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

The correct option is 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-cresolis weaker acid than phenol.

Chlorine has both +R and -I effect, but -I effect predominates. Hence m-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