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brominated and nitrated; when the nitration is carried out in the presence of sulphuric acid, the nitro-groups take up the meta position with regard to the amino-groups. Benzidine finds commercial application since its tetrazo compound couples readily with amino-sulphonic acids, phenol carboxylic acids, and phenol and naphthol-sulphonic acids to produce substantive cotton dyes (see Dyeing). Among such dyestuffs are chrysamine or flavophenine, obtained from salicylic acid and diazotized benzidine, and congo red obtained from sodium naphthionate and diazotized benzidine. On the constitution of benzidine see G. Schultz (Annalen, 1874, 174, p. 227).

The Benzidine and Semidine Change.—Aromatic hydrazo compounds which contain free para positions are readily converted by the action of acids, acid chlorides and anhydrides into diphenyl derivatives; thus, as mentioned above, hydrazo-benzene is converted into benzidine, a small quantity of diphenylin being formed at the same time. The two products are separated by the different solubilities of their sulphates. This reaction is known as the benzidine transformation. If, however, one of the para positions in the hydrazo compound is substituted, then either diphenyl derivatives or azo compounds are formed, or what is known as the semidine change takes place (P. Jacobson, Berichte, 1892, 25, p. 992; 1893, 26, p. 681; 1896, 29, p. 2680; Annalen, 1895, 287, p. 97; 1898, 303, p. 290). A para mono substituted hydrazo compound in the presence of a hydrochloric acid solution of stannous chloride gives either a para diphenyl derivative (the substituent group being eliminated), an ortho-semidine, a para-semidine, or a diphenyl base, whilst a decomposition with the formation of amines may also take place. The nature of the substituent exerts a specific influence on the reaction; thus with chlorine or bromine, ortho-semidines and the diphenyl bases are the chief products; the dimethylamino, −N(CH3)2, and acetamino, −NHCOCH3, groups give the diphenyl base and the para-semidine respectively. With a methyl group, the chief product is an ortho-semidine, whilst with a carboxyl group, the diphenyl derivative is the chief product. The ortho- and para- semidines can be readily distinguished by their behaviour with different reagents; thus with nitrous acid the ortho-semidines give azimido compounds, whilst the para-semidines give complex diazo derivatives; with formic or acetic acids the ortho-semidines give anhydro compounds of a basic character, the para-semidines give acyl products possessing no basic character. The carbon disulphide and salicylic aldehyde products have also been used as means of distinction, as has also the formation of the stilbazonium bases obtained by condensing ortho-semidines with benzil (O. N. Witt, Berichte, 1892, 25, p. 1017).

Structurally we have:—

1911 Britannica-Benzidine.png

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.).