Question

The correct order of \(\mathrm{pK_a}\) values for the following compounds is:

• A. \(a > b > c > d\)
• B. \(c > a > d > b\)
• C. \(b > d > a > c\)
• D. \(b > a > d > c\)

Hint:

Electron-withdrawing groups (NO2) stabilise the phenoxide ion and increase acidity (lower pKa); electron-donating groups (alkyl, NMe2) decrease acidity (higher pKa).

Answer 1

The Correct Option is C

Acidity of substituted phenols depends on the stability of the phenoxide ion.
(c) p-nitrophenol: \(-\mathrm{NO_2}\) withdraws electrons by \(-I\) and \(-M\) effects and strongly stabilises the phenoxide ion, so it is the most acidic (lowest pKa).
(a) phenol: reference compound.
(d) p-alkylphenol: the alkyl group is electron donating (\(+I\), hyperconjugation), which destabilises the phenoxide ion, so it is less acidic than phenol.
(b) p-dimethylaminophenol: \(-\mathrm{N(CH_3)_2}\) is a strong \(+M\) donor, so it destabilises the phenoxide ion the most and is the least acidic (highest pKa).
Acidity order: \(c > a > d > b\).
pKa is inversely related to acidity, so pKa order: \(b > d > a > c\).
Answer: (3) \(b > d > a > c\)