Electronic Displacement Effects - Resonance Effect

Description:

It is defined as the polarity produced in molecule  by interaction of two π-bonds (resonance effect) or a π bond and lone pair of electrons (mesomeric effect).

Alternate single and double bonds in open chain or cyclic compounds results in conjugation. In systems with conjugation, π-electrons are delocalised and polarity is developed.

Resonance effect can be of two types −

• +R/+M effect − The transfer of electrons is away from the substituent group attached to the conjugated system.

  Groups exhibiting +R/+M effect − −X,−OH,−OR,−OCOR,−NH₂ − NHCOR

  Example: Aniline

  

  In +R effect, negative charge is developed at ortho and para positions.

  

• −R/−M effect − The transfer of electrons is towards the substituent group attached to the conjugated system.

  Groups exhibiting −R/−M effect − −COOH,−CHO,Co,−CN,−NO₂

  Example: Benzonitrile

  In –R effect, positive charge is developed at ortho and para postions.