Amines
1.0 Introduction
2.0 Preparation of Amines
2.1 Nucleophilic substitution
2.2 Reduction of Nitro Compounds
2.3 Reduction of Nitriles
2.4 Reduction of Amides
3.0 Physical Properties
4.0 Basic Nature of Amines
5.0 Reactions of Amines
5.1 Reaction of ${1^ \circ }$ and ${2^ \circ }$ amines with aldehydes and ketones
5.2 Acylation Reaction
5.3 Carbylamine Reaction
5.4 Reaction with Nitrous Acid
5.5 Substitution Reactions of Aryl Diazonium Salts
5.6 Electrophilic substitution reaction of Aniline
5.6 Electrophilic substitution reaction of Aniline
2.2 Reduction of Nitro Compounds
2.3 Reduction of Nitriles
2.4 Reduction of Amides
5.2 Acylation Reaction
5.3 Carbylamine Reaction
5.4 Reaction with Nitrous Acid
5.5 Substitution Reactions of Aryl Diazonium Salts
5.6 Electrophilic substitution reaction of Aniline
Aniline is a resonance hybrid of five structures. in which Ortho- and para-positions to the $–NH_2$ group become centres of high electron density. Thus $–NH_2$ group is ortho and para directing and a powerful activating group.
Because of very high reactivity aromatic amines substitution tends to occur at ortho- and para-positions. In order to prepare monosubstituted aniline derivative, we have to reduce the effect of $–NH_2$ group which can be done by protecting the $-NH_2$ group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine.
Following reactions helps us in understanding what kind of products are achieved in the reactions and how to proceed when you are given some reagents/products and you have to find the unknowns.
1. Bromination
Aniline reacts with bromine water at room temperature to give a white precipitate of $2,4,6-$tribromoaniline.
As it is seen that, in the above reaction the major product is $2,4,6-$Tribromoaniline. In order to achieve monosubstituted aniline derivative, we have to perform the reaction in the way shown below.
2. Nitration
Due to the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing which results in the meta derivatives along with ortho and para derivatives as shown.
By acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the $para$-nitro derivative can be obtained as the major product as shown below.
3. Sulphonation
Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid produces para-aminobenzene sulphonic acid, also known as sulphanilic acid, as the major product.
Note:
Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.
