Question

Choose the correct statement(s) regarding reverse hyperconjugation :

• A. It is a destabilizing effect.
• B. Takes place when $\alpha$-hydrogens are there in a conjugated system.
• C. The electron density flows from $\sigma$-orbital to empty p-orbital.
• D. $Ph-CC{l}_3$ shows $+H$ effect (hyperconjugation) due to presence of $\alpha$-chlorines.

Answer 1

The correct option is A It is a destabilizing effect.

Reverse hyperconjugation is the donation of electron density from a filled $\pi$ orbital to a neighbouring ${\sigma }^{\ast }$-orbital.
It is a destabilizing effect.
In reverse hyperconjugation, the electron density flows in the opposite direction (from $\pi$-orbital to empty ${\sigma }^{\ast }$-orbital) than it does in the more common hyperconjugation (from $\sigma$-orbital to empty p-orbital).

$Ph-CC{l}_3$ shows reverse hyperonjugation due to presence of chlorine as shown :