benzidine

Benzidine
IUPAC name	4,4'-diaminobiphenyl
Other names	Benzidine, di-phenylamine, diphenylamine
Identifiers
CAS number	92-87-5 Y
PubChem	7111
SMILES	c2c(c1ccc(N)cc1)ccc(N)c2
InChI	1/C12H12N2/c13-11-5-1-9(2-6-11)10-3-7-12(14)8-4-10/h1-8H,13-14H2
InChI key	HFACYLZERDEVSX-UHFFFAOYAX
ChemSpider ID	6844
Properties
Molecular formula	C12H12N2
Molar mass	184.24 g/mol
Appearance	white solid
Melting point	122-125 °C
Solubility in water	0.94 g/100 mL at 100 °C
Related compounds
Related compounds	biphenyl
Hazards
Main hazards	carcinogenic
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Benzidine, the trivial name for 4,4'-diaminobiphenyl, is the organic compound with the formula (C₆H₄NH₂)₂. This aromatic amine is a component of a test for cyanide and also in the production of dyes. Benzidine has been linked to bladder and pancreatic cancer.^[1]

Synthesis and properties

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.^[2] The benzidine rearrangement, which proceeds intramolecularly, is a classic mechanistic puzzle in organic chemistry.^[3]

The conversion is described as a [5,5]sigmatropic reaction ^{[4] [5]}.

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 diprotonated species has pK_a's of 9.3 x 10^-10 and 5.6 x 10^-11. Its solutions react with oxidizing agents to give deeply coloured quinone-related derivatives.

Applications

As with some other aromatic amines such as 2-aminonaphthalene, benzidine has been significantly withdrawn from use in most industries because it is so carcinogenic. 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 Such applications have largely been replaced by methods using phenolphthalein/hydrogen peroxide and luminol.

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.^[2] These derivatives include, in order of scale, the following: 3,3'-dichlorobenzidine (CAS# 91-94-1, m.p. 132 – 133 °C), o-tolidine (2,2'-dimethyl-4,4’-benzidine, CAS# 119-93-7, m.p. 130 °C), and o-dianisidine (2,2'-dimethoxy-4,4’-benzidine, CAS# 119-90-4, m.p. 133 °C). 3,3',4,4'-Tetraaminodiphenyl (CAS# 91-95-2, m.p. 178 °C) is a precursor to polybenzimidazole fiber, a high-strength, flame-resistant material.

References