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