Symmetry

Structures such as those shown below for nitromethane are referred to as equivalent contributing structures and the resonance hybrid is the single structure which represents the equal contribution of both of the contributing structures. It is very important to note that a resonance hybrid is not the average of a number of rapidly interconverting forms. The hybrid is the molecule and the resonance forms which we draw are simply devices to represent the limits of electron distribution within the hybrid. By convention, resonance forms are shown connected by "double-headed arrows" to stress the fact that they are not separate valence isomers in some sort of rapid equilibrium. Resonance hybrids are useful to draw because they can often be utilized to predict regions of electron density, or of cationic character which can be useful in predicting or explaining the effects of the structure of organic molecules on their reactivity.

A device which is often used to show the interconversion of resonance forms is the "curved arrow" (shown above). These arrows are designed to show the movement of valence electrons and should begin centered on a bond or a pair of electrons, and end in the final position of the electrons flow.

The flow of electrons among resonance forms follows a set of simple rules; electrons may flow:

from an atom to an adjacent bond, or
from a bond to an adjacent atom, or
from a bond to an adjacent bond.

Since the atoms involved must remain bonded, the bonds which are most commonly involved in constructing resonance forms are double and triple bonds and the most common source of electrons on individual atoms are unshared pairs.

While the first two resonance forms shown above are clearly equivalent forms, the third form, involving electron flow from the nitrogen, differs from the first two and is classified as a nonequivalent form. Guidelines for estimating the relative importance of various resonance forms follows:

For Resonance forms,

equivalent structures contribute equally,
structures in which all atoms have filled valence shells contribute more than structures in which one or more valence shells are partially filled,
structures which involve the generation and separation of charge contribute less than neutral structures, or structures with the same charge, and,
structures with negative charges on electronegative atoms contribute more than structures with negative charges on more electropositive atoms.

For the example shown above, the third (nonequivalent) form involves the generation of new charge and will contribute less than the first two. In the example shown below for acetophenone (acetylbenzene), the first two resonance forms involve equivalent, neutral structures in which electrons are simply moved between adjacent sp² centers and are the most important. The third form, shown below, involves generation of charge, with the electrons on the more electronegative oxygen atom and would contribute more than a form in which the oxygen carried a positive charge, with the carbon being anionic. You should note that all of these are "legal" resonance forms, but that the first two are considered the "major" forms. The third form is useful in explaining why anionic compounds often attack carbonyl carbons (they bear a partial positive charge), but the form shown is of much less importance in the "real" structure of the hybrid than are the first two (neutral) forms. When you are asked to write "major resonance forms", you should focus on those which most clearly follow the guidelines given above.