Question
Chemistry Question on Aldehydes, Ketones and Carboxylic Acids
What will be the correct decreasing order of acid strength of the hydroxybenzoic acids? (Symbols and notations carry their usual meanings)
p-Hydroxybenzoic acid > benzoic acid >m-hydroxybenzoic acid >o-hydroxybenzoic acid
o-Hydroxybenzoic acid >m-hydroxybenzoic acid > benzoic acid >p-hydroxybenzoic acid
o-Hydroxybenzoic acid > benzoic acid >m-hydroxybenzoic acid >p-hydroxybenzoic acid
m-Hydroxybenzoic acid > benzoic acid >o-hydroxybenzoic acid >p -hydroxybenzoic acid
o-Hydroxybenzoic acid >m-hydroxybenzoic acid > benzoic acid >p-hydroxybenzoic acid
Solution
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