Aniline

Overview
Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. It is the simplest and one of the most imporant 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 a somewhat unpleasant odour of rotten fish and also has a burning aromatic taste; it is a highly acrid poison. It ignites readily, burning with a smoky flame.

Structure and synthesis
Consisting of a phenyl group attached to an amino group, aniline is produced industrially in two steps from benzene:



First, benzene is nitrated using a mixture of concentrated mixture of nitric acid and sulfuric acid at 50 - 60 °C, which gives the electrophile NO2+ that attacks the benzene, displacing a proton H+ from that particular carbon atom. The resulting nitrobenzene is then treated with hydrogen gas, typically at 600 °C in presence of a nickel catalyst to give aniline, this conversion being called hydrogenation.

Derivatives
Many derivatives of aniline can be prepared similarly. 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.

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, while arsenic acid produces the violet-colouring matter violaniline. Chromic acid converts it into quinone, while 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 generally much weaker bases than aliphatic amines. Aniline reacts with strong acids to form anilinium (or phenylammonium) ion (C6H5-NH3+). 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 CH3-CO-NH-C6H5 is acetanilide. Antifebrin or acetanilide is obtained from 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(NHC6H5)2, which may be decomposed into phenyl isothiocyanate, C6H5CNS, and triphenyl guanidine, C6H5N=C(NHC6H5)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 which were widely used as antibacterials in the early 20th century. Reaction with sulfuric acid at 180&deg;C produces sulfanilic acid, NH2C6H4SO3H.

Diazotization
Aniline and its ring-substituted derivatives react with nitrous acid to form diazonium salts. Through these, the -NH2 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
Originally 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 which 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 is 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.

Currently 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 (Pogg. Ann., 1826, 8, p. 397), who named it crystalline. In 1834, Friedrich Runge (Pogg. Ann., 1834, 31, p. 65; 32, p. 331) isolated from coal tar a substance which produced a beautiful blue colour on treatment with chloride of lime; this 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, 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.