BENZOIC ACID occurs naturally in some resins, especially in gum benzoin (from Styrax benzoin), in dragon's blood, and as a benzyl ester in Peru and Tolu balsams. It crystallizes from water in glistening leaflets which melt at 121.4° C. and boil at 249. 2 ° C. Its specific heat is 0.1946. It sublimes readily and is volatile in steam. It is readily soluble in hot water and the ordi nary organic solvents. Benzoic acid is the simplest representative of the organic acids in the aromatic series (see CHEMISTRY, Or ganic). It may be regarded as derived from the hydrocarbon benzene, through the replacement of one hydrogen atom by the acidic (carboxyl) group, —CO.OH; its formula is thus When heated with lime, it is decomposed, benzene being formed; if its vapours are passed over heated zinc dust, it is converted into benzaldehyde (A. Baeyer, 1866). Distillation of its calcium salt gives benzophenone (q.v.) with small quantities of other substances, but if the calcium salt be mixed with calcium formate and the mixture distilled, benzaldehyde is produced. By the action of sodium amalgam on an aqueous solution of the acid, benzyl alcohol, tetrahydrobenzoic acid and hexahydrobenzoic acid are formed. The salts of benzoic acid, known as benzoates, are mostly soluble in water. They are decomposed by mineral acids with the production of benzoic acid, and on addition of ferric chloride to their neutral solutions give a reddish-brown pre cipitate of ferric benzoate. Benzoic acid is usually prepared from toluene, this hydrocarbon being chlorinated to benzotrichloride, this is heated with water or aqueous alkali, milk of lime being employed commercially, and the resulting calcium ben zoate is then decomposed by mineral acid to yield free benzoic acid. It is also obtained by oxidizing benzaldehyde (q.v.), benzyl alcohol, or cinnamic acid. It also arises in many reactions of aro matic substances, as for instance by hydrolysis of benzonitrile ; by the action of carbon dioxide on benzene in the presence of aluminium chloride (C. Friedel and J. M. Crafts, 1888); by the action of carbon dioxide on monobrombenzene in the presence of sodium ; by passing carbon dioxide into an ethereal solution of phenyl magnesium bromide (see GRIGNARD REAGENTS) ; by con densing benzene and carbonyl chloride in presence of aluminium chloride, the benzoyl chloride formed being subsequently hydro lyzed; and similarly from benzene and chlorformamide with subsequent hydrolysis (q.v.) of benzamide. It may also be pre pared by boiling benzyl chloride with dilute nitric acid (G. Lunge, 1877), or by fusing sodium benzene sulphonate with sodium formate. Benzoic acid can be extracted from the hippuric acid found in the urine of the herbivorae. This urine is concentrated and the hippuric acid, precipitated by the addition of hydrochloric acid, is then boiled with concentrated hydrochloric acid until hydrolyzed into benzoic and aminoacetic acids.
Benzoic anhydride prepared by the action of benzoyl chloride on sodium benzoate, or by heating benzoyl chloride with anhydrous oxalic acid (R. Anschiitz, 1884), crystal lizes in needles, melting at 42° C., and boiling at 36o° C. It is insoluble in water but readily soluble in alcohol and ether.
Benzoyl chloride, a typical acid chloride, is formed by distilling a mixture of phosphorus pentachloride and benzoic acid; by the action of chlorine on benzaldehyde, or by passing a stream of hydrogen chloride over a mixture of benzoic acid and phosphoric oxide heated to zoo° C. (C. Friedel, 1869.) It is a colourless liquid of very unpleasant smell, which boils at 198° C., and solidifies in a freezing mixture, the crystals obtained melting at — I ° C. When triturated with powdered silver nitrate, benzoyl chloride is concerted into benzoyl nitrate (F. Francis, 1900). This nitrate explodes on heating, but in non hydroxylic media it serves as a nitrating agent for aromatic sub stances; it slowly changes into its isomeride meta-nitrobenzoic acid, Ethyl benzoate, prepared by boiling benzoic acid and alcohol with a small quantity of sulphuric acid for some hours (E. Fischer and A. Speier, 1896), is a colourless liquid of boiling point 213° C.
Benzamide, prepared by the action of benzoyl chloride on ammonia or ammonium carbonate, or from ethyl benzoate and ammonia, crystallizes from water in glistening leaf lets which melt at 130° C. and boil at 288° C. (See Aar, AMIDES. ) Chlor-, brom-, iodo- and fluor-benzoic acids are obtained by oxidizing the corresponding halogenated toluenes, or from the amino acids, or by substitution. Nitration of benzoic acid gives chiefly meta-nitrobenzoic acid. The ortho- and para-nitrobenzoic acids are obtained by oxidizing ortho- and para-nitro-cinnamic acids. Ortho-amino-benzoic acid (anthranilic acid) : is closely related to indigo (q.v.).
Gum benzoin, which contains from 12 to 20% of benzoic acid, is used in medicine as the essential constituent of benzoated lard, Adeps benzoatus, which owes its antiseptic properties to benzoic acid; and in friar's balsam, Tinctura benzoini composita, which is an ancient and valuable medicament, still largely used for inhalation in cases of laryngitis, bronchitis and other inflamma tory or actually septic conditions of the respiratory tract. It owes its value to the benzoic acid which it contains. A fluid drachm of friar's balsam may be added to a pint of water at a temperature of about 140°, and the resultant vapour may be inhaled from the spout of a kettle or from a special inhaler. Benzoic acid itself, ammonium benzoate and sodium benzoate are all administered internally in doses of from five to 3o grains. The ammonium salt is most often employed, owing to the stim ulant character of the ammonium base. The acid itself is a powerful antiseptic. When administered internally, it causes the appearance of hippuric acid in the urine. This is due to its combination in the body (kidney) with glycine. The hippuric acid in the urine acts as a stimulant and disinfectant to the urinary mucous membrane. Benzoic acid is also excreted by the bronchi and tends to disinfect and stimulate the bronchial mucous membrane. Hence the value of friar's balsam. The acid and its salts are antipyretic and were used in Germany instead of salicylates in rheumatic fever. But the most important fact is that ammonium benzoate is largely used—often in combina tion with urinary anodynes such as tincture of hyoscyamus—as a urinary antiseptic in cases of cystitis (inflammation of the bladder) and pyelitis (inflammation of the pelvis or the kidney).