Which of the following compounds is most reactive toward a nucleophilic…

Concept

In a nucleophilic addition to a carbonyl compound, the nucleophile attacks the electrophilic carbonyl carbon. Greater electron deficiency at that carbon generally increases the reaction rate.

When condensed aryl-aldehyde formulas omit locants, the standard comparison treats ring positions and steric conditions as comparable and ranks the compounds by the substituents’ dominant electronic effects.

Application

The nitro group has a strong –I effect at every ring position; at positions conjugated with the aldehyde group, it also withdraws by resonance. Under the comparable-position convention, NO2–C6H4–CHO therefore has the greatest carbonyl electron deficiency in this set.

Contrast

• C6H5–CHO has no additional ring substituent, so it represents the baseline reactivity of benzaldehyde.

• The methyl group (–CH3) donates electron density through its +I effect and hyperconjugation, reducing carbonyl electrophilicity.

• The methoxy substituent donates by resonance at conjugating ring positions; at a meta position only its much weaker –I effect operates. It does not produce exceptional carbonyl electron deficiency in this standard substituent comparison.

Cross-check

Across standard ring-substituent comparisons, nitro is markedly more electron-withdrawing than hydrogen, methyl, or methoxy. Thus NO2–C6H4–CHO is the most reactive compound toward nucleophilic addition in this set.