When an alkyl group is attached to a ∏ system, it acts as an electron-donor group by the process of hyper conjugation. To understand this concept better, let us take the example of propene.

In hyper conjugation, the sigma electrons of the C-H bond of an alkyl group are delocalised. This group is directly attached to an atom of an unsaturated system. The delocalisation occurs because of the partial overlap of a sp³-s sigma bond orbital with an empty p orbital of the ∏ bond of an adjacent carbon atom.

The process of hyper conjugation in propene is shown as :

This type of overlap leads to delocalisation (also known as no-bond resonance) of the ∏ electrons, making the molecule more stable.