The reaction of
HNO3 with phenol is given.
As a result of +R effect of the (OH) group, the electron density in the benzene ring increases, thereby facilitating the attack by an electophile. In other words, the 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 and therefore, electrophilic substitution occurs mainly at o- and p-position.