RESONANCE STRUCTURES

The language of organic chemistry does not permit the description of the exact electronic configuration of molecules in terms of a single Lewis structure. To illustrate this point, imagine a language which has words for blue and yellow but none for green. To describe green, one could do it in terms of various proportions of blue and yellow. Similarly, organic chemists describe molecules in terms of imaginary extreme electronic structures termed resonance structures.

1. Resonance structures are written for compounds that cannot be described by a single Lewis structure.

2. Resonance structures are imaginary structures.

3. Resonance structures are described by using the double-headed arrow formalism. This formalism does not indicate that the molecule is resonating between the two extreme Lewis structures. It is the easiest way that chemists can describe different contributions to the total picture of the hybrid molecule. In the same way that green does not change between blue and yellow, a molecule does not shift from one resonance structure to the other. Green always looks green and an electronic structure of a molecule always is the same. It just may be difficult for us to describe it using Lewis structure language.

4. The hybrid molecule is the only one that has real existence and a resonance hybrid is always characterized by having lower energy and greater thermodynamic stability than any of its resonance contributors.

5. Resonance contributors cannot involve any changes in the position of the nuclei.

6. Resonance contributors involve only electron reorganization. As such, lone pairs and multiple bond electrons are usually the ones involved.

7. Resonance structures must have the same number of paired or unpaired electrons, but not necessarily the same number of bonds. RELATIVE IMPORTANCE OF RESONANCE STRUCTURES

1. Homovalent structures are more important than heterovalent ones.

2. Structures in which all first row (second period) atoms have filled octets are more important that structures without octets.

3. The more important structures are the ones involving no charge separation or minimum charge separation.

4. Some Lewis structures with complete octets cannot be written without charge separation. In such cases, the more important structure is that which the negative charge is borne by the most electronegative element.

5. Elements beyond the second period may have structures with apparent expansion of their octets as resonance contributors.