Page 193 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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440 Organic Chemical Systems, Theory

optimization. This approach is known as molecular me- tures. Usually, it results in a stabilization, referred to as
chanics and provides a good approximation, particularly resonance energy.
for hydrocarbons, for which extensive and carefully opti- Resonance energy plays a particularly important role in
mized parameter sets are available. It runs into difficulties cyclic systems. Those cyclic systems with two equivalent
with molecules in which more than one classical valence doubly bonded structures that contain 4N+2 electrons in
structure is important. the perimeter (N is an integer) exhibit a large stabilization
and are known as aromatic. The stabilization is known as
the aromatic resonance energy. The archetypical example
C. Resonance (Mesomerism)
is benzene (14). Those containing 4N electrons in the
An important concept in classical structural theory is res- perimeter are actually destabilized (antiaromatic); a good
onance (mesomerism). This is related to the fact that more example is cyclobutadiene (15):
than one classical structure can normally be written for a
molecule. A double-headed arrow is usually placed be- or
tween the structures to indicate that all of them contribute
to the description of bonding in the molecule. It is im- 14
portant to note that all of the structures refer to the same
molecular geometry.
15
The situation is most easily exemplified in the case of
multiple bonds (e.g., 10–12). The existence of two or more In the case of polycyclic ring systems of conjugated
valence structures can also be indicated by curved arrows, double bonds it is more difficult to specify the degree of
as in 10c to 12c. aromatic stabilization or antiaromatic destabilization. A
H 3 C H 3 C H 3 C good rule of thumb is to count the number of possible

C O C O or C O structures, ignoring those with an even number of double
H 3 C H 3 C H 3 C
10a 10b 10c bonds located inside any single ring. The higher the num-
H 5 C 2 C 2 H 5 H 5 C 2 C 2 H 5 H 5 C 2 C 2 H 5 ber, the larger the stabilization. These kinds of problems

B N B N or B N
are far more efficiently handled by the more advanced
C 2 H 5 C 2 H 5 C 2 H 5
H 5 C 2 H 5 C 2 H 5 C 2
11a 11b 11c quantum mechanical theories of molecular structure.
CH CH CH CH CH CH In order to simplify notation and to avoid writing a large
O
H 2 N CH O H 2 N CH or H 2 N CH O
12a 12b 12c number of contributing structures, it is customary to draw
a circle inside an aromatic ring, as shown for 14.
The relative importance of several contributing struc-
Ambiguities in the writing of molecular structures exist
tures is dictated primarily by their energies (i.e., the ener-
even in compounds containing only single bonds. In prin-
gies of hypothetical molecules in which only the structure
−
+
+
ciple, it is possible to write the structures H H ,H H ,
−
in question would contribute). The energies can be esti-
and H H for molecular hydrogen. The charge-separated
mated qualitatively. Low energy is favored by the pres-
structures are ordinarily not written, and their existence
ence of a large number of bonds in the structure (a highly
is tacitly understood when H H is written; this is true of
twisted double bond does not yield much stabilization—
all other bonds as well. In some cases the need to write
steric inhibition of resonance), by the absence of unfavor-
more than one equivalent classical structure for a molecule
able charge separations, and by the presence of negative
containing only single bonds is not so easily avoided, and
charges on electronegative atoms and positive charges on
such structures are often called nonclassical (e.g., struc-
electropositive atoms. +
tures 16a–16d for the CH cation and structures 17a–17d
Two structures contribute equally when they are equiv- 5
for the norbornyl cation):
alent by symmetry, as in the allyl radical 13:
H H H H H
CH CH CH H H H
or H C H C H C or H C
H 2 C H 2 C H 2 C
CH 2
CH 2
CH 2
H H H H
13a 13b 13c H H H H
16a 16b 16c 16d
A bond that is single in some and absent in other con-
tributing resonance structures is called a partial bond and
is often drawn with a dashed line.
or
The existence of several contributing structures is asso-
17a 17b 17c 17d
ciated with a more or less significant effect on the thermo-
dynamic stability of the molecule relative to that expected Just as in the case of resonance involving double bonds,
from the simple rules for any one of the contributing struc- such systems with several equivalent structures are