12.10 Explain why alkyl groups act as electron donors when attached to a system.
When an alkyl group is attached to the system, it acts as an electron donor group by the property of hyperconjugation.
for example in propene,
In figure, you can see that the sigma electrons of C-H bonds of the alkyl group are delocalised because of the partially overlapping of the sigma bond orbital with the empty p orbital of the bond of the adjacent carbon atom. It is also known as no-bond resonance.