Aromatic Amines

AROMATIC AMINES

Aromatic amines are organic compounds that contain an amine group (-NH2) attached to an aromatic ring, such as benzene or naphthalene. They are also known as arylamines. Aromatic amines can be synthesized by the reduction of nitro compounds or by the reaction of primary amines with aromatic compounds.

Aromatic amines have a wide range of applications in various industries, including dyes, pharmaceuticals, pesticides, and rubber chemicals. However, many aromatic amines have been identified as carcinogenic and mutagenic. Exposure to aromatic amines has been linked to an increased risk of bladder cancer, lung cancer, and other types of cancer.

Some common examples of aromatic amines include aniline, toluidine, and naphthylamine. These compounds are often used as intermediates in the synthesis of dyes and other organic compounds. It is important to handle aromatic amines with care and to follow proper safety protocols to minimize the risk of exposure.

 

Hoffmann bromamide degradation reaction for aromatic amines

 

Gabriel Synthesis Reaction

 

 

Basicity of aromatic amine

 Comparison of basic strength of aryl amines and alkylamines: Generally aryl amines are considerably less basic than alkyl amines .Taking an example of aniline and ethylamine it is observed that ethyl amine is more basic than aniline. In aniline –NH2 group is directly attached to benzene ring. Hence unshared pair of electron on nitrogen is less available for protonation because of resonance. Below mentioned are resonating structures of aniline.

In the above resonating structures there is a positive charge on nitrogen atom making the lone pair less available for protonation. Hence aniline is less basic than ethyl amine which has no resonating structures. Less basicity of aniline can also be explained by comparing the relative stability of aniline and anilinium ion obtained by accepting a proton. Greater the number of resonating structures, greater is the stability of that species.
Aniline is resonance hybrid of five resonating structures whereas anilinium ion has only two resonating structures.

Thus aniline has less tendency to accept a proton to form anilinium ion.

 

Effect of substituent on basic character of amines: Electron donating or electron releasing group/groups (EDG) increases basic strength while electron withdrawing (EWG) decreases basic strength.

 

Chemical reactions of aromatic amines

Electrophilic substitution of aromatic amine

Aniline, due to high electron density at ortho and para-positions, is ortho and para directing towards electrophilic substitution reaction.

Bromination of aniline

Aniline reacts with bromine water at room temperature to give a white precipitate of 2, 4, 6-tribromoaniline

In order to stop reaction at monosubstitution activating effect of –NH2 group is reduced by acetylation. This prevents di and tri substituted products. Acetyl group is removed by hydrolysis.

Nitration of aniline

(a) Under strongly acidic medium aniline gets protonated to form anilinium ion, which is deactivating group and is meta directing. Hence minitroaniline is also formed in 47% along with ortho and para products.

Aromatic amines cannot be nitrated directly because HNO3 being a strong oxidising agent oxidises it forming black mass.

(b) Nitration by protecting the –NH2 group by acetylation reaction with acetic anhydride:

Sulphonation of aniline

Aniline reacts with conc. H2SO4 to form aniliniumhydrogensulphate which on heating with sulphuric acid at 453-473K produces p-amino benzene sulphonic acid, commonly known as sulphanilic acid, as the major product.