Question

Which of the following functional groups will show -R effect?

• A. -OH
• B. -OCH\(_3\)
• C. -NH\(_2\)
• D. -NO\(_2\)
• E. -NHCOCH\(_3\)

Hint:

Groups with a double/triple bond to an electronegative atom (like -NO\(_2\), -CN, -CHO, -COOH) show -R and -I effects.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The resonance effect (R effect) describes the polarity produced in a molecule by the interaction of lone pairs of electrons with \(\pi\)-bonds or the interaction of two \(\pi\)-bonds in adjacent atoms. It is transmitted along a conjugated system.
- +R effect (electron-donating): A group donates electrons to the conjugated system. - -R effect (electron-withdrawing): A group withdraws electrons from the conjugated system. This is also known as the -M (mesomeric) effect.
Step 2: Analysis of Options:
A group shows a -R effect if it is attached to a conjugated system and has a multiple bond to a more electronegative atom. This allows it to pull \(\pi\)-electron density from the system towards itself. Let's analyze the groups:
(A) -OH, (B) -OCH\(_{3}\), and (C) -NH\(_{2}\): In these groups, the atom directly attached to the conjugated system (O or N) has lone pairs of electrons. These lone pairs can be delocalized into the conjugated system, thus donating electron density. They all exhibit a +R effect.
(E) -NHCOCH\(_{3}\): The nitrogen atom has a lone pair, which it can donate to the aromatic ring (+R effect). Although the C=O group is withdrawing, the overall effect of the group on an aromatic ring is electron-donating via resonance.
(D) -NO\(_{2}\) (nitro group): The structure is -N(=O)(-O). The nitrogen atom is attached to the conjugated system and is also double-bonded to a more electronegative oxygen atom. This arrangement allows the nitro group to strongly withdraw electron density from the conjugated system through resonance. Therefore, the -NO\(_{2}\) group shows a strong -R effect.
Step 3: Final Answer:
The -NO\(_{2}\) group is a powerful electron-withdrawing group that exhibits the -R effect. This corresponds to option (D).