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what is Resonance effect or Mesomeric effect ?

Resonance effect :

When two or more structures that differ only in the distribution of electrons can be written for a molecule, no single Lewis structure is sufficient to describe it’s true electron distribution. The true structure is said to be a resonance hybride of the various Lewis formulas.

 The various Lewis formulas called resonating structure/contributing structure/canonical structure, that can be written for a molecule.

Resonating structures are hypothetical but contribute to the real structure, which is called resonance hybrid. The resonance hybrid is more stable than any other resonating structures.

   

The most stable resonating structure contribute maximum to the resonance hybrid and less stable resonating structure contribute minimum to resonance hybrid.

 

Conjugation:

 A given atom or group is said to be in conjugation with an unsaturated system if:-

            (i) It is directly linked to one of the atoms of the multiple bond through a single bond.

            (ii) It has  bond, positive charge, negative charge, odd electron or lone pair electrons.

Types of Conjugation:

  1. Conjugation between C=C and C=C ( )
  2. Conjugation between +ve charge and C=C ( )
  3. Conjugation between lone pair and C=C ( )
  4. Conjugation between odd electron and C=C (CH2=CH– –CH=CH2)
  5. Conjugation between negative charge and C=C ( )

 

The Rules of Resonance:

            (a)   All the canonical forms (resonating structure) must have proper lewis structure. For instance none of them may have a carbon atom with five bonds.

            (b)   The positions of the nuclei of the atoms must remain the same in all of the structures. Structures 3 is not a resonance structure of 1 or 2, for example, because in order to form it we would have to move a hydrogen atoms and this is not permitted:

            (c) All atoms taking part in the delocalisation must lie in a plane so that orbitals overlaping become parallel to each other.

i.e.                   

            (d) All canonical forms must have the same number of unpaired electron.

            (e) The energy of actual molecule is lower than of any form. Therefore delocalisation is a stabilizing phenomenon.

            (f) All canonical forms do not contribute equally to the true molecule. The more stable structure is the greater contributor to its resonance hybrid.