Which of the following is an isomer of 2-methylprop-1-ene?

The name "prop" indicates a 3-carbon chain. "1-ene" indicates a double bond starting at the first carbon. "2-methyl" indicates a methyl group ($\text{CH}_3$) on the second carbon.
The structure is:

Counting the atoms:

• Carbon atoms: 3 (in the main chain) + 1 (in the methyl group) = 4 carbon atoms.
• Hydrogen atoms: 2 (on $\text{CH}_2$) + 3 (on $\text{CH}_3$ attached to C2) + 3 (on $\text{CH}_3$ at the end) = 8 hydrogen atoms. The molecular formula for 2-methylprop-1-ene is $\text{C}_4\text{H}_8$.

• A) 3-methyl but-1-ene: "but" means 4 carbons in the main chain, plus 1 methyl group, totaling 5 carbons. Its formula is $\text{C}_5\text{H}_{10}$.
• B) 2-methyl but-1-ene: "but" means 4 carbons in the main chain, plus 1 methyl group, totaling 5 carbons. Its formula is $\text{C}_5\text{H}_{10}$.
• C) but-2-ene: "but" means 4 carbons in the main chain. "2-ene" means a double bond between the second and third carbons. The structure is: $$\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3$$ Counting the atoms:
  • Carbon atoms: 4 carbon atoms.
  • Hydrogen atoms: 3 (on first $\text{CH}_3$) + 1 (on first $\text{CH}$) + 1 (on second $\text{CH}$) + 3 (on second $\text{CH}_3$) = 8 hydrogen atoms. The molecular formula for but-2-ene is $\text{C}_4\text{H}_8$.
• D) pent-2-ene: "pent" means 5 carbons in the main chain. Its formula is $\text{C}_5\text{H}_{10}$.

An isomer has the same molecular formula but a different structural formula. Both 2-methylprop-1-ene and but-2-ene have the molecular formula $\text{C}_4\text{H}_8$.

The final answer is $\boxed{\text{C}}$.

One more to go today! Send it over.