7.6 Stability of Alkenes

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Stability of Alkenes

More Substituted Alkenes are More Stable

The general principle behind the basic stability of alkenes is the more substituted the alkene the more stable the alkene.  By more substituted we mean the number of carbon substituents.  In the C=C double bond of an alkene you have two sp2 hybridized carbon atoms.  Besides the double bond to each other they are both bonded to two additional atoms.  If the four additional atoms these two sp2 hybridized carbon atoms are bonded to are all carbon atoms, we say this alkene is tetra-substituted which is the greatest degree of substitution.  If all four of these atoms are hydrogen atoms we say this alkene is unsubstituted which is the least degree of substitution.

 

Earlier in the course you learned that the more substituted carbocation is also more stable and it turns out it is for the same reason...hyperconjugation.

carbocation stability trend

Hyperconjugation

The idea behind hyperconjugation stabilizing a carbocation is that there is an empty p-orbital perpendicular to the plan made by the three bonds coming from the sp2 hybridized (trigonal planar geometry) carbon atom.  If one of these bonds is to another carbon atoms then there will be some degree of overlap between the bonding molecular orbitals coming from one of these adjacent carbon atoms and the empty p-orbital of the carbocation (see image on the left below).  This overlap leads to delocalization and donation of electron density to the carbocation making it less positive and more stable.  But if one of these bonds coming from the sp2 hybridized carbon atom is to a hydrogen atom no such overlap occurs as the hydrogen is not bonded to any additional atoms.

 

The same occurs with the p-orbitals in the C=C double bond of an alkene.  Bonding to additional carbon atoms from the sp2 hybridized carbon atoms of the alkene will lead to hyperconjugation in exactly the same fashion which lowers the energy of the pi electrons thereby increasing their stability (see image on the right below).

hyperconjugation carbocation
hyperconjugation alkene stability