KCET · Chemistry · Alcohols Phenols and Ethers
Arrange the following compounds in the increasing order of their acidic strength :
i. m-nitrophenol
ii. m-cresol
iii. phenol
iv. m-chlorophenol
- A iii \( < \mathrm{ii} < \mathrm{i} < \mathrm{iv} \)
- B ii \( < \) iv \( < \mathrm{iii} < i \)
- C \( \mathrm{ii} < \mathrm{iii} < \mathrm{iv} < \mathrm{i} \)
- D ii \( < \mathrm{iii} < \mathrm{i} < \mathrm{iv} \)
Answer & Solution
Correct Answer
(C) \( \mathrm{ii} < \mathrm{iii} < \mathrm{iv} < \mathrm{i} \)
Step-by-step Solution
Detailed explanation
Nitro group has both \( -R \) effect and \( -1 \) effect; but \( -R \) effect predominates. Due to stronger electron withdrawing nature of
\( \mathrm{NO}_{2} \) group, phenoxide ion is stabilized. Hence, m-nitrophenol is more acidic than phenol. Methyl group destabilizes the
phenoxide ion by \( +I \) effect and hyperconjugation. Hence m-cresol is weaker acid than phenol. Chlorine have both +R
and \( -1 \) effect but \( -1 \) effect predominates
Hence m-chlorophenol is more acidic than phenol. \( -R \) effect of nitro group is stronger than \( -1 \) effect of chlorine hence, m-
nitrophenol is more acidic than m-chlorophenol.
Therefore, the correct order of acidic strength is \( m \)-nitrophenol \( >m \)-chlorophenol \( >p h e n o l>m \)-cresol.
(II)
(III)
(IV)
(I)
\( \mathrm{NO}_{2} \) group, phenoxide ion is stabilized. Hence, m-nitrophenol is more acidic than phenol. Methyl group destabilizes the
phenoxide ion by \( +I \) effect and hyperconjugation. Hence m-cresol is weaker acid than phenol. Chlorine have both +R
and \( -1 \) effect but \( -1 \) effect predominates
Hence m-chlorophenol is more acidic than phenol. \( -R \) effect of nitro group is stronger than \( -1 \) effect of chlorine hence, m-
nitrophenol is more acidic than m-chlorophenol.
Therefore, the correct order of acidic strength is \( m \)-nitrophenol \( >m \)-chlorophenol \( >p h e n o l>m \)-cresol.
(II)
(III)
(IV)
(I)
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