2-methylprop-1-ene is a structural isomer of?

Analysis

To solve this, we need to determine the molecular formula of 2-methylprop-1-ene and compare it with the molecular formulas of the given options. Structural isomers are compounds that have the same molecular formula but different structural arrangements of atoms.

Concept: Molecular Formula of Alkenes

• 2-methylprop-1-ene is an alkene.
• The general formula for acyclic alkenes (with one double bond) is $C_nH_{2n}$.
• "Prop" indicates a chain of 3 carbons, and "methyl" adds 1 carbon, making a total of 4 carbons ($n=4$).
• Therefore, the molecular formula is $C_4H_{2(4)} = \mathbf{C_4H_8}$.

To be an isomer, the other compound must also have the molecular formula $C_4H_8$.

Step-by-Step Solution

1. Analyze 2-methylprop-1-ene: • Structure: $CH_2=C(CH_3)_2$ • Count: 4 Carbons, 8 Hydrogens ($C_4H_8$).

2. Evaluate the Options: • A. But-1-yne: This is an alkyne (triple bond). General formula $C_nH_{2n-2}$. For $n=4$, it is $C_4H_6$. (Incorrect) • B. 2-methylbut-1-ene: This has 5 carbons (4 in the butane chain + 1 methyl group). Formula is $C_5H_{10}$. (Incorrect) • C. 3-methylprop-1-yne: This has 4 carbons but is an alkyne. Formula is $C_4H_6$. (Incorrect) • D. But-2-ene: This is an alkene with 4 carbons. Formula is $C_4H_8$. (Correct)

Conclusion

Since But-2-ene ($CH_3-CH=CH-CH_3$) has the same molecular formula ($C_4H_8$) as 2-methylprop-1-ene, they are structural isomers. Specifically, they are chain/positional isomers of each other.

Exam Tip

When asked for isomers, always count the number of carbon atoms first. If the number of carbons doesn't match, they cannot be isomers! If the carbon count matches, check the functional group (alkane, alkene, or alkyne) to ensure the hydrogen count matches the general formula.