Benzidine is prepared in a two step process from nitrobenzene. First, the nitrobenzene is converted to 1,2-diphenylhydrazine, usually using iron powder as the reducing agent. Treatment of this hydrazine with mineral acids induces a rearrangement reaction to 4,4'-benzidine. Smaller amounts of other isomers are also formed.[3] The benzidine rearrangement, which proceeds intramolecularly, is a classic mechanistic puzzle in organic chemistry.[4]
In terms of its physical properties, 4,4'-benzidine is poorly soluble in cold water but can be recrystallized from hot water, where it crystallises as the monohydrate. It is dibasic, the deprotonated species has Ka values of 9.3 × 10−10 and 5.6 × 10−11. Its solutions react with oxidizing agents to give deeply coloured quinone-related derivatives.
Applications
Conversion of benzidine to the bis(diazonium) salt was once an integral step in the preparation of direct dyes (requiring no mordant). Treatment of this bis(diazonium) salt with 1-aminonaphthalene-4-sulfonic acid gives the once popular congo red dye. In the past, benzidine was used to test for blood. An enzyme in blood causes the oxidation of benzidine to a distinctively blue-coloured derivative. The test for cyanide relies on similar reactivity. Such applications have largely been replaced by methods using phenolphthalein/hydrogen peroxide and luminol.
The popular dye congo red is derived from benzidine.
Related 4,4’-benzidines
A variety of derivatives of 4,4’-benzidine are commercially produced on the scale of one to a few thousand kilograms per year, mainly as precursors to dyes and pigments.[3] These derivatives include, in order of scale, the following:
As with some other aromatic amines such as 2-naphthylamine, benzidine has been significantly withdrawn from use in most industries because it is so carcinogenic. In August 2010 benzidine dyes were included in the U.S. EPA's List of Chemicals of Concern.[7]
The manufacture of Benzidine has been illegal in the UK since at least 2002 under the Control of Substances Hazardous to Health Regulations 2002 (COSHH).
^March, J. (1992). Advanced Organic Chemistry (5th ed.). New York: J. Wiley and Sons. ISBN0-471-60180-2.
^Shine, H. J.; Zmuda, H.; Park, K. H.; Kwart, H.; Horgan, A. G.; Collins, C.; Maxwell, B. E. (1981). "Mechanism of the benzidine rearrangement. Kinetic isotope effects and transition states. Evidence for concerted rearrangement". Journal of the American Chemical Society. 103 (4): 955–956. doi:10.1021/ja00394a047..
^Shine, H. J.; Zmuda, H.; Park, K. H.; Kwart, H.; Horgan, A. G.; Brechbiel, M. (1982). "Benzidine rearrangements. 16. The use of heavy-atom kinetic isotope effects in solving the mechanism of the acid-catalyzed rearrangement of hydrazobenzene. The concerted pathway to benzidine and the nonconcerted pathway to diphenyline". Journal of the American Chemical Society. 104 (9): 2501–2509. doi:10.1021/ja00373a028.
