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Hyperconjugation is a phenomenon in chemistry where electrons in sigma bonds, typically adjacent to an atom with an available p orbital or a pi system, are delocalized. This delocalization involves the interaction of the filled sigma bonding orbital with the empty or partially filled orbital of the adjacent atom or group. It is often described as a "no-bond resonance" effect, as it can be visualized as a partial sharing of electrons between the sigma bond and the adjacent orbital.

The most common example of hyperconjugation is observed in alkyl-substituted carbocations, alkenes, and radicals. In a carbocation, the filled sigma orbitals of adjacent alkyl groups overlap with the empty p orbital of the positively charged carbon. This overlap stabilizes the carbocation by spreading the positive charge over a larger area. Similarly, in alkenes, hyperconjugation can occur between sigma bonds of adjacent alkyl substituents and the pi system, contributing to the stability of the double bond. For radicals, hyperconjugation involves the overlap of filled sigma orbitals with the half-filled p orbital of the radical center.

Hyperconjugation is a weaker interaction than true resonance but is significant in explaining many chemical properties. It influences the reactivity and stability of organic molecules, affecting reaction rates, equilibrium positions, and conformational preferences. For instance, the observed stability order of tertiary carbocations over secondary and primary carbocations is largely attributed to hyperconjugation. The effect is also crucial in understanding the reactivity of allylic and benzylic systems. Computational studies and spectroscopic evidence support the existence and importance of hyperconjugation in organic chemistry.