Rate determining step in nitration of benzene is

Options

A. formation of `NO_(2)^(+)`
B. formation of carboncation
C. replacement of H atom
D. none of these

Explanation

To determine the rate-determining step in the nitration of benzene, we need to analyze the reaction mechanism step by step.

### Step-by-Step Solution:

1. **Identify the Reaction Components**:
- The nitration of benzene involves the reagents nitric acid (HNO3) and sulfuric acid (H2SO4). The goal is to introduce a nitro group (NO2) into the benzene ring.

2. **Formation of the Electrophile**:
- The first step in the mechanism is the generation of the nitronium ion (NO2+), which is the active electrophile. This is formed when HNO3 reacts with H2SO4:
\[ \text{HNO3} + \text{H2SO4} \rightarrow \text{NO2}^+ + \text{H3O}^+ \]
- This step is fast and does not control the rate of the overall reaction.

3. **Electrophilic Attack on Benzene**:
- The second step involves the electrophilic attack of the nitronium ion (NO2+) on the benzene ring. The benzene acts as a nucleophile and forms a carbocation intermediate:
\[ \text{C6H6} + \text{NO2}^+ \rightarrow \text{C6H5NO2}^+ \]
- This step is slow and is considered the rate-determining step because it involves the formation of a new bond and the transition state has a higher energy compared to the other steps.

4. **Deprotonation to Form Nitrobenzene**:
- The final step involves the loss of a proton from the carbocation to regenerate the aromatic system, forming nitrobenzene:
\[ \text{C6H5NO2}^+ \rightarrow \text{C6H5NO2} + \text{H}^+ \]
- This step is relatively fast and does not control the rate of the overall reaction.

5. **Conclusion**:
- The rate-determining step in the nitration of benzene is the second step, which is the formation of the carbocation from the electrophilic attack of NO2+ on the benzene ring.

### Final Answer:
The rate-determining step in the nitration of benzene is the formation of the carbocation.

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