Friday, January 25, 2008

IIT JEE Revision Basicity of Amines

Chapter Amines

Basicity of substituted anilines and aliphatic amines

Nitrogen has a lone pair in Ammonia are sharing an electron with each of the hydrogen atoms.

Due to the presence of lone pair on nitrogen atom of the -NH2 group, the aliphatic amines are generally basic in nature.

They have a tendency to donate this lone pair of electrons to different electron deficient compounds and, therefore, behave as bases.

The amines react with water to from hydroxides which inonise to furnish hydroxyl (OH-) ions.

The basic strength is expressed in terms of dissociation constant Kb for the dissociation reaction which is a reversible reaction.

Greater the Kb value, which means greater the dissociation at equilibrium, the stronger the base.

Aliphatic amines are stronger bases than ammonia.

It is observed that secondary amines are the strongest bases, next come the primary amines and the weakest among the three are tertiary amines.

The reasons for the strength order of basicity is explained in terms of steric hindrance and solvation of ions.

Aromatic amines are also bases.

Among aniline (NH2 substituted in benzene) and ethylamine, aniline is a weaker base. This is due to the fact tath among the resonating structures of aniline three of them have positive charge on nitrogen. In these cases the lone pair is not available for protonation (for donating the lone pair to the H+ ion). Hence aniline is less basic than ethylamine.

In substituted amines, electron releasing groups like -OCH3, -CH3 increase the basic strength. Electron withdrawing groups like -X (halogen), -NO2, -CN decrease basic strength.

The effects of substituents is more marked at p-position than a m-position.

Every o-substituted aniline (electron releasing or electron withdrawing) is less basic than aniline due to ortho effect.

Base weakening effect of electron withdrawing group is very large at o-position.

When hydrogen atoms of the amino group of arylamines are replaced by electron donating alkyl groups, the basic character of the resulting arylamine increases.

For example, N-methylaniline is a stronger base than aniline and N,N-dimethylaniline is even stronger.

In contrast, if hydrogen atoms of the amino group of arylamines are replaced by electron withdrawing groups (such as phenyl group-aryl group), the basic character of the resulting arylamine increases.

Aniline is stronger than diphenylamines and this is a stronger base than triphenylamine.

In case of aralkylamines where amine group is attached to an alkyl group attached to the carbon of the benzene ring, the lone pair of nitrogen atom is not delocalized and hence readily available for protonation (accepting the H+ ion). Therefore aralkylamines are more basic than arylamines.

Hence benzylamines is a stronger base than aniline.

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