Question

An alkyne has the molecular formula C₆H₁₀. The number of 1-alkyne isomers (excluding stereoisomers) possible for it is

• A. 2
• B. 5
• C. 3
• D. 4

Hint:

To find all structural isomers systematically, start with the longest possible straight chain and then progressively shorten it, adding the removed carbons as branches in all unique positions.

Answer 1

The Correct Option is D

Step 1: Identify the Requirement: Formula: \( \text{C}_6\text{H}_{10} \) (Degree of unsaturation = 2, consistent with 1 triple bond). Requirement: "1-alkyne". This means the triple bond must be at the terminal position (\( -\text{C}\equiv\text{CH} \)).
Step 2: Determine Structure Framework: The structure must be \( \text{R}-\text{C}\equiv\text{CH} \). Total carbons = 6. Triple bond carbons = 2. So, the alkyl group \( \text{R} \) must have \( 6-2=4 \) carbon atoms (\( \text{C}_4\text{H}_9 \), Butyl group).
Step 3: Count Isomers of Butyl Group (-R): We need to find how many structural isomers exist for the butyl group attached to the alkyne unit. The butyl group (\( \text{C}_4\text{H}_9- \)) has 4 isomers: 1. n-Butyl: \( \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2- \) Compound: 1-Hexyne. 2. Isobutyl: \( (\text{CH}_3)_2\text{CH}-\text{CH}_2- \) Compound: 4-Methyl-1-pentyne. 3. sec-Butyl: \( \text{CH}_3-\text{CH}_2-\text{CH}(\text{CH}_3)- \) Compound: 3-Methyl-1-pentyne. 4. tert-Butyl: \( (\text{CH}_3)_3\text{C}- \) Compound: 3,3-Dimethyl-1-butyne.
Step 4: Total Count: There are 4 distinct structural isomers.