Question

Choose the more stable alkene in each of the following pairs. Explain your reasoning. (a) 1-Methylcyclohexene or 3-methylcyclohexene (b) Isopropenylcyclopentane or allylcyclopentane (c) or

• A. 1-Methylcyclohexene
• B. 3-methylcyclohexene
• C. Isopropenylcyclopentane
• D. Allylcyclopentane
• E. The first figure
• F. The second figure

Answer 1

(a) 1-Methylcyclohexene (b) Isopropenylcyclopentane (c) The first figure

Answer 2

The stability of alkenes generally increases with the number of alkyl substituents attached to the double bond carbons. This is due to hyperconjugation and the inductive effect of alkyl groups. A tetrasubstituted alkene is more stable than a trisubstituted alkene, which is more stable than a disubstituted alkene, which is more stable than a monosubstituted alkene.

(a) 1-Methylcyclohexene is a trisubstituted alkene because the double bond carbons are attached to one methyl group and two ring carbons. 3-methylcyclohexene is a disubstituted alkene as the methyl group is on a carbon adjacent to the double bond, and the double bond carbons are only substituted by ring carbons. Therefore, 1-methylcyclohexene is more stable.

(b) Isopropenylcyclopentane has the structure Cyclopentyl-C(CH₃)=CH₂. The double bond carbons are substituted by a methyl group and the cyclopentyl group, making it a disubstituted alkene. Allylcyclopentane has the structure Cyclopentyl-CH₂-CH=CH₂. The double bond carbons are substituted by one alkyl group (the CH₂ attached to the ring) and two hydrogen atoms, making it a monosubstituted alkene. Therefore, isopropenylcyclopentane is more stable.

(c) The first figure represents 1-methylcyclohexene, which is a trisubstituted alkene. The second figure represents 3-methylcyclohexene, which is a disubstituted alkene. Therefore, the first figure represents the more stable alkene.