Q18 / 180 · COMEDK 2024 (Afternoon Shift) · Chemistry
Arrange the following compounds in the decreasing order of their acidic strength.
(I) m-cresol
(II) Phenol
(III) m-aminophenol
(IV) m-methoxyphenol
- AI > II > III > IV
- Correct answerBIV > III > II > I
- CI > III > IV > II
- DIII > I > II > IV
Solution
At meta position, only inductive effect (I-effect) operates, not resonance.
m-methoxyphenol: $-OCH_3$ has strong $-I$ effect (O is highly electronegative) $\to$ stabilizes phenoxide $\to$ most acidic.
m-aminophenol: $-NH_2$ has $-I$ effect but weaker than $-OCH_3$ (N less electronegative than O) $\to$ more acidic than phenol.
Phenol: reference compound, no substituent.
m-cresol: $-CH_3$ has $+I$ effect $\to$ destabilizes phenoxide $\to$ least acidic.
Decreasing order: IV $>$ III $>$ II $>$ I
m-methoxyphenol: $-OCH_3$ has strong $-I$ effect (O is highly electronegative) $\to$ stabilizes phenoxide $\to$ most acidic.
m-aminophenol: $-NH_2$ has $-I$ effect but weaker than $-OCH_3$ (N less electronegative than O) $\to$ more acidic than phenol.
Phenol: reference compound, no substituent.
m-cresol: $-CH_3$ has $+I$ effect $\to$ destabilizes phenoxide $\to$ least acidic.
Decreasing order: IV $>$ III $>$ II $>$ I
Hence, the correct answer is option: 'B'.