Aniline

Aniline is an 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 has the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds. Aniline is an 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 has the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds. Aniline is a slightly pyramidalized molecule, with hybridization of the nitrogen somewhere between sp3 and sp2. The amine is flatter than an aliphatic amine, owing to conjugation of the lone pair with the aryl substituent. Thus, the experimental geometry reflects a balance between the stabilization of lone pairs in orbitals with higher s character and better stabilization via conjugation with the aryl ring for an orbital of pure p character. 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 methylamine, this value is ~125°, while a planar nitrogen like that of formamide has an angle of 180°.) The C−N distance is correspondingly shorter. In aniline, the C−N and C−C distances are close to 1.39 Å, indicating the π-bonding between N and C. 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: The reduction of nitrobenzene to aniline was first performed by Nikolay Zinin in 1842, using inorganic sulfide as a reductant (Zinin reaction). The reduction of nitrobenzene to aniline was also performed as part of reductions by Antoine Béchamp in 1854, using iron as the reductant (Bechamp reduction). 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 (échappés) of the fuchsine fusion. 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. Alternatively, using Buchwald-Hartwig coupling or Ullmann reaction approaches, aryl halides can be aminated with aqueous or gaseous ammonia