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The
six leftover carbon electrons in benzene are not tied up in three
double bonds. Instead, all six electrons are completely spread out
or delocalized around the carbon ring. Every C-C bond is approximately
a "one-and-a-half bond," as is suggested by the observed bond length
of 1.39 A, which is intermediate between a single bond, 1.54 A, and
a double bond, 1.35 A. One consequence of this delocalization or "spreading
out" of the six leftover electrons is that the benzene molecule is
40 kcal/mole lower in energy, or more stable, than would be calculated
from the alternating bond models at the upper left and from known
C-C and C=C bond energies. This is an important general principle:
Whenever electrons are delocalized in a molecule, the molecule becomes
more stable. Delocalization is possible whenever single and double
bonds alternate along a chain: |
