Question

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

• A. $iii<ii<i<iv$
• B. $ii<iv<iii<i$
• C. $ii<iii<iv<i$
• D. $ii<iii<i<iv$

Answer 1

Answer: (C)

$ii<iii<iv<i$

Nitro group has both $-R$ effect and $-I$ effect, but $-R$ effect predominates. Due to stronger electron withdrawing nature of $-N{O}_2$ group, phenoxide ion is stabilized more. Hence nitrophenol is more acidic than phenol.

Methyl group destabilizes the phenoxide ion by $+I$ effect and hyper conjugation. Hence w-cresol is weaker acid than phenol.

Chlorine have both $+R$ and $-I$ effect, but $-I$ effect predominates. Hence w-chlorophenol is more acidic than phenol.

$-R$ effect of nitro group is stronger than $-I$ effect of chlorine, hence m-nitrophenol is more acidic than m-chlorophenol.
Therefore the correct order of acidic strength is:

$m-nitrophenol>m-chlorophenol>phenol>m-cresol$

Acidic strength in increasing order is-
$ii<iii<iv<i$