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Benzoic Acid

BENZOIC ACID, C7H6O2 or C6H5COOH, the simplest representative of the aromatic acids. It 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 can be prepared by the oxidation of toluene, benzyl alcohol, benzaldehyde and cinnamic acid; by the oxidation of benzene with manganese dioxide and concentrated sulphuric acid in the cold (L. Carius, Ann. 1868, 148, p. 51); by hydrolysis of benzonitrile or of hippuric acid; by the action of carbon dioxide on benzene in the presence of aluminium chloride (C. Friedel and J.M. Crafts, Ann. chim. phys. 1888 [6], 14, p. 441); by the action of carbon dioxide on monobrombenzene in the presence of sodium; by condensing benzene and carbonyl chloride in presence of aluminium chloride, the benzoyl chloride formed being subsequently hydrolysed; and similarly from benzene and chlorformamide: -

C6H6 + Cl·CONH2 = HCl + C6H5CONH2,

the benzamide being then hydrolysed. It may also be prepared by boiling benzyl chloride with dilute nitric acid (G. Lunge, Berichte, 1877, 10, p. 1275); by fusing sodium benzene sulphonate with sodium formate: C6H5SO3Na + HCO2Na = C6H5COONa + NaHSO3; by heating calcium phthalate with calcium hydroxide to 330°-350° C.; by heating benzotrichloride with water in a sealed tube, and from the hippuric acid which is found in the urine of the herbivorae. For this purpose the urine is concentrated and the hippuric acid precipitated by the addition of hydrochloric acid; it is then filtered and boiled for some time with concentrated hydrochloric acid, when it is hydrolysed into benzoic and amido-acetic acid. It is made commercially by boiling benzotrichloride (obtained from toluene) with milk of lime, the calcium benzoate so obtained being then decomposed by hydrochloric acid

2C6H5CCl3 + 4Ca(OH)2 = (C6H5COO)2Ca + 3CaCl2 + 4H2O.

Benzoic acid crystallizes in glistening leaflets (from water) which melt at 121.4° C. and boil at 249.2° C. (H. Kopp). Its specific heat is 0.1946. It sublimes readily and is volatile in steam. It is readily soluble in hot water and the ordinary organic solvents, but is only slightly soluble in cold water. 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, Ann. 1866, 140, p. 296). 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 are known as the benzoates and are mostly soluble in water. They are readily decomposed by mineral acids with the production of benzoic acid, and on addition of ferric chloride to their neutral solutions give a reddish-brown precipitate of ferric benzoate.

Benzoic anhydride, (C6H5CO)2O, is prepared by the action of benzoyl chloride on sodium benzoate, or by heating benzoyl chloride with anhydrous oxalic acid (R. Anschütz, Ann. 1884, 226, p. 15). It crystallizes in needles, melting at 42° C., and boiling at 360° C. It is insoluble in water but readily soluble in alcohol and ether.

Benzoyl chloride, C6H5COCl, 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 hydrochloric acid gas over a mixture of benzoic acid and phosphorus pentoxide heated to 200° C. (C. Friedel, Ber. 1869, 2, p. 80). 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 −1° C. It shows all the characteristic properties of an acid chloride.

Ethyl benzoate, C6H5COOC2H5, is best prepared by boiling benzoic acid and alcohol with a small quantity of sulphuric acid for some hours (E. Fischer and A. Speier, Berichte, 1896, 28, p. 3252). It is a colourless liquid of boiling point 213° C.

Benzamide, C6H5CONH2, is prepared by the action of benzoyl chloride on ammonia or ammonium carbonate, or from ethyl benzoate and ammonia. It crystallizes (from water) in glistening leaflets which melt at 130° C. and boil at 288° C. Its silver salt behaves as if it were the salt of an imido benzoic acid, since it yields benzimido ethyl ether C6H5·C( : NH)·OC2H5 with ethyl iodide (J. Tafel and C. Enoch, Berichte, 1890, 23, p. 1550).

Chlor-, brom-, iodo- and fluor-benzoic acids are known and can be obtained by oxidizing the corresponding halogen toluenes, or from the amido acids, or by substitution. Nitration of benzoic acid gives chiefly meta-nitro-benzoic acid. The ortho- and para-nitro-benzoic acids can be obtained by oxidizing ortho-and para-nitro-cinnamic acids. Ortho-amino-benzoic acid, C6H4·NH2·COOH (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 inflammatory 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° F., 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 thirty grains. The ammonium salt is most often employed, owing to the stimulant 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 with glycocoll. The combination probably occurs in the kidney. 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 combination 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 of the kidney).

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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