Question

Are aromatic amines less basic?

Answer 1

Aromatic amines are less basic than aliphatic amines.

There are three factors to consider:

• Hybridization
• Resonance
• Stability of the ions being formed

Let's compare methylamine and aniline.

Hybridization

In methylamine the C atom is `sp^3` hybridized (25 % `s` character). In aniline, the C atom is `sp^2` hybridized (33 % `s` character).

Since `s` electrons spend more of their close to the nucleus, `sp^2` carbons are more electronegative.

They pull electrons away from the N atom.

There is less electron density on the N atom, so aniline is predicted to be a weaker base than methylamine.

Resonance

There is no resonance in methylamine, but the methyl group is electron-donating.

This increases the electron density on the N atom and enhances its basicity.

In aniline, the lone pair electrons on N are resonance-delocalized into the ring.

In addition to the two Kekulé forms, there are three contributors with electron density in the ring.

Most of the electron density is still on N, but it has been decreased somewhat, so aniline is a weaker base.

Stability of the ions

In the equilibrium

`"RNH"_2 + "H"_2"O" ⇌ "RNH"_3^+ + "OH"^-`

The position of equilibrium depends on the stabilities of the product ions compared with the stabilities of reactants.

The anilinium ion has only the two Kekulé resonance contributors, so it is less stable than aniline.

The position of equilibrium moves to the left, so aniline is a weaker base.