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: