During nitration, the electrophile $\mathrm{NO}_2^{+}$ attacks the benzene ring. Nitration will be easier if the benzene ring shows increased electron density. This happens when electron releasing groups such as $\text{–R, –}\mathrm{NH}_2\text{,–}\mathrm{NHCOCH}_3$, –OH, –OMe etc. are attached to the ring whereas, the attachment of electron withdrawing groups such as – $\mathrm{NO}_2$, –CHO, –COR, –COOH reduces the electron availability for $\mathrm{NO}_2^{+}$ and nitration becomes difficult. Therefore, relative ease of nitration of given molecules may be arranged as: