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What is the minimum number of carbon atoms required for an alkane, alkene, and alkyne, respectively, to exhibit chain isomerism?

1. 4, 4, 4
2. 4, 3, 3
3. 4, 4, 3
4. 4, 4, 5

Correct answer: 4, 4, 4

Solution

For alkanes, C4 gives n-butane and isobutane (different skeletons). For alkenes, C4 gives but-1-ene (or but-2-ene, linear chain) and 2-methylpropene (branched chain). For alkynes, C4 gives but-1-yne (linear) and 2-methylprop-1-yne (branched). All three classes need a minimum of 4 carbon atoms for chain isomerism.

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