Benzidine

BENZIDINE is an important chemical base of the aromatic or coal-tar group crystallizing in colourless plates that melt at 12 7 28 ° C. ; it owes its origin to a noteworthy series of chemical changes arising from the reduction of nitro benzene in alkaline media. Removal of oxygen from two molecular proportions of nitrobenzene leads to red azobenzene (see Azo-coM POUNDS), 2C6H5•NO2—NC6H5•N and addition of hydrogen to azobenzene gives rise to colourless hydrazobenzene, a product which undergoes a singular molecular rearrangement on treatment with mineral acids, and has by analogy the constitution being, in fact, di-para-diamino-diphenyl.

The Benzidine Conversion or Rearrangement.—In this conversion the nitrogen atoms part company and the phenyl resi dues swing round, to join together again mainly, in dipara-posi tions with respect to the resulting amino-groups thus giving ben zidine, and, to a less extent, to bring the para-position of one phenyl group into the ortho-position on the other so as to form a small proportion of diphenyline.

Diphenyline is technically unimportant, but benzidine is employed on a considerable scale in the manufacture of azo-colours having the valuable property of dyeing directly on cotton without the in tervention of a mordant (see DYES, SYNTHETIC).

Benzirline

is manufactured by heating together a mixture of nitrobenzene (produced by nitrating benzene, q.v.), zinc dust and aqueous caustic soda, preferably with addition of alcohol. The colourless stage of the reduction being reached, the alcohol is distilled off, the hydrazobenzene is separated mechanically from zinc and zinc compounds, and dissolved in cold hydrochloric acid. Addition of sodium sulphate to this acid solution precipitates sparingly soluble benzidine sulphate. This salt is suspended in water and decomposed with soda ash, the free benzidine being col lected, dried and purified by distillation under reduced pressure.

When cheap electrical energy is available, benzidine is manufac tured electrolytically. Nitrobenzene is floated on a strong solu tion of caustic soda in a one-compartment electrolytic cell and reduced by the nascent hydrogen generated at the cathode; the oxygen simultaneously evolved at the anode is collected and utilized. In order to maintain the hydrazobenzene in the liquid phase a suitable solvent such as o-dichlorobenzene is added. On treating this solution of hydrazobenzene with cold hydrochloric acid, the benzidine conversion takes place and the base is isolated as in the foregoing chemical reduction.

The electrolytic process is also effective in the case of tolidine or 2 : 2'-dimethylbenzidine (I), a base which is also used exten sively in colour manufacture.

Another valuable diamine of this series is dianisidine or 2 : 2'-dime thoxybenzidine (2), which gives rise to direct-cotton colours hav ing special tinctorial properties. This technical use of benzidine and its allies is based on their diazotization to bisdiazonium salts, (see DlAzo-00MP0uNDs), which are then combined with various naphthylaminesulphonic acids and naph tholsulphonic acids giving rise to bisazo dyes which are substan tive on unmordanted cotton.