Q.Write the order of basicity of aliphatic alkylamine in gaseous phase.

Step 1. Recall the two competing effects from section 13.5.1. In AQUEOUS solution, basic strength is governed by a combination of the +I inductive effect (favouring more alkyl groups) AND solvation of the conjugate acid by water (favouring FEWER alkyl groups, i.e. more N-H hydrogens available to hydrogen-bond the solvent) -- these two effects pull in opposite directions, and it is specifically the POOR solvation of the tertiary amine's conjugate acid that makes it weaker than the secondary amine in water.

Step 2. Remove the solvation effect for the gas phase. In the GAS phase, there is no solvent water present at all, so the solvation effect described in section 13.5.1(b) simply does not operate.

Step 3. Apply only the remaining effect. With solvation removed, ONLY the +I inductive-effect argument of section 13.5.1(a) remains -- and that effect alone predicts a smooth, MONOTONIC increase in basic strength as more electron-donating alkyl groups are added to stabilise the conjugate acid. …

Recognising that gas-phase basicity removes the solvation effect of section 13.5.1(b), leaving only the +I inductive-effect argume …

• Simply repeating the AQUEOUS-solution order (NH3 < RNH2 < R2NH > R3N) without recognising that the question is specifically asking about the GAS phase, where the solvation effect responsible for the tertiary amine's dip does not apply at all. …