7.5: Relative Stabilities of Alkenes
The relative stability of alkenes can be determined by comparing their heats of hydrogenation. The lower heat of hydrogenation indicates the more stable alkene. The three main factors determining the relative stability of alkenes are i) the number of substituents attached to the double-bond carbon atoms, ii) hyperconjugation, and iii) the stereochemistry of the double bond.
- Number of substituents across the double bond: An alkene with two smaller substituents is more stable than its isomer having one large substituent. For example, 2-butene is more stable compared to 1-butene. The highly substituted alkenes have a higher ratio of sp2–sp3 bonds, which are lower in energy and are stronger as compared to the sp3–sp3 bonds. Thus, a tetrasubstituted alkene is more stable than a tri-, di-, or monosubstituted alkene.
- Hyperconjugation: Hyperconjugation is a stabilizing interaction of the delocalized electron density between the carbon–carbon π bond and the adjacent carbon–hydrogen σ bonds on the substituent. Thus, a higher number of alkyl substituents across the double bond suggests greater hyperconjugation, resulting in a more stable alkene.
- Stereochemistry: The spatial arrangement of the substituents also contributes to the stability of alkenes. The cis isomer exhibits steric strain because of the crowding of the substituents on the same side of the double bond and therefore is less stable compared to the trans isomer.