Q37 / 180 · AIIMS 2011 · Chemistry

$$ \text { The correct decreasing order of } \mathrm{p} K_a \text { is } $$ Figure

  • Correct answer
    A
    II $>$ IV $>$ I $>$ III
  • B
    IV $>$ II $>$ III $>$ I
  • C
    III $>$ II $>$ IV $>$ I
  • D
    IV $>$ I $>$ II $>$ III

Solution

Weaker acids have higher $\mathrm{p} K_a \cdot-\mathrm{OCH}_3$ at meta-position exerts only $-I$ effect, hence increase the acidity. $-I$ effect order : $-\mathrm{NO}_2>-\mathrm{OCH}_3>-\mathrm{Cl}$. $-\mathrm{CH}_3$ has $+I$ effect. So order is (1).

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

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