Q34 / 150 · MHT CET 2025 (20 Apr Shift 2) · Chemistry

Which from following is the most stable species?

  • A
    $\mathrm{NH}_4^{+}$
  • B
    $\mathrm{R}-\stackrel{+}{\mathrm{N}} \mathrm{H}_3$
  • C
    $\mathrm{R}_2 \stackrel{+}{\mathrm{N}} \mathrm{H}_2$
  • Correct answer
    D
    $\mathrm{R}_3 \stackrel{+}{\mathrm{N}} \mathrm{H}$

Solution

Ammonium ion stability depends on three competing factors: the inductive effect of alkyl groups, solvation through hydrogen bonding, and steric hindrance to solvation.

Alkyl groups donate electron density inductively, stabilizing positive charge. Solvation in aqueous environments favors ions with more hydrogens for hydrogen bonding. Steric hindrance from bulky groups reduces solvation effectiveness.

Experimental pKa values for methyl-substituted ammonium ions reveal the stability order:

$\mathrm{pKa}[(\mathrm{CH}_3)_2 \stackrel{+}{\mathrm{N}} \mathrm{H}_2] (10.73) > \mathrm{pKa}[\mathrm{CH}_3 \stackrel{+}{\mathrm{N}} \mathrm{H}_3] (10.64) > \mathrm{pKa}[(\mathrm{CH}_3)_3 \stackrel{+}{\mathrm{N}} \mathrm{H}] (9.80) > \mathrm{pKa}[\mathrm{NH}_4^{+}] (9.25)$

The secondary ammonium ion $\mathrm{R}_2 \stackrel{+}{\mathrm{N}} \mathrm{H}_2$ achieves optimal balance between inductive stabilization (two alkyl groups) and solvation capacity (two hydrogens), with minimal steric interference.

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

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