Question

Which of the following phenols has the largest $p{K}_a$ value (i.e., least acidic)?

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

$Acidicstrength\propto K_a\propto \frac{1}{p{K}_a}$

The electron density on oxygen atom is stabilised by Electron withdrawing group and destabilised by Electron donating group at para position.

$N{O}_2,CNandCl$ are electron withdrawing group, hence it will stabilise the conjugate base and it will be more acidic. Thus its $p{K}_a$ value will be low.
$N{O}_{2}andCN$ shows -R effect which will decrease the electron density on $O^{-}$ and stabilise the conjugate base and are more acidic. Hence they will have less $p{K}_a$ value.
$C{H}_3$ is an electron donating group, hence it will destabilise the conjugate base and it will be less acidic. Thus its $p{K}_a$ value will be high.