Hyperconjugation (or no-bond resonance) is the delocalization of $\sigma$ electrons of $\alpha -C-H$ bonds.

• Mechanism: It involves the overlap of the filled $\sigma$ bonding orbital of a $C-H$ bond with an adjacent empty $p$ orbital (in carbocations) or a partially filled $p$ orbital (in free radicals) or an antibonding ${\pi }^{\ast }$ orbital (in alkenes).
• This effect stabilizes the system and explains why $3^{\circ }$ carbocations (having more $\alpha -H$ for hyperconjugation) are more stable than $1^{\circ }$.