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Organic and biochemistryNational 2025

How many stable isomers of $\ce{C4H8}$ are there?

11% of national qualifiers got this right

Explanation

draft

$\ce{C4H8}$ has molecular formula $\text{C}_4\text{H}_8$ , giving one degree of unsaturation (either a double bond or a ring). Count all distinct structures:

Alkenes (C=C, chain isomers):
- 1-butene: $\ce{CH2=CHCH2CH3}$
- cis-2-butene
- trans-2-butene (cis/trans are distinct stable isomers)
- 2-methylpropene (isobutylene): $\ce{(CH3)2C=CH2}$

That's 4 alkene isomers.

Cycloalkanes (ring, no double bond):
- cyclobutane
- methylcyclopropane

That's 2 ring isomers.

Total: $4 + 2 = 6$ stable isomers of $\ce{C4H8}$ .

Counts only 3 — likely lists just 1-butene, 2-butene (treating cis/trans as one), and isobutylene, forgetting to split cis/trans and omitting both cycloalkanes.

Counts 4 — captures only the four alkene isomers (1-butene, cis-2-butene, trans-2-butene, isobutylene) but forgets the two cyclic isomers (cyclobutane and methylcyclopropane).

Counts 5 — includes the four alkenes plus only one cycloalkane (usually cyclobutane), missing methylcyclopropane as a separate ring isomer.

Correct: 4 alkene isomers (1-butene, cis-2-butene, trans-2-butene, isobutylene) plus 2 cycloalkanes (cyclobutane, methylcyclopropane) gives 6 total stable isomers.

Concept · Structural and stereoisomers of alkenes/cycloalkanes (C4H8)