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,it acts as an electron-donor group by the process of hyperconjugation.
In hyperconjugation,the $\sigma$ electrons of the $C-H$ bond of an alkyl group are delocalized. This group is directly attached to an atom of an unsaturated system. The delocalization occurs because of a partial overlap of an $sp^3-s$ $\sigma$ bond orbital with an empty $p$ orbital of the $\pi$ bond of an adjacent carbon atom.
This type of overlap leads to a delocalization (also known as no-bond resonance) of the $\pi$ electrons,making the molecule more stable. This effect is illustrated by the hyperconjugative structures of propene,where the $\sigma$ electrons of the $C-H$ bond shift to form a $\pi$ bond,resulting in the displacement of the existing $\pi$ electrons.