Stability of Alkenes

Not all alkenes were created equal. Like carbocations, alkenes are stabilized through a phenomenon called hyperconjugation

Hyperconjugation allows adjacent -R groups (mostly C-C and C-H σ-bonds) to create shared molecular orbitals with π-bonds, stabilizing the bond.

So basically, the more –R groups attached to the double bond, the more stable the double bond will be.

Specifically, when it comes to di-substituted bonds, the order os stability is gem > trans >  cis.

  • trans > cis due to steric hindrance. Groups have less freedom of movement in the cis position.
  • gem > trans due to better hyperconjugation. The full explanation for this trend is beyond the level of this course. Just memorize it.

Example: Rank the following alkenes in order of lowest to highest heat of combustion.