Which among the following molecules has lowest $\mathrm{pK}_{\mathrm{b}}$ value?

Which among the following molecules has lowest $\mathrm{pK}_{\mathrm{b}}$ value?
  1. $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2$
  2. $\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_3 \mathrm{~N}$
  3. $\left(\mathrm{CH}_3 \mathrm{CH}_2\right)_2 \mathrm{NH}$
  4. $\mathrm{CH}_3 \mathrm{NH}_2$

Solution

$\begin{aligned} & \mathrm{CH}_3 \mathrm{NH}_2 < \mathrm{C}_2 \mathrm{H}_5 \mathrm{NH}_2 < \left(\mathrm{CH}_3 \mathrm{CH}_2\right)_3 \mathrm{~N} < \left(\mathrm{C}_2 \mathrm{H}_5\right)_2 \mathrm{NH} \\ & \mathrm{pK}_{\mathrm{b}}=3.34 ; \mathrm{pK}_{\mathrm{b}}=3.29 ; \mathrm{pK}_{\mathrm{b}}=3.24 ; \mathrm{pK}_{\mathrm{b}}=3.0\end{aligned}$

Asked in: MHT CET 2022 (08 Aug Shift 2)

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