Explain why alkyl groups act as electron donors when attached to a $\pi$ system.

(N/A) When an alkyl group is attached to a $\pi$ system,the $\sigma$ electrons of the $C-H$ bond can delocalize into the adjacent $\pi$ orbital.
This phenomenon is known as hyperconjugation.
Specifically,the $\sigma$ orbital of the $C-H$ bond aligns parallel to the empty or partially filled $2p$ orbital of the $\pi$ system.
This allows for the partial overlap and donation of electron density from the $C-H$ $\sigma$ bond into the $\pi$ system,thereby stabilizing the system through resonance.