Illustrated Glossary of Organic Chemistry

Hyperconjugation: The interaction of an unoccupied or partially filled p orbital, π molecular orbital, or π* molecular orbital with an adjacent sigma bond. The sigma bond is usually a C-C or C-H bond. Explains how increasing degree of substitution stabilizes carbocations and radicals.

Hyperconjugation in the tert-butyl carbocation. The unoccupied p orbital (shown in red) on the carbon with the positive formal charge overlaps with an adjacent C-H sigma bond orbital. This increases electron delocalization and increases stability. Increasing the number of C-C or C-H bonds adjacent to the empty p orbital increases the number of stabilizing interactions, resulting in the correlation between degree of substitution of a carbocation (or radical) and its stability.