Aniline

For other uses, see Aniline (disambiguation).

Aniline
Aniline	Aniline
IUPAC name Phenylamine
Other names Phenylamine Aminobenzene Benzenamine
style="background: #F8EABA; text-align: center;" colspan="2" | Identifiers
CAS number	62-53-3 Y
SMILES
style="background: #F8EABA; text-align: center;" colspan="2" | Properties
Molecular formula	C6H7N
Molar mass	93.13 g mol−1
Appearance	colourless liquid
Density	1.0217 g/ml, liquid
Melting point	-6.3 °C, 267 K, 21 °F
Boiling point	184.13 °C, 457 K, 363 °F
Solubility in water	3.6 g/100 mL at 20°C
Solubility in methanol	methanol 10.97 M
Basicity (pKb)	9.3
Viscosity	3.71 cP (3.71 mPa·s at 25 °C
style="background: #F8EABA; text-align: center;" colspan="2" | Thermochemistry
Std enthalpy of combustion ΔcHo298	-3394 kJ/mol
style="background: #F8EABA; text-align: center;" colspan="2" | Hazards
EU classification	Toxic (T) Carc. Cat. 3 Muta. Cat. 3 Dangerous for the environment (N)
R-phrases	R23/24/25 R40 R41 R43 R48/23/24/25 R68 R50
S-phrases	(S1/2) S26 S27 S36/37/39 S45 S46 S61 S63
NFPA 704	75px 2 3 0
style="background: #F8EABA; text-align: center;" colspan="2" | Related compounds
Related aromatic amines	1-Naphthylamine 2-Naphthylamine
Related compounds	Phenylhydrazine Nitrosobenzene Nitrobenzene
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

Aniline, phenylamine or aminobenzene is an organic compound with the formula C₆H₅NH₂. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane. Like most volatile amines, it possesses the somewhat unpleasant odour of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds. Aniline is colorless, but it slowly oxidizes and resinifies in air, giving a red-brown tint to aged samples.

Production

Aniline is mainly 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 200-300 °C in presence of various metal catalysts:

C₆H₅NO₂ + 3 H₂ → C₆H₅NH₂ + 2 H₂O

Originally, the reduction was effected with a mixture of ferrous chloride and iron metal via the Bechamp reduction.

As an alternative, aniline is also prepared from phenol and ammonia, the phenol being derived from the cumene process.

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.

Related aniline derivatives

Many derivatives of aniline can be prepared in similar fashion from nitrated aromatic compounds. Nitration followed by reduction of toluene affords toluidines. Nitration of chlorobenzene and related derivatives gives aniline derivatives, e.g. 4-chloroaniline.

Reactions

The chemistry of aniline is extremely rich because the compound has been cheaply available for many years. Below are some classes of its reactions.

Oxidation

The oxidation of aniline has been heavily investigated, and can result in reactions localized at nitrogen or more commonly results in the formation of new C-N bonds. In alkaline solution, azobenzene results, whereas arsenic acid produces the violet-coloring 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. Oxidation with persulfate affords a variety of polyanilines compounds. These polymers exhibit rich redox and acid-base properties.

Electrophilic reactions at carbon

Like phenols, aniline derivatives are highly susceptible to electrophilic substitution reactions. Its high reactivity reflects that it is an enamine, which enhances the electron-donating ability of the ring. For example, reaction of aniline with sulfuric acid at 180 °C produces sulfanilic acid, H₂NC₆H₄SO₃H, which can be converted to sulfanilamide. Sulfanilamide is one of the sulfa drugs, which were widely used as antibacterials in the early 20th century. The largest scale industrial reaction of aniline involves its alkylation with formaldehyde:

2 C₆H₅NH₂ + CH₂O → CH₂(C₆H₄NH₂)₂ + H₂O

The resulting diamine is the precursor to 4,4'-MDI and related diisocyanates.

Reactions at nitrogen

Basicity

Aniline is a weak base. Aromatic amines such as aniline are, in general, much weaker bases than aliphatic amines because of the electron-withdrawing effect of the phenyl group. 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. The weak basicity is due to a negative inductive effect as the lone pair on the nitrogen is partially delocalised into the pi system of the benzene ring.

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 (acetanilide), an anti-pyretic and analgesic, is obtained by the reaction of acetic acid and aniline.

N-Alkylation

N-methylation of aniline with methanol at elevated temperatures over acid catalsts gives N-methylaniline and dimethylaniline:

C₆H₅NH₂ + 2 CH₃OH → C₆H₄N(CH₃)₂ + H₂O

Methyl and dimethylaniline are colourless liquids with b.p. of 193-195 °C and 192 °C, respectively. These derivatives are of importance in the colour industry. Aniline combines directly with alkyl iodides to form secondary and tertiary amines.

Carbon disulfide derivatives

Boiled with carbon disulfide, it gives sulfocarbanilide (diphenylthiourea) (CS(NHC₆H₅)₂), which may be decomposed into phenyl isothiocyanate(C₆H₅CNS), and triphenyl guanidine (C₆H₅N=C(NHC₆H₅)₂).

Diazotization

Aniline and its ring-substituted derivatives react with nitrous acid to form diazonium salts. Through these intermediates, 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. Hydrogenation gives cyclohexylamine.

Being a standard reagent in laboratories, aniline is used for many niche reactions. Its acetate is used in the Aniline acetate test for carbohydrates, identifying pentoses by conversion to furfural. It is used to stain neural RNA blue in the Nissl stain.

Uses

The largest application of aniline is for the preparation of methylene diphenyl diisocyanate (MDI). The majority of aniline serves this market. Other uses include rubber processing chemicals (9%), herbicides (2%), and dyes and pigments (2%). As additives to rubber, aniline derivatives such as phenylenediamine and diphenylamine, are antioxidants. Illustrative of the drugs prepared from aniline is paracetamol (acetaminophen, Tylenol). The principal use of aniline in the dye industry is as a precursor to indigo, the blue of blue jeans.

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, from Portuguese anil "the indigo shrub" from Arabic an-nil "the indigo" assimilated from al-nil, from Persian nila, from nili "indigo" with Indigofera anil, anil being derived from the Sanskrit नीली 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.

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 an enormous range of dyestuffs, such as fuchsine, safranine and induline. 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.

Toxicology

Aniline is toxic by inhalation of the vapour. The IARC lists it in Group 3 (not classifiable as to its carcinogenicity to humans) due to the limited and contradictory data available. The early manufacture of aniline resulted in increased incidents of bladder cancer, but these effects are now attributed to naphthylamines, not anilines.

External links

• International Chemical Safety Card 0011
• Aniline electropolymerisation
• U.S. National Library of Medicine: ChemIDplus - Aniline

 This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. ar:أنيلين bg:Анилин ca:Anilina cs:Anilin da:Anilin de:Anilin et:Aniliin el:Ανιλίνη es:Anilina fr:Aniline ko:아닐린 io:Anilino it:Anilina he:אנילין lv:Anilīns hu:Anilin mk:Анилин nl:Aniline ja:アニリン no:Anilin pl:Anilina pt:Anilina ro:Anilină ru:Анилин sk:Anilín sl:Anilin fi:Aniliini sv:Anilin tr:Anilin uk:Анілін vi:Anilin zh:苯胺