question_answer

Arrange the following compounds in the increasing order of their acidic strength: (i) m-Nitrophenol    (ii) m-Cresol (iii) Phenol            (iv) m-Chlorophenol

A)  \[ii<iv<iii<i\]

B)  \[ii<iii<i<iv\]

C)  \[iii<ii<i<iv\]

D)  \[ii<iii<iv<i\]

Correct Answer: D

Solution :

: Nitro group has both -R effect and -J effect, but -R effect predominates. Due to stronger electron withdrawing nature of -N02 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