Phenol may be regarded as a hybrid of structures I-V, show below.
As a result of +R effect of the -OH group, the electron density in the benzene ring increases thereby facilitating the attack of an electrophile. In other words, presence of - OH group, activates the benzene ring towards electrophilic substitution reactions. Further, since the electron density is relatively higher at the two O-and one p- position, therefore electrophilic substitution occurs mainly at O-and p-positions.
