Which of the following group directs the incoming electrophile at meta position in benzene ring ?

Question

Accepted Answer

In the context of electrophilic aromatic substitution reactions, the directing effects of substituents on a benzene ring are crucial in determining the position where the incoming electrophile will attach. Substituents can be classified as either ortho/para-directing or meta-directing based on their electronic effects.

1. **Option 4: -COR (Correct Answer)**
   - The -COR group is a carbonyl-containing substituent, which is an electron-withdrawing group due to the presence of the electronegative oxygen atom. This group withdraws electron density from the benzene ring through resonance and inductive effects. As a result, the electron density on the ortho and para positions is reduced, making them less favorable for electrophilic attack. Consequently, the meta position becomes relatively more electron-rich compared to the ortho and para positions, directing the incoming electrophile to the meta position. This is why -COR is considered a meta-directing group.

2. **Option 1: -C₂H₅**
   - The -C₂H₅ (ethyl) group is an alkyl group, which is electron-donating through hyperconjugation and inductive effects. Alkyl groups increase the electron density on the benzene ring, particularly at the ortho and para positions, making them more reactive towards electrophiles. Therefore, -C₂H₅ is an ortho/para-directing group, not a meta-directing group.

3. **Option 2: -NHCOCH₃**
   - The -NHCOCH₃ group is an acylated amine, which can donate electron density to the benzene ring through resonance. The lone pair on the nitrogen can participate in resonance, increasing electron density at the ortho and para positions. This makes -NHCOCH₃ an ortho/para-directing group, not a meta-directing group.

4. **Option 3: -OCH₃**
   - The -OCH₃ (methoxy) group is an electron-donating group due to the lone pairs on the oxygen atom, which can participate in resonance with the benzene ring. This increases electron density at the ortho and para positions, making them more favorable for electrophilic attack. Thus, -OCH₃ is an ortho/para-directing group, not a meta-directing group.

In summary, the -COR group is the only option among the given choices that directs the incoming electrophile to the meta position due to its electron-withdrawing nature, which decreases electron density at the ortho and para positions, making the meta position relatively more favorable for electrophilic substitution.