Benzidine
Encyclopedia
Benzidine, the trivial name for 4,4'-diaminobiphenyl
, is the solid 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
.
Since August 2010 benzidine dyes are included in the EPA's List of Chemicals of Concern.
. 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. The benzidine rearrangement, which proceeds intramolecularly, is a classic mechanistic
puzzle in organic chemistry
.
The conversion is described as a [5,5]sigmatropic reaction
.
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 x 10−10 and 5.6 x 10−11. Its solutions react with oxidizing agents to give deeply coloured quinone-related derivatives.
s such as 2-Naphthylamine
, benzidine has been significantly withdrawn from use in most industries because it is so carcinogenic.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 Such applications have largely been replaced by methods using phenolphthalein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
.
(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.
Biphenyl
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...
, is the solid organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
(C6H4NH2)2. This aromatic amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
is a component of a test for cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
and also in the production
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
of dyes. Benzidine has been linked to bladder
Bladder cancer
Bladder cancer is any of several types of malignant growths of the urinary bladder. It is a disease in which abnormal cells multiply without control in the bladder. The bladder is a hollow, muscular organ that stores urine; it is located in the pelvis...
and pancreatic cancer
Pancreatic cancer
Pancreatic cancer refers to a malignant neoplasm of the pancreas. The most common type of pancreatic cancer, accounting for 95% of these tumors is adenocarcinoma, which arises within the exocrine component of the pancreas. A minority arises from the islet cells and is classified as a...
.
Since August 2010 benzidine dyes are included in the EPA's List of Chemicals of Concern.
Synthesis and properties
Benzidine is prepared in a two step process from nitrobenzeneNitrobenzene
Nitrobenzene is an organic compound with the chemical formula C6H5NO2. It is a water-insoluble pale yellow oil with an almond-like odor. It freezes to give greenish-yellow crystals. It is produced on a large scale as a precursor to aniline. Although occasionally used as a flavoring or perfume...
. 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
Rearrangement reaction
A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule...
to 4,4'-benzidine. Smaller amounts of other isomers are also formed. The benzidine rearrangement, which proceeds intramolecularly, is a classic mechanistic
Reaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
puzzle in organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
.
The conversion is described as a [5,5]sigmatropic reaction
Sigmatropic reaction
A sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular process. The name sigmatropic is the result of a compounding of the long-established sigma designation from single carbon-carbon...
.
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 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 amineAromatic amine
An aromatic amine is an amine with an aromatic substituent - that is -NH2, -NH- or nitrogen group attached to an aromatic hydrocarbon, whose structure usually contains one or more benzene rings. Aniline is the simplest example....
s such as 2-Naphthylamine
2-Naphthylamine
2-Naphthylamine is an aromatic amine. It is used to make azo dyes. It is a known human carcinogen and has largely been replaced by less toxic compounds. 2-Naphthylamine is prepared by heating 2-naphthol with ammonium zinc chloride to 200-210 °C; or in the form of its acetyl derivative by heating...
, 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
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
. An enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
in blood causes the oxidation of benzidine to a distinctively blue
Blue
Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal...
-coloured derivative. The test for cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
relies on similar reactivity. Such applications have largely been replaced by methods using phenolphthalein
Phenolphthalein
Phenolphthalein is a chemical compound with the formula C20H14O4 and is often written as "HIn" or "phph" in shorthand notation. Often used in titrations, it turns colorless in acidic solutions and pink in basic solutions...
/hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
and luminol
Luminol
Luminol is a versatile chemical that exhibits chemiluminescence, with a striking blue glow, when mixed with an appropriate oxidizing agent...
.
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. These derivatives include, in order of scale, the following: 3,3'-dichlorobenzidine (CAS# 91-94-1, m.p. 132 – 133 °C), o-tolidineTolidine
Tolidine is a group of isomeric organic compounds, the most prevalent of which is 2-tolidine .-Chemistry:2-Tolidine is slightly soluble in water and has a melting point of 129 °C. It readily forms salts with acids, such as the hydrochloride which is commercially available...
(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
Polybenzimidazole fiber
Polybenzimidazole fiber is a synthetic fiber with a very high melting point that also does not readily ignite, because of its exceptional thermal and chemical stability. The U.S...
, a high-strength, flame-resistant material.