: 230
Hyperconjugation is the delocalization of sigma C-H bond electrons into an adjacent empty or partially filled p-orbital. It requires alpha-hydrogen atoms next to an sp2 center (double bond, carbocation, or radical). The number of hyperconjugative structures equals the number of alpha-C-H bonds — tert-butyl cation has 9 (3 CH3 groups x 3 H each), making it the most stable simple carbocation. Hyperconjugation explains several phenomena: (1) Alkene stability increases with substitution (Zaitsev's rule basis), (2) trans-2-butene is more stable than cis-2-butene, (3) C-C bonds adjacent to C=C are shortened (1.50 vs 1.54 Angstrom), and (4) Baker-Nathan order (CH3 > C2H5 > i-Pr > t-Bu for electron donation into pi systems — opposite to +I order). The Baker-Nathan order arises because methyl has 3 alpha-H for hyperconjugation while tert-butyl has zero alpha-H despite stronger +I. In electrophilic aromatic substitution rates, this order is sometimes observed when hyperconjugation dominates over induction. Reverse hyperconjugation (C-X sigma bonds donating into electron-deficient systems) is a minor effect primarily of theoretical interest for JEE.