Question

Mark the correct order of decreasing acid strength of the following compounds.

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

Answer 1

The correct option is A $b>a>c$

Phenols are ​more acidic than alcohol & water due to
(i) the higher electronegativity of $s{p}^2$ ​carbon to which OH ​group is attached.
(ii) the electron withdrawing nature of the benzene ring​.

Conjugate base of phenol is stabilised by conjugation of negative charge in the ring​. Acidic strength is inversely propotional to EDGs and directly propotional to EWGs.

Resonating structures of phenoxide ion:

Hence, b will be the most acidic because of presence of $-N{O}_2$. As $C{H}_3$ is an electron donating group it will destabilise the phenoxide ion. Therefore, the correct acidic order will be $b>a>c$.