What is the structural isomer of 2-Methylprop-1-ene?

The name "prop-1-ene" indicates a 3-carbon chain with a double bond between the first and second carbons.
The "2-methyl" indicates a methyl group (CH$_3$) attached to the second carbon.

The structure is:

$$\text{CH}_2=\text{C}(\text{CH}_3)-\text{CH}_3$$

Counting the carbons: There are 3 carbons in the main chain and 1 carbon in the methyl group, totaling 4 carbons.
Counting the hydrogens:

• The first carbon (CH$_2$) has 2 hydrogens.
• The second carbon (C) is bonded to the first carbon (double bond), the third carbon (single bond), and the methyl group (single bond). It has no hydrogens.
• The third carbon (CH$_3$) has 3 hydrogens.
• The methyl group (CH$_3$) has 3 hydrogens. Total hydrogens = $2 + 0 + 3 + 3 = 8$. So, the molecular formula of 2-Methylprop-1-ene is $\text{C}_4\text{H}_8$.

A) but-1-yne
"But-1-yne" indicates a 4-carbon chain with a triple bond at the first carbon. Alkynes have the general formula $\text{C}_n\text{H}_{2n-2}$. For $n=4$, the formula is $\text{C}_4\text{H}_{2(4)-2} = \text{C}_4\text{H}_6$. This is not $\text{C}_4\text{H}_8$.

B) 2-methylbut-1-ene
"But-1-ene" indicates a 4-carbon chain with a double bond at the first carbon. "2-methyl" indicates a methyl group at the second carbon.
The structure is:

$$\text{CH}_2=\text{C}(\text{CH}_3)-\text{CH}_2-\text{CH}_3$$

Counting carbons: 4 carbons in the main chain + 1 carbon in the methyl group = 5 carbons.
Alkenes have the general formula $\text{C}_n\text{H}_{2n}$. For $n=5$, the formula is $\text{C}_5\text{H}_{10}$. This is not $\text{C}_4\text{H}_8$.

C) 3-methylprop-1-yne
"Prop-1-yne" indicates a 3-carbon chain with a triple bond at the first carbon. "3-methyl" indicates a methyl group at the third carbon.
The structure would be:

$$\text{CH}\equiv\text{C}-\text{CH}(\text{CH}_3)-\text{CH}_3$$

This molecule has a longest chain of 4 carbons containing the triple bond, making it 3-methylbut-1-yne.
It is an alkyne with 4 carbons, so its molecular formula is $\text{C}_4\text{H}_{2(4)-2} = \text{C}_4\text{H}_6$. This is not $\text{C}_4\text{H}_8$.

D) but-2-ene
"But-2-ene" indicates a 4-carbon chain with a double bond at the second carbon.
The structure is:

$$\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3$$

Counting carbons: 4 carbons.
Counting hydrogens: $3 + 1 + 1 + 3 = 8$ hydrogens.
So, the molecular formula of but-2-ene is $\text{C}_4\text{H}_8$.

Both 2-Methylprop-1-ene and but-2-ene have the molecular formula $\text{C}_4\text{H}_8$. Since they have the same molecular formula but different structural arrangements (one is branched, the other is a straight chain), they are structural isomers.

The final answer is $\boxed{\text{D}}$