Q27 / 150 · MHT CET 2024 (10 May Shift 1) · Chemistry

Identify the correct decreasing order of $\mathrm{pK}_{\mathrm{b}}$ values of compounds from following.

  • Correct answer
    A
    $\mathrm{NH}_3\gt\mathrm{R}-\mathrm{NH}_2\gt\mathrm{R}_2 \mathrm{NH}$
  • B
    $\mathrm{R}_2 \mathrm{NH}\gt\mathrm{R}-\mathrm{NH}_2\gt\mathrm{NH}_3$
  • C
    $\mathrm{R}-\mathrm{NH}_2\gt\mathrm{R}_2 \mathrm{NH}\gt\mathrm{NH}_3$
  • D
    $\mathrm{R}_2 \mathrm{NH}\gt\mathrm{NH}_3\gt\mathrm{R}-\mathrm{NH}_2$

Solution

Basic strength increases as we move from $\mathrm{NH}_3$ to $\mathrm{R}-\mathrm{NH}_2$ and from $\mathrm{R}-\mathrm{NH}_2$ to $\mathrm{R}_2 \mathrm{NH}$, but basic strength decreases as we move from $\mathrm{R}_2 \mathrm{NH}$ to $\mathrm{R}_3 \mathrm{~N}$. Order of basic strength: $\mathrm{NH}_3 \lt \mathrm{R}-\mathrm{NH}_2 \lt \mathrm{R}_2 \mathrm{NH}$
Thus, Order of $\mathrm{pK}_{\mathrm{b}}$ values: $\mathrm{NH}_3\gt\mathrm{R}-\mathrm{NH}_2\gt\mathrm{R}_2 \mathrm{NH}$

Hence, the correct answer is option: 'A'.

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