13'9 Aromatic Compounds and the Structure of Benzene

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13.9 Aromatic Compounds and the Structure of
Benzene

• In the early days the word aromatics was used to
  described many fragrant molecules isolated from
  natural sources. Today the term aromatic is
  used to describe benzene like molecules.

Benzene C6H6
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Notice that in the above picture I have written 3
structures of benzene. The problem representing
benzene (and most aromatic compounds) arises from
its properties that are very particular. To
understand this, lets look at two pieces of
experimental evidence and see where the problem
resides.
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1)
What is strange about this table?

• All bonds in benzene are equal!
• The bond length is between that of a single and a
  double bond!

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The main rxn. of alkenes is
addition
2)

• The equivalent reaction of benzene is

What type of rxn is this?
Aromatic compounds react through substitution in
most cases preserving the aromatic ring.
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• To understand these data it is a good idea to
  look into the electronic structure of benzene.
• We have carbon atoms forming double bonds so we
  should have on each carbon the same set of
  orbitals we had for ethylene

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• As before, the carbon atoms use the sp2 orbitals
  (the ones in green to form the benzene backbone)

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• The remaining p orbitals (in green) are now
  depicted in the following figure before
  overlapping

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• In the case of ethylene, one p orbital in one
  carbon will overlap with the one in the
  neighboring carbon

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• Here we seem to have a much more complicated
  situation

Which pairs will overlap?
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• Notice that these two structures do not account
  for the first observation all the bonds in
  benzene are equal.

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• There is a third possibility All the p orbitals
  could overlap.

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• Notice that this big p orbital is spread evenly
  around the whole molecule.
• Because the electronic density is the same over
  all the molecule all the bonds are equal!

We say that the p electrons are delocalized.
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• Delocalized electrons confer special stability to
  a system.
• Whenever delocalization occurs you will find that
  the energy of the system is lower than expected.

• It is not surprising then that benzene wants to
  keep this structure at all costs and that is why
  benzene will prefer substitution rather than
  addition.

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• This notation indicates that benzene is neither
  structure but actually an average of the two.
• Each of the structures is called a resonance
  hybrid.

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• Does this representation of benzene make sense?

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• For practical purposes only we will write benzene
  using only one of the forms

• However, we should always remember that this is
  an incomplete representation of benzene.

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• Simple aromatic compounds like benzene are
  non-polar, insoluble in water, volatile, and
  flammable.
• Unlike alkenes, several aromatic hydrocarbons are
  toxic. Benzene itself is implicated as a cancer
  causing chemical.