3,3'-Dichlorobenzidine
3,3'-Dichlorobenzidine is a chlorinated aromatic amine derived from benzidine. It is widely used in the production of diarylide yellow pigments used in the production of printing inks. Its use in the production of dyes has been largely discontinued because of concerns about carcinogenicity.
Properties and the market
3,3'-Dichlorobenzidine is a crystalline solid soluble in alcohol, benzene, diethyl ether, and glacial acetic acid, but difficultly soluble in dilute hydrochloric acid and water. It exists in gray-to-purple needle-like crystals. Commercial powder samples may also be light-brown or violet.This compound is often confused with its dihydrochloride salt, even in scientific literature. 3,3'-Dichlorobenzidine dihydrochloride is sparingly soluble in water and readily soluble in alcohol. It is marketed in the form of a colorless to pale gray crystalline powder. Both the commercial 3,3'-dichlorobenzidine and its hydrochloride usually contain 5 to 30% of water.
The so-called urethane quality 3,3'-dichlorobenzidine is anhydrous and free of hydrochloric acid. However, some sources dispute the very existence of the free base on the market and believe the dihydrochloride salt to be the only form available commercially.
Preparation and reactions
3,3'-Dichlorobenzidine is prepared in two steps from 2-nitrochlorobenzene. The first step involves reduction with zinc in base to afford 2,2'-dichlorodiphenylhydrazine. This intermediate undergoes the benzidine rearrangement to afford 3,3'-dichlorobenzidine.Aqueous solutions of 3,3'-dichlorobenzidine degrade in light to monochloro derivative. It undergoes chlorination to give the tetrachloro derivative.
The most widely practiced reaction of 3,3'-dichlorobenzidine is its double diazotization. This bis intermediate is then coupled to derivatives of acetoacetylaminobenzene CH2C. In this way, the diarylide commercial yellow pigments are produced: Pigment Yellow 12, Pigment Yellow 13, Pigment Yellow 14, Pigment Yellow 17, Pigment Orange 13, Pigment Yellow 81, and Pigment Yellow 83.