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An aromatic hydrocarbon is an
(does not contain the maximum
amount of H) cyclic hydrocarbon with a bonding arrangement that allows it to be stable.
The simplest aromatic hydrocarbon is
, C6H6
Benzene is shown as having three double bonds and three single bonds (but not really)
The bond angle around each carbon is
The benzene molecule is also represented as a hexagon with a circle in it. This suggests
that the electrons in the bond are
between all 6 carbon atoms
Rather, benzene’s bonds are formed by
, which are
not fixed to any one carbon atom
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The symbol
for benzene
3D Representation
NAMING AND DRAWING OF AROMATIC COMPOUNDS
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In some cases, benzene can be the parent molecule. There may be functional groups that
act as substituents for benzene.
If there are two or more substituents bonded to the benzene ring, we number the carbon
atoms of the benzene ring starting with the first substituent (alphabetically). The
numbering system goes in the direction of the next closest substituent
In some cases benzene can be the substituent branching from a hydrocarbon chain. In this
case the substituent branch name is phenyl-
chlorobenzene
methylbenzene
nitrobenzene
Example 1: Name the compounds represented by the following structural formulas and
line diagrams
Cl
Br
F
Example 2: Draw the structural formulas for the following compounds
1-fluoro-3-methylbenzene
2-phenylbut-2-ene
PROPERTIES OF AROMATIC HYDROCARBONS
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Most aromatic compounds are liquids at room temperature, some are crystals
They are usually non-polar and generally insoluble in water
If they have an electronegative functional group attached, they may become polar
Benzene has been found to be carcinogenic, and so toluene (methylbenzene) has been
used in its place in many of its industrial uses
They generally have higher boiling points than their corresponding cycloalkanes
REACTIONS OF AROMATIC HYDROCARBONS
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Substitution reactions are characteristic of aromatic hydrocarbons
Aromatic compounds are much less reactive than alkenes, but more reactive than alkanes
Homework: Read Pages 28-30 and do questions #1-6 on Page 31