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

Options

A.
B.
C.
D.

Answer

To solve the problem of which sigma-complex is of lowest energy when an electrophile attacks a benzene ring, we can follow these steps:

### Step 1: Understand the Electrophilic Attack
The electrophile \( E^+ \) is an electron-deficient species that seeks to attack regions of high electron density. In a benzene ring, the presence of pi electrons makes it a good target for electrophilic attack.

**Hint:** Remember that electrophiles are attracted to areas of high electron density.

### Step 2: Identify the Effects of Substituents
When substituents like \( -NO_2 \) (nitro group) are present on the benzene ring, they can influence the electron density of the ring. The nitro group is an electron-withdrawing group (EWG), which decreases the electron density of the ring through resonance and inductive effects.

**Hint:** Consider how different substituents affect the electron density of the benzene ring.

### Step 3: Analyze Resonance Structures
When the electrophile attacks the benzene ring, it can lead to the formation of resonance structures. For a benzene ring with a nitro group, the resonance structures will show that the ortho and para positions relative to the nitro group will develop a positive charge due to the electron-withdrawing nature of the nitro group.

**Hint:** Draw the resonance structures to visualize how the positive charge is distributed in the sigma-complex.

### Step 4: Determine the Preferred Position for Electrophilic Attack
Since the ortho and para positions are destabilized by the presence of the positive charge (due to the nitro group), the electrophile will preferentially attack the meta position, where the electron density is less affected by the nitro group.

**Hint:** Identify which positions on the benzene ring are more favorable for electrophilic attack based on the resonance structures.

### Step 5: Evaluate Stability of Sigma-Complexes
The sigma-complex formed by the electrophile attacking the meta position will be more stable compared to those formed at the ortho and para positions because the positive charge is not directly influenced by the electron-withdrawing nitro group.

**Hint:** Compare the stability of the sigma-complexes formed at different positions to determine which one is the most stable.

### Step 6: Conclusion
Based on the analysis, the sigma-complex formed by the electrophile attacking the meta position is of lowest energy due to the reduced destabilization from the nitro group.

**Final Answer:** The sigma-complex of lowest energy is the one formed at the meta position (Option D).