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Mesomeric Effect (Resonance Effect)

  • Mesomeric effect is valid only for conjugated system.
  • There are two types of Mesomeric effect:

                                          + M Effect (+ R) and – M Effect (- R)

  • Consider the following conjugated system


                              (+M effect of y)

  • Consider another conjugated system


                               (-M effect of NO2)

Mesomeric Effect in Phenol (+ M Effect)

Mesomeric Effect in Aniline (+ M effect)

  • If more of e- towards ring (+ M effect)
  • This effect increases the electron density over benzene ring.

– M effect in Benzaldehyde

Ex. Identify the compound showing +M or –M separately

Sol. (a) (-M)          (b) (-M)        (c) +M

* +M group increases electron density of ring while –M decreases the electron density of benzene ring.

If NO2 is present on the ortho or para position then along with it’s –I effect, it will also show –M effect.

Above compound have +M of -OH and –M of NO2 group.

As we can easily see that –NO2 at Meta position is not attracting e-density towards itself and that’s why it will not show –M effect at meta-position.

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