Stability of Alkenes

In general the more substituted an alkene, the more stable it is.  A more substituted alkene has more carbon containing substituents attached to its sp² carbons.  It is important that you understand this since you will utilize this to predict products of elimination reactions.  There are three factors that influence alkene stability;

1. Sterics
2. Degree of substitution
3. Conjugation

Although we've been saying that alkenes have restricted movement or rotation about their C=C double bonds, when treated with acid they can isomerize.  If you treated pure (cis)-but-2-ene with acid it would slowly equilibrate with its stereoisomeric "brother", (trans)-2-butene. 

At equilibrium you would obtain about 24% of the cis versus 76% of the trans.  Thus the (trans)-but-2-ene is more stable than (cis)-but-2-ene.  You could use ΔG=-RTlnK and obtain ΔG=-2.8 kj/mol.  Since ΔG=ΔH-TΔS and ΔS is likely small (i.e. no phase changes, same # of molecules on each side of reaction, not breaking any rings) the ΔG=ΔH.  So (trans)-but-2-ene is about 2.8 kj/mol more stable (lower in energy) than (cis)-but-2-ene.  This is mainly due to lack steric interaction between the methyl groups in the trans stereoisomer. 

While this is a decent method of assessing stability, a more general method of assessing alkene stability utilizes reaction calorimetry and measuring the heat released upon combustion of hydrogenation.  A reaction calorimeter would be used to measure this heat of reaction.

For example if you hydrogenated in separate experiments (trans)-but-2-ene and then (cisi)-but-2-ene you would see that the cis isomer releases about 3.9 kj/mol more energy than the trans isomer.  Thats pretty close to our first estimate of 2.8 kj/mol we estimated from the equilibrium constant.

The ΔH of hydrogenation has been determined for many alkenes.  The following data clearly show that more highly substituted alkenes are more stable than less substituted alkenes. 

This is usually attributed to;

• hyperconjugation
• bond stability associated bonds to sp² carbons and
• The electron donating ability of alkyl groups - an alkyl group is an EDG (Electron Donating Group).