8.2 Carboxylic acids and their derivative — Chemistry STD 12 Science — Question

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Further, $OH$ group has a strong $+ R$ -efect and a weak -$I$-effect. since at m-position $OH$ group cannot exert its $+ R$ -effect but can only exert its -$l$-effect, therefore, $m$-hydroxybenzoic acid $(III)$ is a stronger acid than p-hydroxybenzoic acid $(IV)$ where $OH$ group exerts its strong $+ R$ -effect as shown

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thus, the overall acid strength decreases in the order:

$\text { 2,6-Dihydroxybenzoic acid (I) }$

$\left(p K_a=4.08\right)$

$ > \text { Salicyclic acid (II) }$

$\left(p K_a=2.98\right)$

$ > \text { m-Hydroxybenzoic acid (III) }$

$\left(p K_a=4.08\right)$

$ > \text { p-Hydroxybenzoic (IV) }$

$\left(p K_a=4.58\right)$