Aromatic Compounds

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• Aromatic Compounds

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Defining Aromatic Compounds

• Historically
• organic compounds had an aroma or odour

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Aromatic Compounds

• unsaturated cyclic hydrocarbons
• contain single rings or groups of rings
• simplest aromatic compound is benzene
• aromatic compounds include benzene or are benzene
  based and have benzene-like structures and
  properties.

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??Sweet Dreams are Made of Benzene ??

• Kekulé
• The idea of the benzene structure came to Kekulé
  in a dream

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Benzene, C6H6

• 6 carbon ring with a hydrogen bonded to each
  carbon
• one electron from each carbon is free to
  participate in a double bond

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Structure of Benzene

• we can draw two different structures with double
  bonds for benzene

or
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Resonance

• occurs when two or more equally valid structures
  can be drawn for a molecule
• benzene exhibits resonance

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• explains unique properties of benzene and other
  aromatics
• benzene is perfectly flat (electrons are
  delocalized, in other words, they are not fixed
  to one carbon but can move around)
• bending or twisting of the aromatic molecule
  would disrupt the electron sharing and the
  stability of the molecule

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Resonance Structure for Benzene

• The following structures represent the resonance
  that exists in the aromatic molecule

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More on Resonance

• The structure with the inserted circle
  illustrates how the electrons can move to form
  resonance structures, but does not illustrate how
  the electrons are involved in bonding.
• molecules that have resonance are more stable
  than molecules without resonance
• ie benzene is not as reactive as a corresponding
  six-carbon alkene

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Properties of Benzene

• Molecular formula of benzene, C6H6, is based on
  its percent composition and molar mass
• MPbenzene 5.5C, BPbenzene80.1C
• Non-polar molecule
• Very unreactive with halogens
• Does not undergo addition reactions.
• All carbon-to-carbon bonds in benzene are the
  same length (determined by x-ray diffraction)
• Empirical evidence shows that all carbons are
  identical and that each carbon is bonded to one
  hydrogen

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Naming Aromatics

• Substituted Benzenes
• Compounds containing substituents (ie alkyl
  groups, other atoms) in place of one hydrogen
  atom are named as derivatives of benzene.

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• Step 1 Identify the branch (alkyl or atom) and
  write it as the first part of the name.
• Step 2 Complete the name with benzene.

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Some examples
methylbenzene
toluene
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• ethylbenzene

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propylbenzene
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Disubstituted Benzenes

• Some derivatives of benzene have two
  substituents
• Known as disubstituted benzenes.

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Isomers

• Three isomers of disubstituted benzenes exist
• Named according to the location of the
  substituents on the benzene ring (IUPAC) or with
  the corresponding prefix (Classical)

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Naming Rules

• Step 1 Number the carbons, starting at one of the
  substituents and numbering in the direction which
  will give the substituents the lowest number.

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• Step 2 Write the locations of the branches
  (IUPAC) or use the appropriate classical prefix.

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• Step 2 Write the location of the branch, followed
  by the name of the branch.
• Step 3 Complete the name with benzene.

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Some Examples
1,2-dimethylbenzene
ortho-dimethylbenzene
o-dimethylbenzene
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1,3-dimethylbenzene
meta-dimethylbenzene
m-dimethylbenzene
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1,4-dimethylbenzene
para-dimethylbenzene
p-dimethylbenzene
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1-methyl-3-ethylbenzene
1-ethyl-2-propylbenzene
m-methylethylbenzene
o-ethylpropylbenzene
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• For larger organic molecules, the benzene is
  considered a branch or an alkyl group
• Called phenyl (C6H5-)

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Here are some examples
3-phenylpentane
2-phenylpropane
2-phenylpentane
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Can you name these?
3-methyl-2-phenylhexane
3-methyl-5-phenyl-3-nonene