# 1911 Encyclopædia Britannica/Benzidine

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​BENZIDINE (Dipara-diamino-diphenyl), NH$_2$·C$_6$H$_4$·C$_6$H$_4$·NH$_2$, a chemical base which may be prepared by the reduction of the corresponding dinitro-diphenyl, or by the reduction of azo-benzene with tin and hydrochloric acid. In this latter case hydrazo-benzene C$_6$H$_5$NH·NH·C$_6$H$_5$ is first formed and then undergoes a peculiar re-arrangement into benzidine (see H. Schmidt and G. Schultz, Annalen, 1881, 207, p. 320; O. N. Witt and Hans v. Helmont, Berichte, 1894, 27, p. 2352; P. Jacobson, Berichte, 1892, 25, p. 994). Benzidine crystallizes in plates (from water) which melt at 122° C., and boil above 360° C., and is characterized by the great insolubility of its sulphate. It is a di-acid base and forms salts with the mineral acids. It is readily ​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(CH$_3$)$_2$, and acetamino, –NHCOCH$_3$, 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:—