Phenol on reaction with conc. $${H_2}S{O_4}$$ gives a mixture of $$o$$- and $$p$$- products (i.e., $$ - S{O_3}H$$ group, occupies $$o$$-, $$p$$- position). At room temperature $$o$$-product is more stable, which on treatment with conc. $$HN{O_3}$$ will yield $$o$$-nitrophenol.