Concept:The formula \( \text{C}_4\text{H}_8 \) matches the alkene general formula \( \text{C}_n\text{H}_{2n} \). Past paper conventions often ask specifically for open-chain structural isomers unless specified otherwise.
Solution:- The acyclic (alkene) structural isomers for \( \text{C}_4\text{H}_8 \) are:
- 1. 1-butene (\( \text{CH}_2=\text{CH--CH}_2\text{--CH}_3 \))
- 2. 2-butene (\( \text{CH}_3\text{--CH}=\text{CH--CH}_3 \))
- 3. 2-methylpropene (isobutylene)
- According to the official exam key provided, the intended answer considers these 3 structural alkene isomers.
Why other options are incorrect:While including cis-trans stereoisomers makes 4, and including cycloalkanes (cyclobutane, methylcyclopropane) makes 6, standard local test keys often only count the structural alkene isomers, landing on 3.
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