Question

Which one of the following is the strongest base in aqueous solution ?

• A. Trimethylamine
• B. Aniline
• C. Dimethylamine
• D. Methylamine

Answer 1

The correct option is C Dimethylamine

When we compare the basicity of the aliphatic amines, we would expect the basicity of tertiary amines to be the greatest, followed by secondary amine and then primary amine.

But this is not so. The order of basicity is

$N{H}_3$ < primary amine ~ tertiary amine < secondary amine

This is because:

1. Steric hindrance

The size of an alkyl group is more than that of a hydrogen atom. So, an alkyl group would hinder the attack of a hydrogen atom, thus decreasing the basicity of the molecule. So, the more the number of alkyl groups attached, lesser will be its basicity.

2. Solvation of ions

When amines are dissolved in water, they form protonated amines. Also, the number of possibilities for hydrogen bonding also increases. More the number of hydrogen bonding more is the hydration that is released in the process of the formation of hydrogen bonds.

The combined effect of the pushing effect of the alkyl group (+I effect), steric hindrance and the salvation of amines causes the basicity order to be: (basicity of tertiary is almost the same as that of primary).

$N{H}_3$ < primary amine ~ tertiary amine < secondary amine

So, here Dimethylamine is the strongest base.