_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).
__ __ __ __ __ __NH2 / \_NH·NH _/ \-->NH2 / \_/ \NH2 and NH2 / \_/ \ \__/ \__/ \__/ \__/ \__/ \__/ Hydrobenzene. Benzidine. Diphenylin. __ __ __ __ __ __ R/ \_NH·NH _/ \--> / \_NH_/ \or R/ \_NH_/ \NH2 \__/ \__/ \__/ \__/ \__/ \__/ Ortho-semidine. Para-semidine.