Question

Which of the following pairs of alkenes is an example of position isomers?

• A. But-1-ene and 2-methylprop-1-ene
• B. But-1-ene and 2-methylbut-1-ene
• C. But-1-ene and but-2-ene
• D. But-2-ene and 2-methylprop-1-ene

Hint:

The IUPAC nomenclature makes finding position isomers very straightforward. If the root word is identical (like "but") indicating the same carbon skeleton, and only the number locant changes (like "1-ene" vs "2-ene"), it is a textbook example of a position isomer.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to identify which pair among the given options represents position isomers.

Step 2: Detailed Explanation:
Position isomers are molecules that share the exact same molecular formula and the identical parent carbon skeleton structure, but differ solely in the position of their functional group (such as a double bond).
Let's evaluate the given options:
1.

But-1-ene and 2-methylprop-1-ene: These differ in their carbon skeleton (straight chain vs. branched), making them chain isomers, not position isomers.
2.

But-1-ene and 2-methylbut-1-ene: These have completely different molecular formulas ($\text{C}_4\text{H}_8$ vs $\text{C}_5\text{H}_{10}$), so they are not isomers of each other at all.
3.

But-1-ene and but-2-ene: Both have the same molecular formula ($\text{C}_4\text{H}_8$) and a straight four-carbon chain skeleton. However, the double bond is located at the first carbon in but-1-ene and at the second carbon in but-2-ene. This perfectly fits the definition of position isomers.

Step 3: Final Answer:
The position isomers are but-1-ene and but-2-ene, matching option (C).