Which of the following can undergo electrophilic substitution when treated with nitrous acid at `0^(@)C`?

Options

A. `C_(6)H_(5)NH_(2)`
B. `C_(6)H_(5)NHCH_(3)`
C. `C_(6)H_(5)N(CH_(3))_(2)`
D. None of the above

Answer

To determine which of the following amines can undergo electrophilic substitution when treated with nitrous acid at 0°C, we will analyze the behavior of primary, secondary, and tertiary amines with nitrous acid.

### Step-by-Step Solution:

1. **Understanding the Reaction with Nitrous Acid**:
- Nitrous acid (HNO2) reacts with amines to form diazonium salts. The general reaction for a primary amine (RNH2) is:
\[ RNH2 + HNO2 \rightarrow R-N2^+ + H2O \]
- The diazonium ion (R-N2^+) can undergo electrophilic substitution reactions.

2. **Analyzing Primary Amines**:
- When a primary amine (like aniline) is treated with nitrous acid, it forms a diazonium salt:
\[ C6H5NH2 + HNO2 \rightarrow C6H5N2^+Cl^- + H2O \]
- In this case, the electrophile (N2^+) does not participate in electrophilic substitution on the benzene ring. Therefore, primary amines do not undergo electrophilic substitution.

3. **Analyzing Secondary Amines**:
- Secondary amines (like N-methyl aniline) also react with nitrous acid but do not form stable diazonium salts. Instead, they form N-nitroso compounds:
\[ R2NH + HNO2 \rightarrow R2N-N=O \]
- Again, the electrophile does not add to the benzene ring, so secondary amines do not undergo electrophilic substitution.

4. **Analyzing Tertiary Amines**:
- Tertiary amines (like N,N-dimethyl aniline) react with nitrous acid to form a nitroso compound:
\[ R3N + HNO2 \rightarrow R3N-N=O \]
- In this case, the nitroso group (NO) can act as an electrophile and can undergo electrophilic substitution on the benzene ring:
\[ C6H5N(CH3)2 + HNO2 \rightarrow C6H5-N=O + H2O \]
- The nitroso group can substitute on the benzene ring, leading to the formation of a nitroso-substituted product.

5. **Conclusion**:
- Among primary, secondary, and tertiary amines, only tertiary amines can undergo electrophilic substitution when treated with nitrous acid at 0°C.

### Final Answer:
The correct answer is **Tertiary Amines**.