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: Understanding the Concept:
The problem asks for the number of structural isomers of C₆H₁₀ that are 1-alkynes (also known as terminal alkynes). A 1-alkyne is a hydrocarbon with a carbon-carbon triple bond at the end of the carbon chain (i.e., between carbon 1 and carbon 2). We need to find all possible arrangements of the 6 carbon atoms that satisfy this condition.
Step 2: Systematic Approach to Finding Isomers:
We will consider different parent carbon chain lengths and place the triple bond at position 1.
1. Parent Chain: Hexane (6 carbons)
The only possibility is a straight chain with the triple bond at the end.
\[ \text{HC}\equiv\text{C}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_3 \] Name: Hex-1-yne. (1 isomer)
2. Parent Chain: Pentane (5 carbons)
We have a 5-carbon chain with the triple bond at C-1, and one methyl group as a substituent. The methyl group cannot be on C-1 or C-2. - Place methyl on C-3:
\[ \text{HC}\equiv\text{C}-\underset{\text{CH}_3}{\underset{|}{C}}\text{H}-\text{CH}_2-\text{CH}_3 \] Name: 3-Methylpent-1-yne. (1 isomer) - Place methyl on C-4:
\[ \text{HC}\equiv\text{C}-\text{CH}_2-\underset{\text{CH}_3}{\underset{|}{C}}\text{H}-\text{CH}_3 \] Name: 4-Methylpent-1-yne. (1 isomer)
3. Parent Chain: Butane (4 carbons)
We have a 4-carbon chain with the triple bond at C-1, and two methyl groups as substituents. The substituents cannot be on C-1 or C-2. Both must be on C-3. - Place two methyls on C-3:
\[ \hspace{1cm}\text{CH}_3
\hspace{1.2cm}|
\text{HC}\equiv\text{C}-\underset{\text{CH}_3}{\underset{|}{C}}-\text{CH}_3 \] Name: 3,3-Dimethylbut-1-yne. (1 isomer)
(Placing one ethyl group on the butane chain is not possible for a 1-alkyne without extending the parent chain).
Step 3: Total Count:
Summing up the isomers from each parent chain: 1 (from hexane) + 2 (from pentane) + 1 (from butane) = 4 isomers.
Step 4: Final Answer:
There are 4 possible 1-alkyne isomers for the formula C₆H₁₀. Therefore, option (D) is correct.