Question

hey guys..........
●Explain why alkyl groups act as electron donors when attached to a
$\pi \: system$
????
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Answer 1

When an alkyl group is attached to a π system, it acts as an electron-donor group by the process of hyperconjugation , let us take the example of propene.  This type of overlap leads to a delocalisation (also known as no-bond resonance) of the π electrons, making the molecule more stable. 
In simple words Because the compound tends to start showing the hypercnjugation effect and +I effect which itself explains that they are the capable of sshowing and acting as the electron donor.

They tends to donate the elctrons from those pi bonds carbon atoms.

The above diagram is only for your understanding purpose