Resonance Effect
The delocalisation of pi or lone pair electrons through a conjugated system, altering electron density at distant atoms.
Formula / equation
−M and +M effects
Unit
Dimensionless
In depth
Also called the mesomeric effect, it operates only where p orbitals can overlap continuously and, unlike induction, does not weaken appreciably with distance. Groups that donate into a system exert +M and activate aromatic rings; those that withdraw exert −M and deactivate them.
Examples in the real world
–OH and –NH₂ activating benzene towards ortho/para substitution; –NO₂ deactivating it and directing meta.