The compound with molecular formula `C_(8)H_(10)` which will give only two isomers on electrophilic substitution with `Cl_(2)//FeCl_(3)` or with `HNO_(3)//H_(2)SO_(4)` is

Options

A. none of these
B. 1,4-dimethyl benzene
C. 1,2-dimethyl benzene
D. ethylbenzene

Answer

To solve the problem, we need to identify the compound with the molecular formula \( C_8H_{10} \) that gives only two isomers upon electrophilic substitution reactions with \( Cl_2/FeCl_3 \) or \( HNO_3/H_2SO_4 \).

### Step 1: Identify possible compounds with the formula \( C_8H_{10} \)
The molecular formula \( C_8H_{10} \) suggests that the compound is likely an aromatic hydrocarbon, specifically a derivative of benzene. The common candidates include:
- Toluene (methylbenzene)
- Dimethylbenzenes (1,2-dimethylbenzene, 1,3-dimethylbenzene, 1,4-dimethylbenzene)

### Step 2: Analyze the isomer formation during electrophilic substitution
Electrophilic substitution reactions on aromatic compounds typically yield different isomers depending on the substituents already present on the benzene ring.

1. **Toluene**: Would yield multiple isomers due to the different positions where chlorine or nitro groups can attach.
2. **1,2-Dimethylbenzene (o-xylene)**: Would yield three isomers.
3. **1,3-Dimethylbenzene (m-xylene)**: Would yield three isomers.
4. **1,4-Dimethylbenzene (p-xylene)**: Would yield only two isomers because the two methyl groups are positioned opposite each other on the benzene ring.

### Step 3: Conclusion
Among the candidates, **1,4-dimethylbenzene (p-xylene)** is the only compound that will yield only two isomers upon electrophilic substitution. The two isomers formed will be:
- 1,4-dichlorobenzene (from \( Cl_2/FeCl_3 \))
- 1,4-dinitrobenzene (from \( HNO_3/H_2SO_4 \))

Thus, the compound with the molecular formula \( C_8H_{10} \) that gives only two isomers on electrophilic substitution is **p-dimethylbenzene**.

### Final Answer
The answer is **p-dimethylbenzene (1,4-dimethylbenzene)**.

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