Acidity of the methylammonium ions
The explanation is the same as for the basicity of the methylamines, but seen from the conjugate acid: the more stable the cation, the less it tends to give up its proton and the less acidic it is.
| Ion | Methyl groups stabilizing the charge | N–H bonds available to hydrogen-bond with water |
|---|---|---|
| NH4+ | 0 | 4 |
| CH3NH3+ | 1 | 3 |
| (CH3)2NH2+ | 2 | 2 |
| (CH3)3NH+ | 3 | 1 |
- NH4+ is the most acidic: no methyl group stabilizes the charge.
- CH3NH3+: one methyl group stabilizes the cation, the equilibrium shifts to the left and acidity decreases.
- (CH3)2NH2+: the second methyl stabilizes it further, but also makes it more hydrophobic. The two factors cancel out and acidity hardly changes.
- (CH3)3NH+: with a single N–H the cation is poorly solvated; destabilization prevails and acidity increases again.