Which of the following group does not exert +R effect?

Question

Accepted Answer

Option 3, -CN, is the correct answer because it does not exert a +R (positive resonance) effect. The +R effect, also known as the positive resonance effect, occurs when a substituent donates electron density through resonance, typically involving lone pairs or π-electrons that can be delocalized into the conjugated system. Let's examine why -CN does not exhibit this effect and why the other options do:

1. **-OCOR**: This group can exert a +R effect because the oxygen atom has lone pairs that can participate in resonance. The lone pairs on the oxygen can be delocalized into the conjugated system, thereby donating electron density and stabilizing the system through resonance.

2. **-NHCOR**: Similar to -OCOR, the -NHCOR group can also exert a +R effect. The nitrogen atom has lone pairs that can be delocalized into the conjugated system. This delocalization allows the nitrogen to donate electron density through resonance, contributing to the +R effect.

3. **-CN**: The -CN group, or cyano group, does not exert a +R effect. Instead, it is known for its -M (negative mesomeric) effect. The carbon-nitrogen triple bond in the cyano group is highly electronegative, and the nitrogen atom tends to withdraw electron density rather than donate it. This withdrawal of electron density does not support the +R effect, making -CN the correct answer as the group that does not exert a +R effect.

4. **$-NR_{2}$**: The $-NR_{2}$ group can exert a +R effect because the nitrogen atom has lone pairs that can be delocalized into the conjugated system. This delocalization allows the nitrogen to donate electron density through resonance, contributing to the +R effect.

In summary, the -CN group is the only one among the options that does not exert a +R effect due to its electron-withdrawing nature, which is consistent with the correct answer being Option 3.