Basicity of Amines

An amine is basic because nitrogen's lone pair of electrons is available to accept a

proton, forming an alkylammonium (or anilinium) ion. The STRENGTH of that basicity, in

turn, depends entirely on how available -- how energetically favourable to donate -- that

lone pair is, and several structural effects compete to raise or lower it.

Aliphatic amines vs. ammonia (gas phase / isolated molecule). An alkyl group is

electron-donating (positive inductive effect, $+I$), pushing electron density onto

nitrogen and making its lone pair MORE available than in ammonia itself. So, considering

inductive effects alone, basicity should increase steadily as $\text{NH}_3 < \text{CH}_3\text{NH}_2 < (\text{CH}_3)_2\text{NH} < (\text{CH}_3)_3\text{N}$ -- and this

order is indeed observed in the gas phase, free from solvent effects, confirming that the

pure electronic/inductive argument is correct in principle.

In aqueous solution, however, the observed order for simple methylamines is

$(\text{CH}_3)_2\text{NH} > \text{CH}_3\text{NH}_2 > (\text{CH}_3)_3\text{N} > \text{NH}_3$ -- the tertiary amine drops BELOW the secondary and even the primary amine.

The reason is that basicity in water is governed not just by how stable the free amine's

lone pair is, but by how well the resulting PROTONATED cation is stabilised by hydrogen

bonding to surrounding water molecules. A tertiary amine's cation, $\text{R}_3\text{NH}^+$,

has only ONE $\text{N}-\text{H}$ left to hydrogen-bond to water, and its three bulky alkyl

groups sterically hinder solvent molecules from approaching closely -- so despite having

the most electron-rich nitrogen of the series, its cation is the least well solvated,

pulling its aqueous basicity down.

Aromatic amines are much weaker bases than either ammonia or aliphatic amines. In

aniline, nitrogen's lone pair is delocalised by resonance into the aromatic ring (donating

electron density to the ortho/para positions, exactly the effect that makes the ring

itself more reactive toward electrophiles). This delocalisation makes the lone pair …