Aniline

Aniline is a toxic organic compound with the formula C₆H₅NH₂. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Its main use is in the manufacture of precursors to polyurethane and other industrial chemicals. Like most volatile amines, it possesses the odour of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds.^[6]

Production

Industrial aniline production involves two steps. First, benzene is nitrated with a concentrated mixture of nitric acid and sulfuric acid at 50 to 60 °C to yield nitrobenzene. The nitrobenzene is then hydrogenated (typically at 200–300 °C) in the presence of metal catalysts.

^[7]

The reduction of nitrobenzene to aniline was first performed by Nikolay Zinin in 1842 using inorganic sulfide as a reductant (Zinin reaction).

Aniline can alternatively be prepared from ammonia and phenol derived from the cumene process.^[6]

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 analogues of aniline are known where the phenyl group is further substituted. These include toluidines, xylidines, chloroanilines, aminobenzoic acids, nitroanilines, and many others. They often are prepared by nitration of the substituted aromatic compounds followed by reduction. For example, this approach is used to convert toluene into toluidines and chlorobenzene into 4-chloroaniline.^[6]

Reactions

The chemistry of aniline is 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.

Polyanilines can form upon oxidation of aniline.

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.

If bromine water is added to aniline, the bromine water is decolourised and a white precipitate of 2,4,6-tribromophenylamine is formed. The largest scale industrial reaction of aniline involves its alkylation with formaldehyde. An idealized equation is shown:

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⁺).^[8] 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 both an inductive effect from the more electronegative sp² carbon and to a resonance effect, as the lone pair on the nitrogen is partially delocalized into the pi system of the benzene ring.

Acylation

Aniline reacts with carboxylic acids^[9] 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,^[10] is obtained by the reaction of acetic acid and aniline.

N-Alkylation

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

C₆H₅NH₂ + 2 CH₃OH → C₆H₅N(CH₃)₂ + 2H₂O

N-Methylaniline and dimethylaniline are colorless liquids with boiling points of 193–195 °C and 192 °C, respectively. These derivatives are of importance in the color 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. This diazonium salt can also be reacted with NaNO₂ and phenol which produces a dye which is benzeneazophenol, this process is called coupling.

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 dianiline and related compounds by condensation with formaldehyde (as discussed above). The diamines are condensed with phosgene to give methylene diphenyl diisocyanate, a precursor to urethane polymers.^[6] Other uses include rubber processing chemicals (9%), herbicides (2%), and dyes and pigments (2%).^[11] As additives to rubber, aniline derivatives such as phenylenediamines 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.^[6]

Cake of indigo dye, which is prepared from aniline.

Aniline is also used at a smaller scale in the production of the intrinsically conducting polymer polyaniline.

History

Aniline was first isolated in 1826 by Otto Unverdorben by destructive distillation of indigo.^[12] He called it Crystallin. In 1834, Friedlieb Runge isolated a substance from coal tar that turned a beautiful blue color when treated with chloride of lime. He named it kyanol or cyanol.^[13] In 1840, Carl Julius Fritzsche (1808–1871) treated indigo with caustic potash and obtained an oil that he named aniline, after an indigo-yielding plant, Añil (Indigofera suffruticosa).^[14]^[15] In 1842, Nikolay Nikolaevich Zinin reduced nitrobenzene and obtained a base that he named benzidam.^[16] In 1843, August Wilhelm von Hofmann showed that these were all the same substance, known thereafter as phenylamine or aniline.^[17]

Synthetic dye industry

In 1856, whilst trying to synthesise quinine, von Hofmann's student William Henry Perkin discovered mauveine and went into industry producing the first synthetic dye. Other aniline dyes followed, such as fuchsine, safranine, and induline. At the time of mauveine's discovery, aniline was expensive. Soon thereafter, applying a method reported in 1854 by Antoine Béchamp,^[18] it was prepared "by the ton".^[19] The Béchamp reduction enabled the evolution of a massive dye industry in Germany. Today, the name of BASF, originally Badische Anilin- und Soda-Fabrik (English: Baden Aniline and Soda Factory), now among the largest chemical suppliers, echoes the legacy of the synthetic dye industry, built via aniline dyes and extended via the related azo dyes. The first azo dye was aniline yellow.^[20]

Developments in medicine

In the late 19th century, aniline emerged as an analgesic drug, its cardiac-suppressive side effects countered with caffeine.^[21] During the first decade of the 20th century, while trying to modify synthetic dyes to treat African sleeping sickness, Paul Ehrlich – who had coined the term chemotherapy for his magic bullet approach to medicine – failed and switched to modifying Béchamp's atoxyl, the first organic arsenical drug, and serendipitously obtained a treatment for syphilis – salvarsan – the first successful chemotherapy agent. Salvarsan's targeted microorganism, not yet recognized as a bacterium, was still thought to be a parasite, and medical bacteriologists, believing that bacteria were not susceptible to the chemotherapeutic approach, overlooked Alexander Fleming's report in 1928 on the effects of penicillin.^[22]

