Arrange the following compounds in the increasing order of their acidic strength : i. m-nitrophenol ii. m-cresol iii. phenol iv. m-chlorophenol

Question

Arrange the following compounds in the increasing order of their acidic strength : i. m-nitrophenol ii. m-cresol iii. phenol iv. m-chlorophenol, iii $ < \mathrm{ii} < \mathrm{i} < \mathrm{iv} $, ii $ < $ iv $ < \mathrm{iii} < i $, $ \mathrm{ii} < \mathrm{iii} < \mathrm{iv} < \mathrm{i} $, ii $ < \mathrm{iii} < \mathrm{i} < \mathrm{iv} $

MARKS App · Accepted Answer

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)

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