Hyperconjugaatio

Hyperconjugation is a phenomenon in chemistry where sigma electrons of a bond, typically adjacent to an atom with an available p orbital or pi system, are delocalized into that orbital or pi system. This delocalization is a stabilizing interaction. It is often described as a type of resonance, but it involves sigma bonds rather than pi bonds. The most common examples involve the interaction of C-H sigma bonds with an adjacent carbocation, radical, or alkene pi bond.

In the case of carbocations, hyperconjugation can be visualized as the overlap of a filled C-H sigma bond with the empty p orbital of the positively charged carbon. This electron donation from the sigma bond to the p orbital reduces the positive charge on the carbon atom, thereby stabilizing the carbocation. The more alpha-hydrogens (hydrogens on the carbon adjacent to the carbocation center), the greater the extent of hyperconjugation and thus the greater the stability of the carbocation. This explains why tertiary carbocations are more stable than secondary, and secondary are more stable than primary.

Similarly, hyperconjugation plays a role in the stability of free radicals, where sigma electrons can delocalize into the half-filled p orbital of the radical center. It also contributes to the stability of alkenes, where C-H sigma bonds adjacent to the pi system can overlap with the pi orbitals. This explains why more substituted alkenes are generally more stable. While often explained as overlap, it can also be understood as a consequence of the delocalization of electrons. Hyperconjugation is a significant factor in determining the relative stabilities of organic molecules and reaction intermediates.