During nitration, the electrophile NO2+ attacks the benzene ring. Nitration will be easier if the benzene ring shows increased electron density. This happens when electron releasing groups such as –R,–NH2,–NHCOCH3, –OH, –OMe etc. are attached to the ring whereas, the attachment of electron withdrawing groups such as – NO2, –CHO, –COR, –COOH reduces the electron availability for NO2+ and nitration becomes difficult.
Therefore, relative ease of nitration of given molecules may be arranged as: