The mesomeric effect in chemistry is a property of substituents or functional groups in a chemical compound. It is defined as the polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and lone pair of electrons present on an adjacent atom.

What is +M and effect?

If the π electrons move away from the group and towards the rest of the molecule, the effect is called a +M effect. An example is the donation of electrons from an amino group into a benzene ring, putting δ− charges on the ortho and para positions.

What is +M mesomeric effect?

When the electrons or the pi electrons are transferred from a particular group towards a conjugate system, thus increasing the electron density of the conjugated system then such a phenomenon is known as (+M) effect or positive mesomeric effect.