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The polarity developed between atoms of a conjugated system by the electron transfer or pi-bond electron transfer is known as the mesomeric effect. Mesomeric effect occurs when pi bonded electrons move away or toward a substituent group in a conjugated orbital system.

The mesomeric effect can be classified into two types - +M effect and -M effect.

+M effect - positive mesomeric effect.

-M effect - negative mesomeric effect.

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