The molecular formula of benzene is C6H6. Its structure is very
important, and I think in a way, elegant.
|It has a six-carbon ring. Each of the carbons has sp2
hybridization. That gives it 120 degree bond angles all around the molecule. That
makes it a perfect hexagon with a hydrogen atom sticking out from each point. Note that
all these atoms are in the same plane. But that in itself is not what makes it special.
|If we turn the molecule on its side you can see that the sp2
hybridization leaves each carbon atom with one half-filled p orbital. Each of these forms
pi bonds to neighbors on both sides, and the pi bonding goes all
the way around the molecule.
|There are no individual localized pi bonds, just a big super pi bond. It
is actually called a delocalized pi bond to distinguish it from the pi
bonds that are localized alongside two carbon atoms to form the double and triple bonds
that we dealt with earlier in this lesson. This super or delocalized pi bond is unusually
stable. It will not react with bromine like the localized pi
bond that is found in alkenes. Another way of saying this is that because the pi bond is
not localized between, or associated with, just two carbon atoms, we have
not really formed a double bond as such. Because of this, the electrons are not
concentrated enough or available enough to react with the bromine.
|This picture shows a close-up view of how the p orbitals overlap
one another to form pi bonding all the way around the benzene molecule. The sigma bonds
between adjacent carbon atoms are not shown in this picture.
Some chemists use a different approach to describing the bonding in benzene which they
call resonance. I think it is unnecessary and I mention it primarily
because you may run into it in other books or in other classes.
|If you take a look at this diagram (or the one in Example 14a in your
workbook) you can see one way that is sometimes used to show the bonding in benzene. This
way is not really very good because it shows three double bonds and three single bonds
between the carbon atoms. Those single and double bonds are just not there.
|Quite often the benzene ring is shown as a hexagon with alternating single
and double lines. Each corner represents a carbon atom with a single bond to a hydrogen
atom, but neither the C nor the H are written. You may run into that symbol for the
benzene ring quite often.
|This idea of resonance is indicated in this way. Notice
that two diagrams are shown here. The only difference between them is where those double
bonds are shown. At one time it was thought that the locations of the double bonds
oscillated back and forth. Next it was thought that the benzene molecule is something like
the double-bond arrangement on the left and something like the double-bond arrangement on
the right, but more like a merging of both of these. The double-headed
arrow is intended to indicate this.
|I tend to make fun of resonance diagrams but many organic chemists have
found them very useful in determining the location of the best reacting site
for some kinds of reactions involving derivatives of benzene. Resonance diagrams helped
make sense of a lot of organic reactions after they were introduced.
|It was not until sometime later that people came up with the idea of the pi
bonding going all the way around, rather than alternating back and forth. One way
of showing this is shown here (and in Example 14c in your workbook). That dotted or dashed
line going all the way around the inside of the hexagon is used to indicate that the pi
bonding is going all the way around the molecule. Usually, it is easier to draw a
continuous line than a dotted line, so the diagram to the right is the one you will quite
frequently see associated with benzene. That symbol is the structural formula for benzene.
Notice that it does not show any of the carbon or hydrogen atoms. You have to get used to
realizing that each corner of the hexagon is a carbon atom with a hydrogen atom attached
to it unless something else is shown.
|Please note that the structure of benzene is the same whichever symbol is
used to represent it.
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