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There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of benzene. Its cyclic structure containing alternating C - C single and C = C double bonds shown below is inadequate for explaining its characteristics properties.
As per the above representation benzene should exhibit two different bond lengths, due to C - C single and C = C double bonds. However, as determined experimentally benzene has a uniform C - C bond distances of 139 pm, a value intermediate between the C - C single and C = C double bonds. Thus, the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II.
Therefore, according to the resonance theory (Unit 6) the actual structure of benzene cannot be adequately represented by any of these structures, rather it is a resonance hybrid of the two structures (I and II) called resonance structures. The resonance structures (canonical structures or contributing structures) are hypothetical and individually do not represent any real molecule. They contribute to the actual structure in proportion to their stability.

Another example of resonance is provided by nitromethane(CH3NO2) which can be represented by two Lewis structures. (I and II). There are two types of N - O bonds in these structures.
However, it is known that the two N - O bonds of nitromethane are of the same length (intermediate between a N -O single bond and a N = 0 double bond. The actual structure of nitromethane is therefore a resonance hybrid of the two canonical forms I and II.

The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures. The difference in energy of resonance structures is called the resonance stabilization energy or simply the resonance energy. The more the number of important contributing structures, the more is the resonance energy. Resonance is particularly important when the contributing structures are equivalent in energy.

The following rules are applied while writing resonance structures:

The resonance structures have (i) the same positions of nuclei and (ii) the same number of unpaired electrons. Among the resonance structures, the one which has, more number of covalent bonds, less separation of opposite charges, a negative charge if any on more electropositive atom and more dispersal of charge is more stable than others.

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