Diphenylbenzidine, reaction with

Reduction to quinol can also be carried out with powdered zinc. In 75% sulfuric acid, oxidation of diphenylbenzidine with quinones is accompanied by the formation of a blue-violet color. Anthraquinone, diacetyl, and benzil do not react, but with all other organic oxidants the reaction is smooth (32). In a similar manner the reaction with p-dimethylaminodiphenylmethane can also be used for the detection of chloranil (33). Quinones, similar to 1,2-diketones, catalyze the slow reaction of formaldehyde with o-dinitro-benzene, in which the violet salt of the acid form of o-isonitrosobenzene (34) is formed. It seems, however, that certain phenols react as well (35). 

The redox reaction between diphenylbenzidine and thiosulfate is of first order with respect to thiosulfate, diphenylbenzidine, and hydrogen ions.301 Zinc-diamine-catalysed reduction of various ketones with polymethylhydrosiloxane in protic conditions show moderate enantioselectivities. Probable mechanisms are proposed.302 Reduction of non-activated aryl and alkyl halides by a neutral ground-state organic molecule (13) afforded the corresponding indolines in excellent yields. A tentative mechanism has been suggested (Scheme 13).303... 

Similar considerations apply to redox titrations, where the indicator dye should possess differently coloured oxidation states and a redox potential which is appropriate for the reaction to be studied. A large number of dyes are known which are suitable for this purpose. Ferroin (Scheme 7), a complex between iron(ll) and phenanthrolin with a deeply red colour, upon oxidation forms a pale blue iron(lll) complex. Another redox indicator is diphenylamine (Scheme 8), which is colourless in the reduced state. It is oxidized in acidic solution irreversibly to diphenylbenzidine (Scheme 9) which can be further oxidized reversibly to the intensely coloured diphenylbenzidine violet (Scheme 10). 

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