MCQ
Considering the basic strength of amines in aqueous solution, which one has the smallest $pK_b$ value ?
- ✓$(CH_3)_2NH$
- B$CH_3NH_2$
- C$(CH_3)_3N$
- D$C_6H_5NH_2$
✓
Answer
Correct option: A.
$(CH_3)_2NH$
a
Order of basic strength of aliphatic amine in aqueous solution is as follows (order of $K_{b}$ )
Order of basic strength of aliphatic amine in aqueous solution is as follows (order of $K_{b}$ )
$\left(\mathrm{CH}_{3}\right)_{2} \bar {N}H>\mathrm{CH}_{3} \bar {N}H_{2}>$$\left(CH_{3}\right)_{3} \bar {N}>\mathrm{C}_{6}\ mathrm{H}_{5} \bar {N} \mathrm{H}_{2}$
As we know $\mathrm{pk}_{\mathrm{b}}=-\log \mathrm{K}_{\mathrm{b}}$
So $ (CH_3) _{2}$ $NH$ will have smallest $pK_{b}$ value. In the case of phenylamine, $N$ is attached to $s p^{2}$ hybridised carbon, hence it has highest $p K_{b}$ and least basic strength.
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