Ethyne adds on HCl to first give a-

Options

A. Carbanion
B. A free radical
C. A vinylic cation
D. A biradical

Answer

To solve the question about the addition of HCl to ethyne, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants**: We have ethyne (C2H2) and hydrochloric acid (HCl). Ethyne has a triple bond between the two carbon atoms.

2. **Write the Structure of Ethyne**: Ethyne can be represented as:
\[ \text{H-C} \equiv \text{C-H} \]

3. **Addition of HCl**: When HCl is added to ethyne, the hydrogen (H) from HCl acts as an electrophile and the triple bond (π bond) in ethyne acts as a nucleophile.

4. **Formation of the Intermediate**: The π bond between the two carbon atoms will break, and one of the carbon atoms will bond with the hydrogen atom from HCl. This results in the formation of a vinylic cation (a positively charged intermediate):
\[ \text{CH}_2\text{=C}^+\text{H} \]
This is a vinylic cation because the positive charge is on a carbon atom that is part of a double bond.

5. **Nucleophilic Attack by Chloride Ion**: The chloride ion (Cl-) from HCl will then attack the positively charged carbon atom in the vinylic cation. This results in the formation of the final product:
\[ \text{CH}_2\text{=CCl-H} \]
Which can be simplified to:
\[ \text{CH}_2\text{=CHCl} \]

6. **Final Product**: The final product of the reaction is vinyl chloride (CH2=CHCl).

7. **Conclusion**: The intermediate formed during this reaction is a vinylic cation.

### Final Answer:
Ethyne adds on HCl to first give a vinylic cation.

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