7.5 : יציבות יחסית של אלקנים

The energy released when an alkene reacts with hydrogen is called the heat of hydrogenation. 

The relative stability of alkenes is determined by comparing their heats of hydrogenation. The smaller the heat of hydrogenation, the higher the stability of the alkene.

Increasing the substitution across the double bond is found to decrease the heat of hydrogenation, revealing that the tetrasubstituted alkene is more stable than the tri-, di-, and monosubstituted alkenes.

The reason is that the more-substituted alkenes have a higher ratio of sp²–sp³ bonds than sp³–sp³ bonds. The sp²–sp³ bond is lower in energy and stronger than the sp³–sp³ bond since it has a higher contribution of s electrons, which are lower in energy.

Another factor responsible for the stability of substituted alkenes is the stabilizing interaction, called hyperconjugation, between the delocalized electron density of the pi bond and the carbon–hydrogen sigma bonds on the substituents. 

Alkyl substituents provide additional hyperconjugation and lead to more stability. Therefore, 2,3-dimethyl-2-butene, with four substituents across the double bond, is more stable than propene, which has just one substituent.   

Further, the stereochemistry of substituents around the double bonds also contributes to the stability of alkenes. trans alkenes typically have a lower heat of hydrogenation and are more stable than the cis isomers.

The reason lies in the steric strain produced by the interaction between the substituents present on the same side of the double bond in the cis isomer.