Everything about Aniline totally explained
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Aniline,
phenylamine or
aminobenzene is an
organic compound with the
formula C
6H
7N. It is the simplest and one of the most important
aromatic amines, being used as a precursor to more complex chemicals. Its main application is in the manufacture of
polyurethane. Like most volatile amines, it possesses the somewhat unpleasant odour of rotten fish and also has a burning aromatic taste; it's a highly- poison. It ignites readily, burning with a smoky flame.
Structure and synthesis
Consisting of a
phenyl group attached to an
amino group, aniline is usually produced in industry in two steps from
benzene:
First, benzene is
nitrated using a concentrated mixture of
nitric acid and
sulfuric acid at 50 to 60 °C, which gives
nitrobenzene. In the second step, the nitrobenzene is
hydrogenated, typically at 600 °C in presence of a
nickel catalyst to give aniline. As an alternative, aniline is also prepared from
phenol and ammonia, the phenol being derived from
cumene.
Derivatives
Many derivatives of aniline can be prepared in similar fashion. In commerce, three brands of aniline are distinguished--aniline oil for blue, which is pure aniline; aniline oil for red, a mixture of equimolecular quantities of aniline and ortho- and
para-toluidines; and aniline oil for
safranine, which contains aniline and ortho-
toluidine, and is obtained from the
distillate (échappés) of the
fuchsine fusion.
Properties
Oxidation
Aniline is colourless, it slowly
oxidizes and resinifies in air, giving a red-brown tint to aged samples. The oxidation of aniline has been carefully investigated. In alkaline solution,
azobenzene results, whereas
arsenic acid produces the violet-colouring matter violaniline.
Chromic acid converts it into
quinone, whereas
chlorates, in the presence of certain metallic salts (especially of
vanadium), give
aniline black. Hydrochloric acid and potassium chlorate give chloranil.
Potassium permanganate in neutral solution oxidizes it to
nitrobenzene, in alkaline solution to
azobenzene, ammonia and
oxalic acid, in acid solution to aniline black.
Hypochlorous acid gives
4-aminophenol and para-amino
diphenylamine.
Basicity
Aniline is a weak
base.
Aromatic amines such as aniline are, in general, much weaker bases than
aliphatic amines. Aniline reacts with strong acids to form
anilinium (or phenylammonium) ion (C
6H
5-NH
3+). The
sulfate forms beautiful white plates. Although aniline is weakly basic, it
precipitates zinc,
aluminium, and
ferric salts, and, on warming, expels
ammonia from its salts.
Acylation
Aniline reacts with carboxylic acids or more readily with
acyl chlorides such as
acetyl chloride to give
amides. The amides formed from aniline are sometimes called
anilides, for example CH
3-CO-NH-C
6H
5 is
acetanilide. Antifebrin (
acetanilide), an anti-pyretic and analgesic, is obtained by the reaction of
acetic acid and aniline.
N-alkyl derivatives
Aniline combines directly with
alkyl iodides to form secondary and tertiary amines. Monomethyl and dimethyl aniline are colourless liquids prepared by heating aniline, aniline hydro-chloride and
methyl alcohol in an
autoclave at 220 °C. They are of great importance in the colour industry. Monomethyl aniline boils at 193-195 °C, dimethyl aniline at 192 °C.
Sulfur derivatives
Boiled with
carbon disulfide, it gives sulfocarbanilide (diphenyl
thiourea), CS(NHC
6H
5)
2, which may be decomposed into phenyl
isothiocyanate, C
6H
5CNS, and triphenyl
guanidine, C
6H
5N=C(NHC
6H
5)
2.
Like
phenols, aniline derivatives are highly susceptible to
electrophilic substitution reactions. For example, sulfonation of aniline produces sulfanilic acid, which can be converted to
sulfanilamide. Sulfanilamide is one of the
sulfa drugs that were widely used as
antibacterials in the early
20th century. Reaction with
sulfuric acid at 180°C produces
sulfanilic acid, NH
2C
6H
4SO
3H.
Diazotization
Aniline and its ring-substituted derivatives react with
nitrous acid to form
diazonium salts. Through these, the -NH
2 group of aniline can be conveniently converted to -OH, -CN, or a
halide via
Sandmeyer reactions.
Other reactions
It reacts with nitrobenzene to produce
phenazine in the
Wohl-Aue reaction.
Uses
The great commercial value of aniline was due to the readiness with which it yields, directly or indirectly,
dyestuffs. The discovery of
mauve in
1856 by
William Henry Perkin was the first of a series of dyestuffs that are now to be numbered by hundreds. Reference should be made to the articles
dyeing,
fuchsine,
safranine,
indulines, for more details on this subject. In addition to its use as a precursor to dyestuffs, it's a starting-product for the manufacture of many drugs, such as
paracetamol (acetaminophen,
Tylenol).
It is used to stain neural
RNA blue in the
Nissl stain.
At the present time, the largest market for aniline is preparation of
methylene diphenyl diisocyanate (MDI), some 85% of aniline serving this market. Other uses include
rubber processing chemicals (9%),
herbicides (2%), and dyes and pigments (2%).
History
Aniline was first isolated from the destructive distillation of
indigo in
1826 by
Otto Unverdorben, who named it crystalline. In
1834,
Friedrich Runge (
Pogg. Ann., 1834, 31, p. 65; 32, p. 331) isolated from
coal tar a substance that produced a beautiful blue colour on treatment with chloride of lime, which he named kyanol or cyanol. In
1841, C. J. Fritzsche showed that, by treating indigo with caustic potash, it yielded an oil, which he named aniline, from the specific name of one of the indigo-yielding plants,
Indigofera anil, anil being derived from the Sanskrit
nīla, dark-blue, and
nīlā, the indigo plant. About the same time
N. N. Zinin found that, on reducing nitrobenzene, a base was formed, which he named benzidam.
August Wilhelm von Hofmann investigated these variously-prepared substances, and proved them to be identical (
1855), and thenceforth they took their place as one body, under the name aniline or phenylamine.
Its first industrial-scale use was in the manufacture of
mauveine, a
purple dye discovered in
1856 by Hofmann's student
William Henry Perkin. At the time of mauveine's discovery, aniline was an expensive laboratory compound, but it was soon prepared "by the ton" using a process previously discovered by
Antoine Béchamp. The synthetic dye industry grew rapidly as new aniline-based dyes were discovered in the late 1850s and 1860s.
p-Toluidine, an aniline derivative, can be used in qualitative analysis to prepare carboxylic acid derivatives.
Toxicology
Aniline is toxic by inhalation of the vapour, absorption through the skin or swallowing. It causes headache, drowsiness,
cyanosis, and mental confusion, and, in severe cases, can cause
convulsions. Prolonged exposure to the vapour or slight skin exposure over a period of time affects the nervous system and the blood, causing tiredness, loss of appetite, headache, and dizziness.
Oil mixtures containing
rapeseed oil denatured with aniline have been clearly linked by
epidemiological and analytic chemical studies to the
toxic oil syndrome that hit
Spain in the spring and summer of 1981, in which 20,000 became acutely ill, 12,000 were hospitalized, and more than 350 died in the first year of the epidemic. The precise
etiology though remains unknown.
Some authorities class aniline as a
carcinogen, although the
IARC lists it in
Group 3 (
not classifiable as to its carcinogenicity to humans) due to the limited and contradictory data available.
Further Information
Get more info on 'Aniline'.
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