Which hydrocarbon does NOT exhibit structural isomerism?

Analyze the question: We need the hydrocarbon that has only ONE possible structure, so it has no structural isomers.

Concept: Structural isomers have the same molecular formula but different arrangements of atoms (chain, position, or functional-group isomerism). We count the possible structures for each formula.

Solution:

• A. C3H6: The possible structures are propene (CH2=CH-CH3) and cyclopropane. If we consider only alkenes, propene is the only one. But cyclopropane has the same formula, so strictly C3H6 does have isomers (ring-chain isomerism). In the usual secondary-school treatment, though, C3H6 is taken as the alkene propene, which has no chain or position isomers.
• B. C4H6: This has 1-butyne, 2-butyne, 1,3-butadiene, and others. It shows isomerism.
• C. C4H8: This has 1-butene, 2-butene, and 2-methylpropene. It shows isomerism.
• D. C4H10: This has butane and 2-methylpropane. It shows isomerism.
• E. C5H12: This has pentane, 2-methylbutane, and 2,2-dimethylpropane. It shows isomerism.

Compare with the key: Options B to E clearly have multiple structural isomers. Option A is the only one that, at the secondary-school level (considering the alkene series), has just one structure, propene.

A small caution: strictly speaking, cyclopropane is a ring-chain isomer of propene, so A is the best answer by the exam's intended reasoning rather than an absolute one. Every other option clearly has chain or position isomers, so A is the one to choose.

Answer: A (C3H6)

Tip: Memorize the isomer counts for alkanes: CH4, C2H6, C3H8 have 1 each; C4H10 has 2; C5H12 has 3; C6H14 has 5. For alkenes, the first with isomers is C4H8 (3 isomers), so C2H4 and C3H6 are the ones with none in the alkene series.