Basicity of the methylamines

Methylamine is more basic than ammonia: its conjugate acid, the methylammonium cation, is more stable than ammonium. If we kept adding methyl groups, we would expect basicity to increase steadily. It does not: dimethylamine is only slightly more basic than methylamine, and trimethylamine is less basic than both.

First effect: methyl groups stabilize the positive charge

We have already seen it with radicals and carbocations: the more substituted an electron-deficient centre, the less unstable it is. The σ C–H bonds of the methyl groups donate some electron density to the nitrogen, spread the positive charge and stabilize the ammonium cation. This effect grows with the number of methyl groups.

Second effect: solvation by water

pK values are measured in water, and water stabilizes ammonium cations by hydrogen bonding to their N–H bonds. Each added methyl group removes one N–H and makes the cation more hydrophobic, so it is solvated less and less well.

The two effects work in opposite directions, and the observed basicity is the result of their balance.