Benzoic Acid

An organic acid, also known as benzene carboxylic acid, with the formula below. Melting point is 250.2°F (121.2°C), and the acid sublimes at 212°F (100°C). Benzoic acid is only slightly soluble in water but is soluble in most organic solvents, and reacts with bases to form the corresponding benzoate salts. Benzoic acid was first obtained by sublimation from gum benzoin. It occurs both free and combined in nature, being found in many berries (cranberries, prunes, cloves) and as the end product of phenylalanine metabolism.

Benzoic acid is prepared in the laboratory by the Grignard reaction, hydrolysis of benzonitrile (C6HsCN), or prolonged oxidation of alkyl benzenes with potassium per­manganate regardless of the length of the alkyl group. Commercially it was previ­ously prepared by the chlorination of toluene (C6H5CH3) with the subsequent hydrol­ysis of the benzotrichloride (C6H5CC13), and by the monodecarboxylation of phthalic anhydride (from naphthalene). Modern preparation is by the catalytic oxidation of toluene at elevated temperatures with a cobalt catalyst, and purification by sublimation.
Benzoic acid undergoes the normal reactions of the aromatic ring (nitration, sul­fonation, halogenation, alkylation). Groups are inserted in the meta position due to the directive influence of the carboxyl group. Substitution occurs less readily than with ortho- or para-directing groups due to the deactivating effect of the meta-directing group. Ortho or para derivatives can be obtained with some starting materials other than the acid. Benzoic acid also undergoes the usual reactions of the carboxyl group, forming acyl halides, anhydrides, amides, esters, and salts. Sodium benzoate is the only salt of importance. It is water-soluble, has antipyretic and antiseptic properties, is useful as a corrosion inhibitor with sodium nitrite if used for iron, and is also used to modify alkyd resins by increasing hardness, adhesion, and gloss.
Esters of benzoic acid are also found in nature. They are almost universally fragrant. Methyl benzoate is the fragrant principle in tuberose. Some esters of benzoic acid are used in the perfume industry, for example, benzyl ester as a fixative. The butyl ester is used as a dye carrier because of its desirable biodegradable properties; and glycol esters are used as plasticizers.
Uses for both benzoic acid and its derivatives include the pharmaceuticals and syn­thetic polymers. Benzoic acid is used in preservatives and many cosmetics. The deriva­tives are used in the dyeing industry, with some applications in the cosmetic industry. Pure benzoic acid is a standard for bomb calorimetry because of its ease of purification by sublimation.