Question

Which of the following is NOT terminal alkyne ?

• A. Ethyne
• B. Propyne
• C. But - 1 - yne
• D. But - 2 - yne

Hint:

A terminal alkyne has its triple bond at the end of the chain, so it has a C-H on a triple-bonded carbon.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
A terminal alkyne has the triple bond at the end of the carbon chain, so one triple-bonded carbon carries a hydrogen. Its general form is \(\text{R-C}\equiv\text{C-H}\).

Step 2: Key Formula or Approach:
Write each structure and see whether the triple bond is at the chain end.

Step 3: Detailed Explanation:
Ethyne, \(\text{HC}\equiv\text{CH}\): both carbons carry H. It is terminal.
Propyne, \(\text{CH}_3\text{C}\equiv\text{CH}\): one triple-bonded carbon carries H. It is terminal.
But-1-yne, \(\text{CH}_3\text{CH}_2\text{C}\equiv\text{CH}\): the triple bond is between C1 and C2, with H on C1. It is terminal.
But-2-yne, \(\text{CH}_3\text{C}\equiv\text{CCH}_3\): the triple bond sits between C2 and C3, and both triple-bonded carbons carry methyl groups. There is no acidic alkyne hydrogen, so it is an internal alkyne.

Final Answer:
But-2-yne is not a terminal alkyne, option (D). \[ \boxed{\text{But-2-yne (D)}} \]