Which of the following $\mathrm{H}$ will have lower $\mathrm{p} K_{\mathrm{a}}$ value in the given compound?

Solution

Lower the $\mathrm{p} K_{\mathrm{a}}$, stronger is the acid $\left(\mathrm{H}_{\mathrm{a}}ight.$ in (I) has lowest) value. I. II. III. IV. Stability order: Aromatic $>$ Non-aromatic $>$ Anti-aromatic $pK_a$ order: $\mathrm{(I)}$ $ <(\mathrm{IV}) <(\mathrm{III}) <$ $\mathrm{(II)}$ $\left(ight.$ or $\left.\mathrm{H}_{\mathrm{a}} <\mathrm{H}_{\mathrm{d}} <\mathrm{H}_{\mathrm{c}} <\mathrm{H}_{\mathrm{b}}ight)$ $\mathrm{H}_{c}$ in $\mathrm{(III)}$ is a stronger acid than $\mathrm{H}_{\mathrm{b}}$ in $\mathrm{(II)}$ because its anion is much more stabilised due to more number of resonating structures. *

Asked in: JEE-TOPICTESTS-CHEMISTRY

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