3,3'-Dichlorobenzidine Dihydrochloride

We want to state this as clearly and as early as possible, the way we do for every carcinogen-classified product in our range: 3,3'-Dichlorobenzidine, and its dihydrochloride salt supplied on this page, carries a harmonised EU CLP classification of Carcinogen Category 1B (H350, "may cause cancer"), an IARC classification of Group 2B ("possibly carcinogenic to humans"), and a US National Toxicology Program listing as "reasonably anticipated to be a human carcinogen." We want to be precise about what tier this is: all three of these are the probable/presumed/possible tier, based on sufficient evidence in animal studies, not the "confirmed/known human carcinogen" tier that substances like vinyl chloride or benzene itself carry. We state this distinction because overstating the classification would be as inaccurate as understating it, and we're committed to neither here.

This product is the dihydrochloride salt of 3,3'-dichlorobenzidine (free base CAS 91-94-1; salt CAS 612-83-9) — the salt form is more water-soluble and is a common commercial form for this diamine. Its real, long-established industrial use is as the diazotisable diamine behind diarylide pigment chemistry: double diazotisation followed by double coupling onto an acetoacetanilide-type component gives Pigment Yellow 12, 13, 14, 17, 55, 63 and 83, and Pigment Orange 13 and 34 — a well-documented family of organic pigments used across printing inks, plastics and coatings.

Where it fits in the synthesis chain

We supply the diamine salt itself. We do not manufacture or supply any of the finished diarylide pigments made from it.

Grades we supply

95% and 98% grades are both seen across the trade for this diamine salt.

3,3'-Dichlorobenzidine is made industrially from 2-nitrochlorobenzene: zinc or an equivalent reducing agent, under basic conditions, reduces it to 2,2'-dichlorohydrazobenzene, which then undergoes an acid-catalysed benzidine rearrangement — a well-known named reaction that converts a hydrazobenzene into the corresponding 4,4'-diaminobiphenyl (benzidine-class) structure.

The free base is then converted to its dihydrochloride salt by treatment with hydrochloric acid, giving the more water-soluble, more commercially convenient crystalline form traded under this product listing.

The finished salt is purified by crystallisation and tested for free-base content, residual starting material, and isomeric purity before release, since diarylide pigment manufacturers require consistent, well-characterised diamine input to achieve reproducible pigment shade.

A regulatory note that matters for how this product is traded

We want to flag an important and genuinely useful distinction for buyers assessing compliance requirements. EU REACH Annex XVII, Entry 43 restricts a list of aromatic amines including this one — but that restriction applies to finished azo dyes and colourants used in textile and leather articles with prolonged skin contact, specifically when reductive cleavage of the azo bond would release the restricted amine above a threshold concentration. It does not restrict the free amine or its salts as a traded chemical intermediate. What we supply here — the diamine itself, for pigment manufacture — is governed by its own CLP carcinogen classification and ordinary REACH registration duties, not by Entry 43's finished-article restriction. We mention this because the two are easy to conflate and the distinction materially affects what compliance documentation is actually required for this specific product.