In 1932, Bayer sought medical applications of its dyes. Gerhard Domagk identified as an antibacterial a red azo dye, introduced in 1935 as the first antibacterial drug, prontosil, soon found at Pasteur Institute to be a prodrug degraded in vivo into sulfanilamide – a colorless intermediate for many, highly colorfast azo dyes – already with an expired patent, synthesized in 1908 in Vienna by the researcher Paul Gelmo for his doctoral research.^[22] By the 1940s, over 500 related sulfa drugs were produced.^[22] Medications in high demand during World War II (1939–45), these first miracle drugs, chemotherapy of wide effectiveness, propelled the American pharmaceutics industry.^[23] In 1939, at Oxford University, seeking an alternative to sulfa drugs, Howard Florey developed Fleming's penicillin into the first systemic antibiotic drug, penicillin G. (Gramicidin, developed by René Dubos at Rockefeller Institute in 1939, was the first antibiotic, yet its toxicity restricted it to topical use.) After World War II, Cornelius P. Rhoads introduced the chemotherapeutic approach to cancer treatment.^[24]

Rocket fuel

In the 1940s and early 1950s, aniline was used with nitric acid or dinitrogen tetroxide as rocket fuel for small missiles and Jet Assisted Take-Off (JATO). The two fuel components are hypergolic, producing a violent reaction on contact. Aniline was later replaced by hydrazine.

It is also used as fuel in the Aerobee rocket's second stage engine.

Toxicology and testing

Aniline is toxic by inhalation of the vapour, ingestion, or percutaneous absorption.^[25]^[26] 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.^[6]

Many methods exist for the detection of aniline.^[27]

