Explain why the order of basicity for methyl amine, N-N-dimethyl amine, N, N, Ntrimethylamine changes in gaseous and aqueous medium.

1) The basic nature of aliphatic amines increases with increase in No. of alkyl groups.
${\mathrm{NH}}_3<{\mathrm{CH}}_3{\mathrm{NH}}_2<{\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}<{\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$
This trend is followed only in gaseous phase.
2) But in aqueous solution this order is not followed.
3) As no. of hydrogen atoms on nitrogen increases, ability to form of hydrogen bond with water increases. ${\mathrm{NH}}_3>1^{\circ }>2^{\circ }>3^{\circ }$
4) As the bulkyness of alkyl group on nitrogen atom increases, basic strength decreases.
${\mathrm{NH}}_3>1^{\circ }>2^{\circ }>3^{\circ }$
Therefore, by the combination of above 3 effects basic strength of amines in aqueous solution is $2^{\circ }>1^{\circ }>3^{\circ }>{\mathrm{NH}}_3$