|
THE
THEORY OF RESONANCE
Dr.
T.H. Black - Department of Chemistry
Eastern Illinois University
Many simple organic compounds, such as methane,
ethene, etc., can be adequately represented by a single Lewis structure:
methane
- CH4
ethene - H2C=CH2 A
large number of substances, however, cannot be accurately represented by a single
valence-bond formula, since some electrons are not constrained to a certain
area of the molecule but rather are delocalized ("spread out") over two or more
atoms. In these cases, two or more structures, called resonance structures,
are drawn; the true nature of electron distribution in the molecule is understood
to be a mixture, or resonance hybrid, of the contributing resonance structures.
It is important to realize that resonance is a stabilizing
phenomenon, since it delocalizes electron density over several nuclei.
Thus, the more resonance structures which can be drawn
for a given species, the greater its stability.
There are two tasks to be accomplished in dealing with resonance: drawing accurate resonance contributors and then weighing their relative importance with regard to their individual contribution to the hybrid (the latter task is not relevant to CHM 2430 and can be ignored). One of the simplest examples of this formalism is benzene, which is represented by the two resonance structures in the left box; benzene thus is understood to exist in reality as a hybrid of these two structures; the distribution of p electrons in the molecule cannot be illustrated with a single structure. Another entity exhibiting resonance is the nitro group, which is drawn as the two contributing resonance structures shown above.
Electron
Shifts in Resonance Structures
Remember
- resonance structures are representations of electronic distribution in a molecule,
thus, they must differ only in the positions of electrons. Atoms
in the molecule cannot change position. Moreover, only p
or nonbonded electrons can participate in resonance; s
bonds can never participate. Electron delocalization is conventionally shown
by small curved arrows which allow a systematic progression from one resonance
symbol to another. These shifts are purely artificial since the electrons do
not really shift, but rather are delocalized over the involved atomic nuclei.
There are only three ways in which electron-shift arrows may be drawn
(outlined in the box):
 A
couple of final points that are obvious but worth repeating in this context:
a) an atom cannot carry more than its quota of electrons (eight for second-row
elements); b) all resonance contributors must have the same overall net charge.
a. diazomethane - H2CN2 - see Exam I
 Study Problems For each of the following molecules, draw all possible resonance structures:
|
