Question

What will be the correct decreasing order of acid strength of the hydroxybenzoic acids? (Symbols and notations carry their usual meanings)

• A. $p$-Hydroxybenzoic acid $>$ benzoic acid $>m$-hydroxybenzoic acid $>o$-hydroxybenzoic acid
• B. $o$-Hydroxybenzoic acid $>m$-hydroxybenzoic acid $>$ benzoic acid $>p$-hydroxybenzoic acid
• C. $o$-Hydroxybenzoic acid $>$ benzoic acid $>m$-hydroxybenzoic acid $>p$-hydroxybenzoic acid
• D. $m$-Hydroxybenzoic acid $>$ benzoic acid $>o$-hydroxybenzoic acid $>p$ -hydroxybenzoic acid

Answer 1

Answer: (B)

In case of hydroxybenzoic acids, they display both kind of effect and there is a decrease in electron density at all positions due to inductive effect of $-OH$ group, but increase in electron density at $o$- and $p$- positions due to resonance effect by $-OH$ group . So, $o$- p-hydroxybenzoic acids should be weaker than m-hydroxybenzoic acid, but $o$-hydroxybenzoic acid is strongest due to stabilisation of anion by hydrogen bonding