Question

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

Answer 1

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

In hyperconjugation, 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 a partial overlap of a $sp ^3 -s$ sigma bond orbital with an empty $p$ orbital of the $\pi$¬ bond of an adjacent carbon atom. The process of hyperconjugation in propene is shown as follows: