Question

Which of the following is not an electron withdrawing group?

• A. \(-CN\)
• B. \(-NO_2\)
• C. \(-COOH\)
• D. \(-COOR\)
• E. \(-OCH_3\)

Hint:

Groups containing carbonyl or nitro functionality are usually electron withdrawing, while groups like \(-OCH_3\) often donate electrons by resonance.

Answer 1

Answer: (E)

Step 1: Understanding the Concept:
This question asks to identify which of the given functional groups is not an electron-withdrawing group (EWG). We need to analyze the electronic effects (inductive and resonance effects) of each group when attached to a benzene ring or a carbon chain. Electron-withdrawing groups decrease electron density, while electron-donating groups (EDG) increase it.
Step 2: Detailed Explanation:
Let's analyze each group:
(A) -CN (Cyano group): The nitrogen atom is more electronegative than the carbon atom, causing a strong dipole and a strong -I (inductive) effect. Additionally, the C\( \equiv \)N triple bond can participate in resonance, withdrawing electron density from a benzene ring (-R or -M effect). Therefore, -CN is a strong electron-withdrawing group.
(B) -NO\(_2\) (Nitro group): Nitrogen and oxygen are highly electronegative, leading to a very strong -I effect. The nitro group can also delocalize electrons from a benzene ring through resonance (-R effect). It is one of the strongest electron-withdrawing groups.
(C) -COOH (Carboxylic acid group): This group has electronegative oxygen atoms, causing a -I effect. The carbonyl (C=O) part can also withdraw electrons via resonance (-R effect). So, -COOH is an electron-withdrawing group.
(D) -COOR (Ester group): Similar to the carboxylic acid group, the ester group has a carbonyl and electronegative oxygens, exhibiting both -I and -R effects. It is an electron-withdrawing group.
(E) -OCH\(_3\) (Methoxy group): The oxygen atom is highly electronegative, so it exerts a -I (inductive) electron-withdrawing effect. However, the oxygen atom also has lone pairs of electrons that can be delocalized into an attached benzene ring or pi-system. This is a strong +R (resonance) or +M (mesomeric) electron-donating effect. For groups attached to a benzene ring, the resonance effect (+R) is generally much stronger than the inductive effect (-I). Therefore, the overall effect of the -OCH\(_3\) group is electron-donating.
Step 3: Final Answer:
The -OCH\(_3\) group is an electron-donating group due to its dominant +R effect, even though it has a -I effect. Therefore, it is not an electron-withdrawing group in the overall sense, especially when considering its effect on aromatic systems.