Question

What is the meaning of Reverse Hyperconjugation?

Answer 1

In the formalism that separates bonds into  and  types, hyperconjugation is the interaction of -bonds (e.g. C-H, C-C, etc.) with a  network. This interaction is customarily illustrated by contributing structures, e.g. for toluene , sometimes said to be an example of "heterovalent" or "sacrificial hyperconjugation", so named because the contributing structure contains one two-electron bond less than the normal Lewis formula for toluene