Which of the following correctly ranks the substituents nitro (${\mathrm{NO}}_2$), methyl (${\mathrm{CH}}_3$), and methoxy (${\mathrm{OCH}}_3$) on a benzene ring in order of increasing electron-donating ability?

• A. Nitro $<$ Methoxy $<$ Methyl
• B. Methoxy $<$ Methyl $<$ Nitro
• C. Nitro $<$ Methyl $<$ Methoxy
• D. Methyl $<$ Nitro $<$ Methoxy

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the nature of each substituent on the benzene ring in terms of their electron-donating or electron-withdrawing effects. The nitro group (\( \mathrm{NO_2} \)) is a strong electron-withdrawing group due to its resonance and inductive effects, the methyl group (\( \mathrm{CH_3} \)) is a weak electron-donating group through hyperconjugation, and the methoxy group (\( \mathrm{OCH_3} \)) is a strong electron-donating group due to resonance donation of lone pairs on oxygen.
2. Step 2: Recall that electron-withdrawing groups decrease electron density on the benzene ring, while electron-donating groups increase it. Therefore, the nitro group will have the least electron-donating ability (actually electron-withdrawing), methyl will have moderate electron-donating ability, and methoxy will have the highest electron-donating ability.
3. Step 3: Rank the substituents from least to greatest electron-donating ability based on their electronic effects: nitro (strongly withdrawing) < methyl (weakly donating) < methoxy (strongly donating).
4. Step 4: Express this ranking using the inequality notation: \( \mathrm{NO_2} < \mathrm{CH_3} < \mathrm{OCH_3} \), where the symbol \( < \) means 'less electron-donating than'.
5. Step 5: Confirm that this order matches the known chemical behavior of these substituents on aromatic rings, which is important for predicting reactivity and directing effects in electrophilic aromatic substitution reactions.