Amines are organic compounds and functional groups which contain a basic nitrogen atom with a lone pair of electrons. They are derivatives of ammonia where one or more hydrogen atom is replaced by substituent such as alkyl or aryl group. They play a very significant role in the survival of life as they are involved in making amino acids. Amino acids are the building blocks of proteins in human beings. Amines act as nucleophiles due to the presence of lone pair of electrons in the nitrogen atom. They are involved in reactions such as acylation, electrophilic substitution, alkylation etc.
Electrophilic substitution reaction
In anilines, the maximum electron density is found at ortho- and para-positions of the –NH2 group. The -NH2 group is ortho and para directing and is a powerful activating group. The following significant reactions take place under this category:
- Bromination – Reaction of aniline with bromine water at room temperature produces a white precipitate of 2,4,6 – tribromoaniline.
During electrophilic reaction, the main problem encountered is the high reactivity of aromatic amines. Substitution occurs at ortho and para positions. If only monosubstituted aniline derivative is required, then the activating effect of –NH2 group can be controlled by protecting the –NH2 group by acetylation with acetic anhydride. The desired substitution reaction is then carried out followed by the hydrolysis of substituted amides to substituted amines.
The lone pair of electrons present on nitrogen of acetanilide interacts with the oxygen atom due to resonance. Hence the lone pair of electrons on nitrogen is less available for donation to the benzene ring. Therefore activating effect of –NHCOCH3 group is less as compared to that of amino group.
- Nitration – If aniline is subjected to direct nitration it yields tarry oxidation products in addition to nitro derivatives. In strongly acidic medium, aniline is protonated to form anilinium ion which is meta directing. This is why apart from ortho and para derivative, a significant amount of meta derivative is also formed.
By protecting the –NH2 group by acetylation reaction with acetic anhydride, nitration reaction is controlled. The p-nitro derivative is obtained as the major product in this case.
- Sulphonation – The reaction of aniline with concentrated sulphuric acid forms anilinium hydrogensulphate, which on heating with sulphuric acid at 453 – 473 K produces p-aminobenzene sulphonic acid which is commonly known as sulphanilic acid.
Aniline does not undergo Friedel- Crafts reaction due to the salt formation with aluminium chloride which acts as a catalyst. Due to this reaction nitrogen acquires a positive charge and hence acts as a strong deactivating group for further reactions.
This article covers the basic details of electrophilic substitution reactions in amines. For any further detail install Byju’s -the learning app.