be adequately represented by any of these structures, rather it is a 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 (CH NO ) 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=O 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 between the actual structure and the lowest energy resonance structure is called the resonance stabilisation 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, all the atoms with octet of electrons (except hydrogen which has a duplet), less separation of opposite charges, (a negative charge if any on more electronegative atom, a positive charge if any on more electropositive atom) and more dispersal of charge, is more stable than others. Benzene Therefore, according to the resonance theory (Unit ) the actual structure of Problem . Write resonance structures of CH COO – and show the movement of electrons by curved arrows. First, write the structure and put unshared pairs of valence electrons on appropriate atoms.
Then draw the arrows one at a time moving the electrons to get