Hyperkonjugiert

Hyperkonjugation is a phenomenon in chemistry that involves the delocalization of electrons that occurs when a bond to an adjacent, unoccupied antibonding molecular orbital is formed. This interaction is most commonly observed between a sigma bond and a pi system or an adjacent sigma bond. It is a stabilizing interaction that can be explained by the overlap of sigma bonding orbitals with adjacent empty or partially filled p orbitals or pi antibonding orbitals.

The effect is particularly noticeable in molecules with alkyl groups adjacent to unsaturated systems, such as carbocations, alkenes, and carbonyl groups. For instance, in a tertiary carbocation, the filled sigma orbitals of the adjacent methyl groups can overlap with the empty p orbital of the carbocation, delocalizing the positive charge and thus stabilizing the carbocation. This delocalization effectively spreads the charge over a larger area, making the species more stable.

Hyperconjugation is often considered an extension of conjugation, but it involves sigma orbitals rather than solely pi orbitals. The extent of hyperconjugation depends on the number of alpha hydrogens (hydrogens on the carbon atom directly attached to the atom involved in the pi system or carbocation). More alpha hydrogens generally lead to greater hyperconjugation and increased stability. This concept is important in understanding the relative stabilities of organic molecules and reaction intermediates, and it is often invoked to explain observed trends in reactivity and physical properties.