The electrophile, `E^(o+)` attacks the benzene ring to generate the intermediate `sigma`-complex. Of the following, which `sigma`-complex is of lowest energy ?

Options

A.
B.
C.
D.

Answer

To determine which sigma-complex is of lowest energy when an electrophile attacks a benzene ring, we need to analyze the stability of the resulting sigma-complexes. The stability of these intermediates is influenced by the degree of charge delocalization and the presence of substituents on the benzene ring.

### Step-by-Step Solution:

1. **Identify the Electrophile and Benzene Ring Interaction**:
- When an electrophile (`E^+`) attacks the benzene ring, it disrupts the aromaticity by forming a sigma-complex (also known as an arenium ion). This complex has a positive charge due to the loss of aromatic stability.

2. **Draw the Sigma-Complex Structures**:
- For each sigma-complex option provided, draw the structure showing the electrophile attached to the benzene ring and indicate the location of the positive charge.

3. **Analyze Charge Delocalization**:
- Examine each sigma-complex for the extent of charge delocalization. The more delocalized the positive charge is, the more stable the complex will be.
- In sigma-complexes, the positive charge can be stabilized by resonance with adjacent double bonds.

4. **Consider the Effects of Substituents**:
- If there are substituents on the benzene ring, consider their electronic effects (either electron-donating or electron-withdrawing). Electron-donating groups (like -OH, -OCH3) can stabilize the positive charge, while electron-withdrawing groups (like -NO2, -CN) can destabilize it.

5. **Compare Energy Levels**:
- After analyzing the structures and their resonance forms, compare the stability of each sigma-complex. The one with the most resonance forms and the least localized positive charge will be of the lowest energy.

6. **Conclusion**:
- Identify which sigma-complex has the lowest energy based on your analysis. This will be the most stable intermediate formed during the electrophilic aromatic substitution.