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 acidÃc 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
para-nitrophenol
(D)
>
ortho-nitrophenol
(B)
>
meta-nitrophenol
(C)
>
phenol
(A)
Hence, the correct pKa value order will be A>C>B>D