Resonance Structures

RESONANCE STRUCTURES

Curved-arrow notation is also a very useful device with which to generate resonance structures. In this application it is truly a bookkeeping system. Since individual canonical forms do not exist but are only thought of as resonance contributors to the description of a real molecule, the use of curved-arrow notation to convert one canonical form to another is without physical significance. Never-theless it provides a useful tool to keep track of electrons and bonds in canonical structures. For example, the structures of carboxylate resonance contributors can be interconverted as follows:

Likewise the Kekule´ forms of benzene can be shown:

Allyl cations, for example, can be shown nicely while keeping track of charges, electrons, and bonds:

Similar considerations can be used for a variety of intermediates and structures. In using curved-arrow notation to generate contributing resonance structures, the same rules of valence, charge, bonding, and so on, must be applied. Given these criteria, however, it is a straightforward exercise to generate complete sets of resonance structures which can then be evaluated.