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@ $K_a$ increases in benzoic acid when substituent " $x$ " is bonded at para-position, then " $x$ " is
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$-\mathrm{OCH}_3$
Larger the value of $\mathrm{p} K_{a r}$ smaller will be its acidity. Out of the four groups, $-\mathrm{COOH},-\mathrm{NO}_2$ and $-\mathrm{CN}$ are $e^{-}$withdrawing which makes benzoic acid more acidic whereas $-\mathrm{OCH}_3$ is $e^{-}$ donating which reduces the acidity (makes $\mathrm{H}^{+}$ less easily available). $\mathrm{p} K_a$ value increases if $\mathrm{OCH}_3$ is present at para-position of benzoic acid.
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