RESONANCE

Sometimes it is found that a single structural formula cannot satisfactorily explain all the observed properties of a given compound. In such cases, the compound is represented by two or more structural formulae which differ only in the arrangement of electrons. None of the structural formula alone can explain all the observed properties of the compound. The compound is then said to show 'Resonance'.The various structures are called resonating structures or canonical forms.The true structure of the molecule is not represented by any of the resonating structures but is considered to be a resonance hybrid of the various resonating structures. The greater the no. of contributing structures, the greater will be the stability of the molecule.

For example, the frequently used structure for CO2 molecule is

  O=C=O   (I)

On the basis of this structure, the heat of formation of CO2 from its elements should be 1463 KJ/mole and C=Obond length should be 1.21 A⁰. But the experimental value for the heat of formation of CO2 is 1588 KJ/mole and C=O bond length is 1.15 A⁰. Thus structure (I) for CO2 cannot explain its properties completely.

 ^-O - C   O^{+                       +}O   C - O ^-

  (II)                                     (III)

Structures (II) and (III) contain triple bond which have shorter bond lengths. But they have shorter bond lengths than 1.15 A. Thus structures (II) and (III) also fail to account for all the observed properties of CO2.

In the light of the above concept, CO2 is regarded as a resonance hybrid of the structures (I), (II),and (III).

Resonance is also called mesomerism.

As a result of resonance, there is shortening of bond length and increased stability, such as in benzene.