Considering the basic strength of amines in aqueous solution, which one has the smallest `pK_b` value?

Options

A. `(CH_3)_2 NH`
B. `CH_3 NH_2 `
C. `(CH_3)_3 N`
D. `C_6 H_5 NH_2`

Answer

A

Explanation

`+I` effect as well as basicity due to the group decreases as `(CH_3)_3 N gt (CH_3)_3 NH gt CH_3 NH_2 gt NH_3` . However, solvation of conjugate acids occurs by hydrogen bonding which depends on the number of hydrogen atoms attached with nitrogen atom. The order of solvation of conjugate base is `(CH_3)_3 N lt (CH_3)_2 NH lt CH_3 NH_2 lt NH_3` . In this way, both factors are against each other and the order of resultant basic strength becomes `(CH_3)_2 NH gt CH_3 NH_2 gt (CH_3)_3 N gt NH_3`.