pKa value is inversely proportional to the acidity, i.e. the smaller the
pKa value, the stronger is the acid.
−NO2 group at ortho and para-position withdraws electron of
−OH group towards itself due to their
−R effect while
−NO2 group at meta-position withdraw electron from -
OH group by the weaker -
I effect. Thus, ortho and para-nitrophenols are more acidic than meta-nitrophenol. Also, para-nitrophenol is more acidíc than ortho-nitrophenol due to intermolecular H-bonding in ortho-nitrophenol which makes loss of proton little more difficult. Phenol is least acidic in nature due to absence of electron-withdrawing group.
So, acidity order will be
Hence, the correct
pKa value order will be
A>C>B>D