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Rank the following alkenes in decreasing order of heat of combustion: (I) 1-butene (terminal alkene, CH2=CH-CH2-CH3) (II) 2-methylpropene (isobutylene, (CH3)2C=CH2, two methyl groups on one double-bond carbon) (III) 2-butene (internal straight-chain alkene, CH3-CH=CH-CH3, trans) (IV) cis-2-butene (CH3-CH=CH-CH3, cis) All are C4 alkenes with 4 carbons.

1. II > III > IV > I
2. II > IV > III > I
3. I > III > IV > II
4. I > IV > III > II

Correct answer: I > IV > III > II

Solution

Heat of combustion decreases as alkene stability increases. More substituted double bonds are stabilised by hyperconjugation, so they burn with less energy release. cis isomers are less stable than trans isomers due to steric strain, so cis-2-butene (IV) has higher heat of combustion than trans-2-butene (III). Thus decreasing order of heat of combustion: I > IV > III > II.

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