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Detailed study of mesomerism

(4) Resonance structures (Contributive structures)

    

Real molecule (or ion) and its contributig (resonance) structures

- The contributing (resonance) structures are represented by Lewis structural formulas: - The contributig (resonance) structures are all formulas that are derived from the →   rules of movement of doublets . - The contributing (resonance) structures do not represent real molecules. - There are no chemical equilibria between contributing (resonance) structures. - The only real species is the average of all contributig (resonance) structures

The weight of contributing (resonance) structures

If 100 nails in a toolbox have a length of 4 cm and one nail has a length of 2 cm, this does not mean that the nails of the box have an average length of 3 cm. The nails of 4 cm have more "weight" in average. In this case the two mesomeric forms obviously have the same weight. They are equally "contributory". The real ethanoate ion has an average charge of $-\frac{1}{2}$ on each oxygen atom. This explains why it is much less reactive than the ethanolate ion $CH_3CH_2O^-$ which keeps a full $-1$ charge on its oxygen

- The symmetric contributing (resonance) structures have the same weight. - A mesomerism between symmetrical mesomeric forms leads to a very stable molecule.

In this case the three contributig (resonance) structures have different weights. Nature does not "not like" the separate charges, but if there are separate charges nature "prefers" the most negative charge on the electronegative atom and the most positive charge on the less electronegative. (3) has therefore an insignificant weight. We often forget to write such a trivial form.

- Less there are separate charges the more weight have contributing (resonance) structures - Contributing (resonance) structures incompatible with electronegativities have a low weight