the action of acetyl chloride on salicylic acid or its sodium salt, crystallises in colourless needles and melts at 132° C (with decomposition).

Applications of Salicylic Acid and Salicylates.—Salicylic acid has a strong inhibitory influence on the growth of micro organisms, and retards the action of unorganised ferments and of the ferments of alcoholic and acetic fermentations. It has a widespread employment as a preservative of foods and bever ages in concentrations of o.5-6 parts per i,000. It is thus used in jams and other fruit preparations, in glues and albuminoid sub stances, and also in milk, beer and wine. The use of salicylic acid as a food preservative was, however, condemned by a commission of the U.S.A. government in 1904.

Applied externally as an antiseptic and antipruritic to wounds and parasitic skin diseases, salicylic acid is less irritating than phenol, but may in strong solution have a destructive action on the horny layer of the epidermis. It is employed to remove corns and warts and in the treatment of lupus. It is a constituent of dental preparations and mouth washes.

For internal use, salicylic acid has been almost completely re placed by its sodium salt (sodii salicylas, official dose o.6 to 2.09 grams) and its acetyl derivative (aspirin), which are less irritant to the mucous membrane of the stomach.

Salicylic acid is the basis of numerous drugs and therapeutic agents, and in addition to the above-mentioned there are also salipyrin, salophen, diuretin and many others.

The greater proportion of salicylic acid is, however, employed in the manufacture of such azo-dyes as alizarin yellow (khaki yel low), chrysamine, anthracene brown, cloth brown, cotton yellow, diamine yellow N, anthracene yellow, etc. Large quantities serve for the preparation of 5-aminosalicylic acid which is an inter mediate in the production of the important wool dye, diamond black.

BIBLIOGRAPHY.

For preparation and chemical properties: T. E. Thorpe, Dictionary of Applied Chemistry, vol. vii. (1926) ; F. Ullmann, Enzyklopddie der technischen Chemie, 2nd. Edition (5928). For appli cation in medicine and therapeutics: W. H. Martindale and W. West cott, Extra Pharmacopoeia, vol. i., 1928. (G. T. M.)