Basicity of a compound is determined by its tendency to donate more electrons. If a compound is more electron rich, its tendency to donate electrons would be more so its basic strength would be more.
If groups showing electron pushing tendency (alkyl groups), are attached to the N atom, electron concentration would be more. Similarly more is the number of such groups, more is the basic strength.
So, 3° amines > 2° amines > 1° amine.
But in aqueous medium, basic strength is also determined by the stability of the protonated amine by hydration effect. Hydration effect refers to the stabilisation of the protonated amine by water molecules. Hydration due to H-bonding is maximum for 1° amine (as there is most number of H atoms on Nitrogen) followed by 2° then 3°.
For methyl amines, whose size is small, there the predominance for stability due to H-bonding is more than that due to –I effect as steric effect is least.
From the compounds given above –
(CH3)3N< CH3NH2< (CH3)2NH
For ethyl amines, the order of basic strength would have been 2°>3°>1° as here ethyl groups are bulkier.
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