Question

Find out the acidic strength order for the different groups in the following compound.

• A. a > b > c >d
• B. b > c > a > d
• C. b > c > d > a
• D. b > d > a > c

Answer 1

The correct option is B b > c > a > d

Carboxylic group is more acidic than phenol, so its $H$ is the most acidic. Comparing groups (a) and (c), $OH$ group at para to $N{O}_2$ group is more acidic than $OH$ at meta position with respect to $N{O}_2$ group because formation of conjugate base ($O^{-}$) is more stable at para position with respect to $N{O}_2$ group due to its $-M$ effect ($-M$ effect is applied only at ortho/para position and not to the meta position.) The $H$ present in $-C\equiv CH$ (group (d)) is least acidic because $H$ is attached in an alkyne system and $C$ is not as electronegative as compared to $O$ atom. So in this case, after removal of $H$, negative charge on $C$ is not stabilised as much as stabilised at more electronegative $O$ atom and also, there is resonace stabilistion of charge in (a) and (c).
Hence, (b) option is correct.