organic chemistry - What is hyperconjugation?

What is hyperconjugation and how do you know if there is hyperconjugation in a compound? Which orbitals are involved in hyperconjugation?

Answer

In the ethyl cation one can write the following resonance structures

The structure on the right can actually be written 3 times since there are 3 hydrogens available. Note also that the "ethylenic-like" structure on the right involves an sp3-p bond (the C-H bond is sp3 hybridized and the cationic carbon has a p orbital), so it is not a true pi bond (p-p) like that found in ethylene. The resonance structure on the right is meant to show the "hyperconjugation" effect of the C-H bond (sp3-s) with the p orbital located on the adjacent cationic carbon. The drawing is a shorthand way of saying that when the C-H bond is rotated into the same plane as the p orbital on the cationic carbon, there can be a stabilizing overlap - and it is called "hyperconjugation". In the 2-propyl cation, there are 6 of these hyperconjugative resonance structures (there are 6 C-H bonds on the 2 methyl groups adjacent to the cationic carbon and in the t-butyl cation there are 9 such resonance structures. This can be used to explain why tertiary carbonium ions are more stable than secondary carbonium ions, which in turn are more stable than primary carbonium ions.