Organic Chemistry Basics 2 Question 21
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21. Among the following compounds, the most acidic is (2011)
======= ####21. Among the following compounds, the most acidic is (2011)
3e0f7ab6f6a50373c3f2dbda6ca2533482a77bed (a) p-nitrophenol
(b) p-hydroxybenzoic acid
(c) o-hydroxybenzoic acid
(d) $p$-toluic acid
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Solution:
- A mono-substituted benzoic acid is stronger than a mono-substituted phenol as former being a carboxylic acid. Among the given substituted benzoic acid, ortho-hydroxy acid is strongest acid although $-\mathrm{OH}$ causes electron donation by resonance effect which tends to decrease acid strength.
It is due to a very high stabilisation of conjugate base by intramolecular $\mathrm{H}$-bond which outweigh the electron donating resonance effect of $-\mathrm{OH}$.
The overall order of acid-strength of given four acids is ortho-hydroxybenzoic acid $\left(\mathrm{p} K_{a}=2.98\right)>$ Toluic acid $\left.\mathrm{p} K_{a}=4.37\right)>p$-hydroxybenzoic acid $\left(\mathrm{p} K_{a}=4.58\right)>$ $p$-nitrophenol $\left(\mathrm{p} K_{a}=7.15\right)$
