Which option is NOT a structural isomer of C4H8?

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Multiple Choice

Which option is NOT a structural isomer of C4H8?

The concept tested is structural (constitutional) isomerism: molecules that share the same formula but differ in how the atoms are connected. For C4H8, there’s one degree of unsaturation, so the molecule can be a straight-chain or branched alkene, or a cyclic structure, all with the same formula. But-1-ene fits as a straight-chain alkene with the double bond at the first carbon, giving C4H8. But-2-ene is also a straight-chain alkene, just with the double bond located between the second and third carbons, still C4H8. Cyclobutane provides a ring form of C4H8, maintaining the same carbon and hydrogen count but changing connectivity from a chain to a ring. Isobutane, however, is C4H10, a saturated alkane with two extra hydrogens, so it does not have the same molecular formula as C4H8 and cannot be a structural isomer of it.

The concept tested is structural (constitutional) isomerism: molecules that share the same formula but differ in how the atoms are connected. For C4H8, there’s one degree of unsaturation, so the molecule can be a straight-chain or branched alkene, or a cyclic structure, all with the same formula.

But-1-ene fits as a straight-chain alkene with the double bond at the first carbon, giving C4H8. But-2-ene is also a straight-chain alkene, just with the double bond located between the second and third carbons, still C4H8. Cyclobutane provides a ring form of C4H8, maintaining the same carbon and hydrogen count but changing connectivity from a chain to a ring. Isobutane, however, is C4H10, a saturated alkane with two extra hydrogens, so it does not have the same molecular formula as C4H8 and cannot be a structural isomer of it.

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