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Chemical Reactions: Basic Character

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Chemical Reactions: Basic Character - Lesson Summary

Amines are basic in nature. Their basic nature is due to the presence of a lone pair of electrons on the nitrogen atom. This lone pair of electrons is donated to the proton of an acid to form a new bond.

Aliphatic amines are stronger bases than ammonia and aromatic amines are weaker bases than ammonia.

Because of inductive effect of an alkyl group of an alkyleamine, the alkyl group tends to donate electrons towards the nitrogen atom and thus makes the lonepair of electrons more available to the proton of the acid. The alkyl ammonium ion formed from the amine is stabilised due to the dispersal of positive charge by the positive inductive effect of alkyl groups. Hence, alkyl amines are stronger bases than ammonia. Thus, the basic nature of aliphatic amines increases with an increase in the number of alkyl groups.

The order of basicity of amines in its aqueous phase is exactly opposite to the order based on inductive effect.

Basicity in gaseous phase:   3° amine  >  2° amine > 1° amine > NH3
Basicity in aqueous phase:   NH3 > 1° amine  >  2° amine > 3° amine

If the size of the alkyl group is larger than the methyl group, then there is steric hindrance to hydrogenbonding in its aqueous phase. As the size of the alkyl group increases, the steric hindrance to hydrogen bonding increases and the basicity decreases.

The protonated aniline (or) anilinium ion obtained by accepting a proton can have fewer resonance structures than aniline. Greater the number of resonance structures, greater is the stability. Thus, aniline is more stable than the anilinium ion. Hence, proton acceptability (or) basic nature of aniline (or) other aromatic amines would be less than ammonia.

In case of substituted aryl amines, electron-donating groups such as methyl, ethyl and methoxy increase the basic strength.

And electron-withdrawing groups like -NO2, -X,-SO3H,-COOH decrease the basic strength.

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