Hyperconjugation involves delocalization of C-H sigma bond electrons into an adjacent empty or partially filled p-orbital. It requires alpha-H atoms next to an sp2 center.

Applications:

1. Carbocation stability: More alpha-H = more hyperconjugative structures = more stable. (CH3)3C+ has 9 alpha-H atoms.
2. Alkene stability: More substituted alkenes are more stable (Zaitsev's rule). trans > cis for 2-butene.
3. Bond length shortening: C-C bond adjacent to C=C is shorter than a normal C-C single bond.
4. Baker-Nathan order: In electrophilic aromatic substitution, CH3 group is better activator than tert-butyl (hyperconjugation > inductive for +M donation into ring).