Which of the following will undergo sulphonation at fastest rate ?

Options

A.
B.
C.
D.

Answer

To determine which compound undergoes sulphonation at the fastest rate, we need to analyze the electron density on the benzene rings of the given compounds. The sulphonation reaction is an electrophilic substitution reaction where the sulphonyl group (SO3H) acts as an electrophile. The rate of sulphonation is influenced by the electron density of the benzene ring; higher electron density leads to a faster reaction.

### Step-by-Step Solution:

1. **Understanding Sulphonation**:
- Sulphonation is an electrophilic substitution reaction where the sulphonyl group (SO3H) replaces a hydrogen atom on the benzene ring. The electrophile (SO3) is generated from concentrated sulfuric acid.

2. **Identify the Compounds**:
- The question provides four compounds, all of which are derivatives of benzene with different alkyl groups attached.

3. **Determine Electron Density**:
- The rate of sulphonation depends on the electron density on the benzene ring. Alkyl groups are electron-donating groups and increase the electron density through inductive (+I) and hyperconjugation effects.

4. **Analyze Each Compound**:
- **Compound A**: Has a single CH3 group. It contributes +I effect but lacks alpha hydrogens for hyperconjugation.
- **Compound B**: Has a CH3 group with three alpha hydrogens. It contributes both +I and significant hyperconjugation, increasing electron density.
- **Compound C**: Has a CH3 group with two alpha hydrogens. It contributes +I and some hyperconjugation but less than compound B.
- **Compound D**: Has a CH3 group with only one alpha hydrogen. It contributes +I and minimal hyperconjugation.

5. **Compare the Effects**:
- The electron-donating effects of the alkyl groups increase the electron density on the benzene ring, making it more reactive towards electrophiles.
- Among the compounds, compound B has the highest electron density due to the maximum number of alpha hydrogens contributing to hyperconjugation.

6. **Conclusion**:
- Since compound B has the highest electron density due to the presence of three alpha hydrogens, it will undergo sulphonation at the fastest rate compared to the other compounds.

### Final Answer:
- The compound that will undergo sulphonation at the fastest rate is **Compound B**.