Aniline
Aniline is an organic compound with the formula. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starting material for fine chemical synthesis. Its main use is in the manufacture of precursors to polyurethane, dyes, and other industrial chemicals. Like most volatile amines, it has the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds. It is toxic to humans.
Relative to benzene, aniline is "electron-rich". It thus participates more rapidly in electrophilic aromatic substitution reactions. Likewise, it is also prone to oxidation: while freshly purified aniline is an almost colorless oil, exposure to air results in gradual darkening to yellow or red, due to the formation of strongly colored, oxidized impurities. Aniline can be diazotized to give a diazonium salt, which can then undergo various nucleophilic substitution reactions.
Like other amines, aniline is both a base and a nucleophile, although less so than structurally similar aliphatic amines.
Because an early source of the benzene from which they are derived was coal tar, aniline dyes are also called coal tar dyes.
Structure
Aryl-N distances
In aniline, the C−N bond length is 1.41 Å, compared to the C−N bond length of 1.47 Å for cyclohexylamine, indicating partial π-bonding between C and N. The length of the chemical bond of in anilines is highly sensitive to substituent effects. The C−N bond length is 1.34 Å in 2,4,6-trinitroaniline vs 1.44 Å in 3-methylaniline.Pyramidalization
The amine group in anilines is a slightly pyramidalized molecule, with hybridization of the nitrogen somewhere between sp3 and sp2. The nitrogen is described as having high p character. The amino group in aniline is flatter stabilization of the N lone pair in an orbital with significant s character favors pyramidalization delocalization of the N lone pair into the aryl ring favors planarity.Consistent with these factors, substituted anilines with electron donating groups are more pyramidalized, while those with electron withdrawing groups are more planar. In the parent aniline, the lone pair is approximately 12% s character, corresponding to sp7.3 hybridization.
The pyramidalization angle between the C–N bond and the bisector of the H–N–H angle is 142.5°. For comparison, in more strongly pyramidal amine group in methylamine, this value is ~125°, while that of the amine group in formamide has an angle of 180°.
Production
Industrial aniline production involves hydrogenation of nitrobenzene in the presence of metal catalysts: Approximately 4 billion kilograms are produced annually. Catalysts include nickel, copper, palladium, and platinum, and newer catalysts continue to be discovered.The reduction of nitrobenzene to aniline was first performed by Nikolay Zinin in 1842, using sulfide salts. The reduction of nitrobenzene to aniline was also performed as part of reductions by Antoine Béchamp in 1854, using iron as the reductant. These stoichiometric routes remain useful for specialty anilines.
Aniline can alternatively be prepared from ammonia and phenol 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 of the fuchsine fusion.
Related aniline derivatives
Many analogues and derivatives of aniline are known where the phenyl group is further substituted. These include toluidines, xylidines, chloroanilines, aminobenzoic acids, nitroanilines, and many others. They also are usually 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. Alternatively, using Buchwald-Hartwig coupling or Ullmann reaction approaches, aryl halides can be aminated with aqueous or gaseous ammonia.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, 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. These polymers exhibit rich redox and acid-base properties.Electrophilic reactions at ortho- and para- positions
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,.If bromine water is added to aniline, the bromine water is decolourised and a white precipitate of 2,4,6-tribromoaniline is formed. To generate the mono-substituted product, a protection with acetyl chloride is required:
The reaction to form 4-bromoaniline is to protect the amine with acetyl chloride, then hydrolyse back to reform aniline.
The largest scale industrial reaction of aniline involves its alkylation with formaldehyde. An idealized equation is shown:
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. Aniline reacts with strong acids to form the anilinium ion.Traditionally, the weak basicity of aniline is attributed to a combination of inductive effect from the more electronegative sp2 carbon and resonance effects, as the lone pair on the nitrogen is partially delocalized into the pi system of the benzene ring. :
Missing in such an analysis is consideration of solvation. Aniline is, for example, more basic than ammonia in the gas phase, but ten thousand times less so in aqueous solution.
Acylation
Aniline reacts with acyl chlorides such as acetyl chloride to give amides. The amides formed from aniline are sometimes called anilides, for example is acetanilide. At high temperatures aniline and carboxylic acids react to give the anilides.''N''-Alkylation
N-Methylation of aniline with methanol at elevated temperatures over acid catalysts gives N-methylaniline and N,''N-dimethylaniline:N''-Methylaniline and N,''N''-dimethylaniline are colorless liquids with boiling points of 193–195 °C and 192 °C, respectively. These derivatives are of importance in the color industry.
Carbon disulfide derivatives
Boiled with carbon disulfide, it gives sulfocarbanilide , which may be decomposed into phenyl isothiocyanate, and triphenyl guanidine.Diazotization
Aniline and its ring-substituted derivatives react with nitrous acid to form diazonium salts. One example is benzenediazonium tetrafluoroborate. Through these intermediates, the amine group can be converted to a hydroxyl, cyanide, or halide group via Sandmeyer reactions. This diazonium salt can also be reacted with Sodium nitrite| and phenol to produce a dye known as benzeneazophenol, in a process called coupling.The reaction of converting primary aromatic amine into diazonium salt is called diazotisation.
In this reaction primary aromatic amine is allowed to react with sodium nitrite and 2 moles of HCl, which is known as "ice cold mixture" because the temperature for the reaction was as low as 0.5 °C. The benzene diazonium salt is formed as major product alongside the byproducts water and sodium chloride.
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.
In addition, aniline is the starting component in the production of diglycidyl aniline. Epichlorohydrin is the other main ingredient.
Uses
Aniline is predominantly used for the preparation of methylenedianiline and related compounds by condensation with formaldehyde. The diamines are condensed with phosgene to give methylene diphenyl diisocyanate, a precursor to urethane polymers.Other uses include rubber processing chemicals, herbicides, and dyes and pigments. As additives to rubber, aniline derivatives such as phenylenediamines and diphenylamine, are antioxidants. Illustrative of the drugs prepared from aniline is paracetamol. The principal use of aniline in the dye industry is as a precursor to indigo, the blue of blue jeans.
Aniline oil is also used for mushroom identification. Kerrigan's 2016 Agaricus of North America P45: "In fact I recommend switching to the following modified test. Frank developed an alternative formulation in which aniline oil is combined with glacial acetic acid in a 50:50 solution. GAA is a much safer, less reactive acid. This single combined reagent is relatively stable over time. A single spot or line applied to the pileus. In my experience the newer formulation works as well as Schaffer's while being safer and more convenient."