References

↑ Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. pp. 416, 668. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4. Aniline, for C₆H₅-NH₂, is the only name for a primary amine retained as a preferred IUPAC name for which full substitution is permitted on the ring and the nitrogen atom. It is a Type 2a retained name; for the rules of substitution see P-15.1.8.2. Substitution is limited to substituent groups cited as prefixes in accordance with the seniority of functional groups explicitly expressed or implied in the functional parent compound name. The name benzenamine may be used in general nomenclature.
1 2 3 4 5 "NIOSH Pocket Guide to Chemical Hazards #0033". National Institute for Occupational Safety and Health (NIOSH).
↑ "Petrucci, Ralph H. General Chemistry: Principles and Modern Applications. Toronto, Ont.: Pearson Canada, 2011. 710. Print"
↑ GOV, NOAA Office of Response and Restoration, US. "ANILINE | CAMEO Chemicals | NOAA". cameochemicals.noaa.gov. Retrieved 2016-06-16.
1 2 3 "Aniline". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH).
1 2 3 4 5 6 Thomas Kahl, Kai-Wilfrid Schröder, F. R. Lawrence, W. J. Marshall, Hartmut Höke, Rudolf Jäckh "Aniline" in Ullmann's Encyclopedia of Industrial Chemistry 2007; John Wiley & Sons: New York.doi:10.1002/14356007.a02_303
↑ Caskey, Douglas C.; Chapman, Douglas W. (Apr 24, 1985), Process for the preparation of arylhydroxylamines, retrieved 2016-06-16
↑ McMurry, John E. (1992), Organic Chemistry (3rd ed.), Belmont: Wadsworth, ISBN 0-534-16218-5
↑ Carl N. Webb (1941). "Benzanilide". Org. Synth. ; Coll. Vol., 1, p. 82
↑ No longer in common medicinal use.
↑ "Aniline". The Chemical Market Reporter. Retrieved 2007-12-21.
↑ Otto Unverdorben (1826). "Ueber das Verhalten der organischen Körper in höheren Temperaturen" [On the behaviour of organic substances at high temperatures]. Annalen der Physik und Chemie. 8 (11): 397–410. Bibcode:1826AnP....84..397U. doi:10.1002/andp.18260841109.
↑ F. F. Runge (1834) "Ueber einige Produkte der Steinkohlendestillation" (On some products of coal distillation), Annalen der Physik und Chemie, 31 : 65–77 (see page 65), 513–524; and 32 : 308–332 (see page 331).
↑
J. Fritzsche (1840) "Ueber das Anilin, ein neues Zersetzungsproduct des Indigo" (On aniline, a new decomposition product of indigo), Bulletin Scientifique [publié par l'Académie Impériale des Sciences de Saint-Petersbourg], 7 (12) : 161–165. Reprinted in:
- J. Fritzsche (1840) "Ueber das Anilin, ein neues Zersetzungsproduct des Indigo," Justus Liebigs Annalen der Chemie, 36 (1) : 84–90.
- J. Fritzsche (1840) "Ueber das Anilin, ein neues Zersetzungsproduct des Indigo", Journal für praktische Chemie, 20 : 453–457. In a postscript to this article, Erdmann (one of the journal's editors) argues that aniline and the "cristallin", which was found by Unverdorben in 1826, are the same substance ; see pages 457–459.
↑ synonym I anil, ultimately from Sanskrit "nīla", dark-blue.
↑ N. Zinin (1842). "Beschreibung einiger neuer organischer Basen, dargestellt durch die Einwirkung des Schwefelwasserstoffes auf Verbindungen der Kohlenwasserstoffe mit Untersalpetersäure" (Description of some new organic bases, produced by the action of hydrogen sulfide on compounds of hydrocarbons and hyponitric acid [H₂N₂O₃]), Bulletin Scientifique [publié par l'Académie Impériale des Sciences de Saint-Petersbourg], 10 (18) : 272–285. Reprinted in: N. Zinin (1842) "Beschreibung einiger neuer organischer Basen, dargestellt durch die Einwirkung des Schwefelwasserstoffes auf Verbindungen der Kohlenwasserstoffe mit Untersalpetersäure," Journal für praktische Chemie, 27 (1): 140–153. Benzidam is named on page 150. Fritzsche, Zinin's colleague, soon recognized that "benzidam" was actually aniline. See: Fritzsche (1842) Bulletin Scientifique, 10 : 352. Reprinted as a postscript to Zinin's article in: J. Fritzsche (1842) "Bemerkung zu vorstehender Abhandlung des Hrn. Zinin" (Comment on the preceding article by Mr. Zinin), Journal für praktische Chemie, 27 (1) : 153.
See also: (Anon.) (1842) "Organische Salzbasen, aus Nitronaphtalose und Nitrobenzid mittelst Schwefelwasserstoff entstehend" (Organic bases originating from nitronaphthalene and nitrobenzene via hydrogen sulfide), Annalen der Chemie und Pharmacie, 44 : 283–287.
↑ August Wilhelm Hofmann (1843) "Chemische Untersuchung der organischen Basen im Steinkohlen-Theeröl" (Chemical investigation of organic bases in coal tar oil), Annalen der Chemie und Pharmacie, 47 : 37–87. On page 48, Hofmann argues that krystallin, kyanol, benzidam, and aniline are identical.
↑ A. Béchamp (1854) "De l'action des protosels de fer sur la nitronaphtaline et la nitrobenzine. Nouvelle méthode de formation des bases organiques artificielles de Zinin" (On the action of iron protosalts on nitronaphthaline and nitrobenzene. New method of forming Zinin's synthetic organic bases.), Annales de Chemie et de Physique, 3rd series, 42 : 186 – 196. (Note: In the case of a metal having two or more distinct oxides (e.g., iron), a "protosalt" is an obsolete term for a salt that is obtained from the oxide containing the lowest proportion of oxygen to metal; e.g., in the case of iron, which has two oxides – iron (II) oxide (FeO) and iron (III) oxide (Fe₂O₃) – FeO is the "protoxide" from which protosalts can be made. See: Wiktionary: protosalt.)
↑ Perkin, William Henry. 1861-06-08. "Proceedings of Chemical Societies: Chemical Society, Thursday, May 16, 1861." The Chemical News and Journal of Industrial Science. Retrieved on 2007-09-24.
↑ Auerbach G, "Azo and naphthol dyes", Textile Colorist, 1880 May;2(17):137-9, p 138.
↑ Wilcox RW, "The treatment of influenza in adults", Medical News, 1900 Dec 15;77():931-2, p 932.
1 2 3 D J Th Wagener, The History of Oncology (Houten: Springer, 2009), pp 150–1.
↑ John E Lesch, The First Miracle Drugs: How the Sulfa Drugs Transformed Medicine (New York: Oxford University Press, 2007), pp 202–3.
↑ "Medicine: Spoils of war", Time, 15 May 1950.
↑ Muir, GD (ed.) 1971, Hazards in the Chemical Laboratory, The Royal Institute of Chemistry, London.
↑ The Merck Index. 10th ed. (1983), p.96, Rahway: Merck & Co.
↑ Basic Analytical Toxicology (1995), R. J. Flanagan, S. S. Brown, F. A. de Wolff, R. A. Braithwaite, B. Widdop: World Health Organization

"Aniline", Encyclopædia Britannica, 9th ed., Vol. II, New York: Charles Scribner's Sons, 1878, pp. 47–48 .
"Aniline", Encyclopædia Britannica, 11th ed., Vol. II, Cambridge: Cambridge University Press, 1911, pp. 47–48 .

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