CAIE A-Level Chemistry 33.1.4 Acidities of Acids, Phenols and Alcohols

Practise ranking carboxylic acids, phenols and alcohols and explaining the order through conjugate-base stability.

Exam points

rank a comparable carboxylic acid above phenol above an alcohol in aqueous solution
stabilise carboxylate by negative-charge delocalisation over two oxygens and phenoxide through the aromatic system
use electron-withdrawing groups to increase acidity and electron-donating alkyl groups to decrease it

[Maximum number: 3]

State the relative acidities of benzoic acid, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}$ , ethanol, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}$ , and phenol, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}$ , in aqueous solution. Explain your answer.
most acidic
least acidic

M1: benzoic acid > phenol > ethanol

M2 / M3: any two [1] any three [2]

- correct link of acidity once AND weakens O-H / OH bond / hydroxyl bond / -O-H

OR $\mathrm{H}^{+}$ more easily lost / (carboxylate) anion stabilised u / c

- (benzoic acid) due to negative inductive effect/ electron withdrawing effect AND of C=O/COOH/carboxyl

- (phenol) as lone pair / p-orbital (electrons) on oxygen (on phenol) / AND overlap / delocalised into the ring / $\pi$ -system

- (ethanol) alkyl / ethyl / R group AND is electron donating / positive inductive effect

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33.1.4—Acidities of carboxylic acids, phenols question 1