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The last class of hydrocarbons we will consider are known as
aromatic compounds, so named originally because of the
aromas they gave off. Aromatic compounds are generally
cyclic conjugated systems. The simples example of
such a system is 1,3,5-cyclohexatriene also known
as benzene, whose formula is C
H
. Although
the classical picture of this molecule is of a resonance
structure:
Figure:
Resonance in benzene.
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However, quantum mechanics gives us a more accurate picture
of the electronic structure of benzene. Each of the 6 carbons is
hybridized. Hence, the remaining unhybridized
orbitals can be combined in much the same way as was
done for 1,3-butadiene. Here, we expect 6 new MOs, which are
shown in the figure below:
Figure:
Molecular orbitals that result in combining the
orbitals in benzene.
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Note that, for each orbital, the plane of the 6 carbons
is a nodal plane!! Unlike with 1,3-butadiene, however, each
of the carbon-carbon bonds in benzene has the same length.
This means that the bonds are really neither single nor double
bonds but something intermediate between single and double.
This is the origin of the third panel in the benzene
resonance picture above.
In fact, we can
represent any organic molecule in this abstract fashion. We would
eliminate the hydrogens and just show the bonding pattern between
the carbons.
Next: Fullerenes
Up: lecture_17
Previous: Alkenes and alkynes
Mark E. Tuckerman
2008-12-17