Explain why the order of basicity for methyl amine, N, N - dimethyl amine and N, N, N - trimethyl amine changes in gasous and a aqueous medium.

Explanation

Given compounds
`CH_(3)NH_(2), (CH_(3))_(2)NH, (CH_(3))_(3)N and NH_(3)`.
In the above compounds methly substituted ammonium ion gets stabilised due to dispersal of the positive charge by the +I effect of the methyl group. Hence methyl amine are stronger bases than ammonia. Basic nature of amines increase with increase of methyl groups. This trend is followed in gaseous phase.
`(CH_(3))_(3)Ngt (CH_(3))_(2)NHgtCH_(3)NH_(2)gtNH_(3)`
In the aqueous phase the substituted ammonium cations get stabilised not only by electron releasing effect of the alkyl group but also by solvation with water molecules. Due to steric hinderance the basic strength of methyl amines in the aqueous state changed as follows
`(CH_(3))_(2)NHgt CH_(3)NH_(2)gt(CH_(3))_(3)NgtNH_(3)`