Resonance effect or mesomeric effect
(1) The effect in which electrons are transferred from a multiple bond to a molecule, or from a multiple bond to a one 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 type of the element with conjugated system of double bonds, the mesomeric effect is transferred 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 sets act as lone pairs of electrons. Sets which decrease the electron density of the rest of the molecule by withdrawing electron pairs are said to have -M effect.
(a) The sets which give electrons to the double bond or to a conjugated system are said to have +M effect or +R effect.
+M effect groups :
(b) The sets which take electrons from the double bond or from a conjugated system towards itself due to resonance are said to have -M effect or -R effect.
-M effect groups :
(3) The inductive and mesomeric effects, when given together, can behave in the same direction or oppose one another. The mesomeric effect is harder than the former. As provided, in vinyl chloride because of - I effect the chlorine atom should develop a negative charge but on account of mesomeric effect it has positive charge.
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