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Basic Organic
Nomenclature
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Aromatic Compounds - Benzene
prefix: phenyl
Aromatic compounds refer to a group of chemicals which rather than having
a strong odor, have a fully conjugated cyclic (or polycyclic) structure.
This arrangement of sp2 hybridized carbon atoms gives the compound
a unique stability, making them less reactive than their alkene counterparts.
All aromatic compounds obey Huckel's rule, possessing 4n + 2 p
electrons (n=1,2,3, ...) in a cyclic arrangement. The simplest carbon example
of an aromatic system is benzene (C6H6, or named
as an alkene, 1,3,5-cyclohexatriene). All of the carbon atoms are sp2
hybridized which results in the planar structure common to aromatic compounds.
There are a number of equivalent ways to draw benzene:
The stability of these compounds is a result of the 3 p
bonds interacting with one another. As can be seen in the left and central
structures above, the double bonds can be drawn between any pair of adjacent
carbon atoms. In fact the carbon - carbon bond lengths in benzene are all
the same, with a bond length between that of a carbon - carbon double bond
and single bond. The structure on the right emphasizes this by the circle
in the center which is meant to indicate that all C -C bonds are equivalent.
Remember benzene (C6H6) is different from cyclohexane
(C6H12), compare the structures below.
Polycyclic Aromatics:
There are a number of polycyclic aromatic compounds, several of the more
common ones are (including the numbering scheme for substituents):
Hetro-aromatics:
There are also a large number of compounds which contain hetro-atoms (O,
N, S) which are also aromatic. This is because the hetro-atom is either
involved in a double bond in the ring, or it can make use of a lone pair
of electrons to interact with the p electrons
in the double bonds within in the ring to satisfy Huckel's rule. Several
of the simpler and more common ones are:
* Purine is a precursor to the DNA bases Adenine and Guanine.
Benzene as a substiuent:
Benzene can of course be a substituent on a molecule. When this occurs
the benzene ring is referred to as a phenyl group. Commonly
it is written as C6H5-R or Ph-R or in a line drawing
as a benzene ring (see top of this page) or 
.
Another common substituent is a R-CH2-C6H5
group which called a benzyl substituent. A general short
hand for any aromatic system is Ar-.
Next Page: Aromatic - II
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