The acidic strength of phenols is effected by the presence of substituents on the ring.An electron-withdrawing substituent helps in greater dispersal of negative charge on the phenoxide ion either by inductive effect (–I) or by resonance effect (–R) or both which result in the increase in acid strength of phenol.And the electron-donating groups due to +I or +R or both effects decreases the acidic strength.
Now halogens have +R and –I-effects, but the –I-effect predominates over the +R-effect. Therefore, all halophenols (except p-fluorophenol) are more acidic than phenol itself.
In case of p- fluorophenol +R-effect and –I-effect of F almost balance each other due to almost identical sizes of 2p -orbitals of C and F and hence it is almost as acidic as phenol itself.Thus p-chlorophenol is more acidic than p-flurophenol.
