Q.Why are primary aliphatic amines stronger bases than ammonia ?

Step 1. Recall the +I inductive-effect argument (section 13.5.1a). An alkyl group has an electron-donating (+I) inductive effect, which stabilises a positive charge on the atom it is bonded to. Moving from ammonia's conjugate acid, NH4(+) (no alkyl group at all), to a primary amine's conjugate acid, RNH3(+) (one alkyl group), this +I effect provides genuine additional stabilisation of the positive charge now sitting on nitrogen.

Step 2. Recall the solvation argument (section 13.5.1b), and check it does not yet work AGAINST the primary amine. Solvation stabilisation depends on how many N-H hydrogens the conjugate acid retains to hydrogen-bond the surrounding water. NH4(+) has four such hydrogens (best solvated of the whole series); RNH3(+) still has THREE (only one fewer), so it remains very well solvated too -- the loss here, going from ammonia to a primary amine, is comparatively small.

Step 3. Combine both effects. Because the +I stabilisation gained by adding one alkyl group clearly OUTWEIGHS the small loss in solvation (four N-H down to three), the primary amine's conjugate acid, RNH3(+), ends up MORE stabilised overall than ammonia's own NH4(+) -- and greater conjugate-acid stabilisation means the protonation equilibrium sits further right, i.e. a STRONGER base. …

Applying section 13.5.1's combined +I-effect-plus-solvation framework specifically to the ammonia-to-primary-amine step, wher …

• Giving only the +I-effect part of the explanation and omitting that the solvation loss at this particular step is small enough not to reverse the trend (unlike the later secondary-to-tertiary step, where it does). …