Arrange the following in the increasing order of their basic strengths :
`CH_(3)NH_(2),(CH_(3))_(2)NH,(CH_(3))_(3)N,NH_(3)`

Options

A. `NH_(3)lt(CH_(3))_(3)NltCH_(3)NH_(2)lt(CH_(3))_(2)NH`
B. `(CH_(3))_(3)NltNH_(3)ltCH_(3)NH_(2)lt(CH_(3))_(2)NH`
C. `CH_(3)NH_(2)lt(CH_(3))_(2)NHlt(CH_(3))_(3)NltNH_(3)`
D. `NH_(3)lt(CH_(3))_(3)Nlt(CH_(3))_(2)NHltCH_(3)NH_(2)`

Answer

A

Explanation

Among the given compounds, `NH_(3)` is the weakest base.
Methyl amines are stronger bases than `NH_(3)` due to the +I effect of `-CH_(3)` group. Thus greater the number of methyl groups, more will be the basicity of the amine due to greater availability of elecron density on nitrogen. Thus, the expected order is
`NH_(3)ltCH_(3)NH_(2)lt(CH_(3))_(2)NHlt(CH_(3))_(3)N`.
However, in tertiary amines, crowding of alkyl groups covers nirogen atom from al sides making the approach and bonding by `H^(+)` relatively difficult, thus the electrons are there but the path is blocked resulting in reduced basicity, thus the order of increasing basicity is:
`NH_(3)lt(CH_(3))_(3)NltCH_(3)NH_(2)lt(CH_(3))_(2)NH`.