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resonance or mesomerism

The effect in which ? electrons are transferred from a multiple bond to an atom, or from a multiple bond to a single covalent bond or lone pair (s) of electrons from an atom to the adjacent single covalent bond is called mesomeric effect or simply as M-effect. In case of the compound with conjugated system of double bonds, the mesomeric effect is transmitted through whole of the conjugated system and thus the effect may better be known as conjugative effect.
 
         (2) Groups which have the capacity to increase the electron density of the rest of the molecule are said to have +M effect. Such groups possess lone pairs of electrons. Groups which decrease the electron density of the rest of the molecule by withdrawing electron pairs are said to have –M effect, e.g.,
 
         (a) The groups which donate electrons to the double bond or to a conjugated system are said to have +M effect or +R effect.


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