Which of the following is active species in sulphonation of benzene ?

Options

A. `overset(o+)SO_(3)H`
B. `SO_(3)`
C. `O=overset(O)overset(||)underset(o+)S=O`
D. `overset(o+)SO_(2)overset(-)OH`

Answer

To determine the active species in the sulfonation of benzene, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Sulfonation**:
- Sulfonation is the process of adding a sulfonyl group (–SO3H) to a benzene ring. This is an electrophilic substitution reaction where a hydrogen atom of benzene is replaced by the sulfonyl group.

2. **Identify the Reagents**:
- The primary reagent used in the sulfonation of benzene is fuming sulfuric acid (H2SO4), which contains sulfur trioxide (SO3) as a key component.

3. **Identify the Electrophile**:
- In electrophilic substitution reactions, the electrophile is the species that accepts electrons. In the case of sulfonation, the active electrophile is the SO3 molecule.

4. **Mechanism of Reaction**:
- The SO3 molecule interacts with the benzene ring. The π electrons of the benzene ring attack the sulfur atom in SO3, leading to the formation of a sigma complex (an arenium ion).
- This results in the loss of a hydrogen atom from the benzene ring, which is then replaced by the sulfonyl group (–SO3H).

5. **Conclusion**:
- Therefore, the active species in the sulfonation of benzene is the SO3 molecule.

### Final Answer:
The active species in the sulfonation of benzene is **SO3**.

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