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Pictures
of the Day 4-17-06 |
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The
acidity of phenol - charge distribution in an aromatic ring |
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Phenol
is relatively acidic, with a pKa near 9.95. Compare this to a normal alcohol
in which the pKa is closer to 16. Phenol is more acidic because the negative
charge of the anion is delocalized around the aromatic ring as described
by the 4 contributing structures above. Note that most of the charge is still
on oxygen, but enough is delocalized in the aromatic pi system to make a
substantial difference. The interesting prediction
of these resonance contributing structures is that the negative charge is
not distributed evenly around the ring, but remains primarily in the ortho
and para positions. This is shown in the electrostatic potential surface
in which the negative charge (red color) is primarily ortho and para to the
oxygen atom. |
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Benzene |
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Above
are representations of benzene. The "flower" representation
on the top right is an electrostatic potential energy surface that answers
the most important question in chemistry, namely where are the electrons
in benzene. The red color indicates the region of high electron density
that is focused on the pi system. The blue color indicates the area of lowest
electron density. This picture makes sense since the pi electrons, depicted
in the lower two pictures, are exposed to solution, not surrounded by atomic
nuclei like electrons in a sigma bond. Thus, these electrons provide for
the red color, and they are able to interact with extremely strong (wicked
strong) electrophiles. Benzene should be considered a very weak nucleophile. |
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Arenium
ion |
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Shown
above are models of the arenium ion produced during the chlorination of benzene. The
chlorine atom is brown. As described by the electrostatic potential energy
surface on the right, the positive charge (blue color) of the arenium ion
is localized at the positions ortho and para to where the electrophile is
attached. You should be able to draw resonance forms that predict the ortho
and para distribution of the positive charge. This ortho and para distribution
of positive charge is important because substituents already on an aromatic
ring must interact with this charge distribution, and the result of that
interaction is a modulation of rate and orientation of reaction. By understanding
these interactions, you will be able to predict accurately the products of
electrophilic substitution reactions that take place on aromatic rings containing
substituents. |
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