Class 11 Chemistry Notes GOC – Mesomeric Effect (Resonance Effect)

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

                             Class 11 Chemistry Notes GOC   Mesomeric Effect (Resonance Effect)  Image by AglaSem

                              (+M effect of y)

  • Consider another conjugated system

                            Class 11 Chemistry Notes GOC   Mesomeric Effect (Resonance Effect)  Image by AglaSem

                               (-M effect of NO2)

Mesomeric Effect in Phenol (+ M Effect)

Class 11 Chemistry Notes GOC   Mesomeric Effect (Resonance Effect)  Image by AglaSem


Mesomeric Effect in Aniline (+ M effect)

Class 11 Chemistry Notes GOC   Mesomeric Effect (Resonance Effect)  Image by AglaSem

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

- M effect in Benzaldehyde

Class 11 Chemistry Notes GOC   Mesomeric Effect (Resonance Effect)  Image by AglaSem

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

Class 11 Chemistry Notes GOC   Mesomeric Effect (Resonance Effect)  Image by AglaSem

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.

Class 11 Chemistry Notes GOC   Mesomeric Effect (Resonance Effect)  Image by AglaSem


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

Class 11 Chemistry Notes GOC   Mesomeric Effect (Resonance Effect)  Image by AglaSem


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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