Relative Stabilities of Alkenes

Relative alkene stability is determined by comparing their heats of hydrogenation, which is the energy released when an alkene reacts with hydrogen. The smaller the heat of hydrogenation, the higher the stability of the alkene. This measurement allows chemists to rank alkenes by their thermodynamic stability and predict their reactivity in introduction to electrophilic addition reactions of alkenes.

Tetrasubstituted alkenes are more stable because they have a higher ratio of sp2–sp3 bonds compared to sp3–sp3 bonds. The sp2–sp3 bond is lower in energy and stronger due to higher s-electron contribution. Additionally, more alkyl substituents provide greater hyperconjugation, a stabilizing interaction between the delocalized pi-bond electron density and adjacent carbon–hydrogen sigma bonds on substituents.

Hyperconjugation is a stabilizing interaction between the delocalized electron density of the pi bond and carbon–hydrogen sigma bonds on alkyl substituents. Alkenes with more alkyl substituents experience greater hyperconjugation, leading to increased stability. For example, 2,3-dimethyl-2-butene with four substituents is more stable than propene with just one substituent due to enhanced hyperconjugation effects.

Trans alkenes are more stable than their cis isomers because cis isomers experience steric strain from substituent crowding on the same side of the double bond. Trans alkenes typically have lower heats of hydrogenation, reflecting their greater thermodynamic stability. The spatial arrangement of substituents significantly influences overall alkene stability.

The sp2–sp3 bond is stronger and lower in energy than the sp3–sp3 bond because it has a higher contribution of s electrons, which are inherently lower in energy. This orbital composition difference makes sp2–sp3 bonds more favorable energetically. Highly substituted alkenes benefit from this bond strength advantage, contributing to their increased stability compared to less-substituted isomers.

2-butene is more stable than 1-butene because it has two smaller substituents across the double bond, while 1-butene has only one substituent. The additional substitution in 2-butene increases the ratio of sp2–sp3 bonds and provides greater hyperconjugation. This makes 2-butene release less energy during hydrogenation, indicating superior thermodynamic stability.

The primary factor is steric strain caused by substituent interactions. In cis alkenes, substituents on the same side of the double bond experience crowding and repulsion, destabilizing the molecule. Trans alkenes avoid this steric strain by positioning substituents on opposite sides, resulting in lower heats of hydrogenation and greater overall stability.