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In which of the following pairs does compound I have a higher heat of hydrogenation than compound II? (A) 1,3-cyclohexadiene and 1,4-cyclohexadiene (B) Benzene and cyclohexene (C) trans-2-butene (CH3-CH=CH-CH3) and 1-butene (CH3-CH2-CH=CH2) (D) Methylenecyclohexane and 1,2-dimethylcyclohex-1-ene

1. Cyclohexadiene (two double bonds) and cyclohexadiene (two double bonds)
2. Benzene and cyclohexene
3. CH3-CH=CH-CH3 and CH3-CH2-CH=CH2
4. Methylenecyclohexane and dimethylcyclohexene

Correct answer: Methylenecyclohexane and dimethylcyclohexene

Solution

Methylenecyclohexane has a monosubstituted exocyclic double bond (less stable) while 1,2-dimethylcyclohex-1-ene has a trisubstituted endocyclic double bond (more stable). Therefore methylenecyclohexane has a higher heat of hydrogenation.

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