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: Use the locant:
In IUPAC names, the locant tells where the triple bond starts. Locant 1 (or none for ethyne and propyne) means it begins at the chain end. Locant 2 means it is in the middle.

Step 2: Apply:
Ethyne: terminal. Propyne: terminal. But-1-yne: terminal.
But-2-yne has the triple bond starting at C2, flanked by a methyl on each side: $\text{CH}_3-\text{C}\equiv\text{C}-\text{CH}_3$.

Step 3: Test with chemistry:
Terminal alkynes give a precipitate with Tollens' reagent or react with sodium (they have an acidic $\equiv$C-H). But-2-yne has no such hydrogen, which confirms it is internal.

Final Answer:
Option (D), but-2-yne. \[ \boxed{\text{But-2-yne (D)}} \]