Arrange the following compounds in increasing order of their acid strengths:
A. o-Cresol, pKa = 10.2
B. m-Nitrophenol, pKa = 8.3
C. Phenol, pKa = 10
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A, C, B
The smaller the pKa, the stronger the acid. In phenols, an electron withdrawing group (such as –\( \ce{NO2} \)) stabilises the phenoxide ion and increases acidity, while an electron donating group (such as –\( \ce{CH3} \)) destabilises it and decreases acidity.
A. o-Cresol, pKa = 10.2: first (weakest acid): the –\( \ce{CH3} \) group donates electrons and destabilises the phenoxide ion
C. Phenol, pKa = 10: second (the parent phenol)
B. m-Nitrophenol, pKa = 8.3: the –\( \ce{NO2} \) group withdraws electrons (−I effect): last (strongest acid)
Shortcut: Increasing acid strength is decreasing pKa: 10.2 > 10 > 8.3, i.e. A, C, B. Only Option (4) starts with A.