Most of these compounds, with up to 5 rings fused have common names which have been accepted into the IUPAC nomenclature system. So, if you need to know them you must learn them!

Ring numbering is shown on the line diagram. Note that only those carbons which can have another atom attached are numbered. Carbons at the fused sites are omitted in the numbering schemes.

There are five sections on this page:

(i)Rings fused in a line
(ii)Rings fused with one 120� angle
(iii)Rings fused in an arc.
(iv)Benzene or naphthalene units in a ring
(v)Other C-6 fused systems
(vi)molecules containing rings other than C-6

(i) Rings fused in a line.

The two and the three ring compounds have common names.

	naphthalene		anthracene

From four rings on a systematic name is given using the suffix -cene with a root that is derived from the number of rings fused together.

	tetracene		pentacene

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(ii) Rings fused with one 120 degree angle at or near the centre of the molecule as in the following patterns have a root for the number of rings and the suffix -phene. The first member (3 rings) has the name phenanthrene.

	pattern for odd number of rings
	pattern for even number of rings

	phenanthrene		tetraphene
	pentaphene		hexaphene

(iii) Rings fused in an arc.

Three rings in an arc is phenanthrene (above).

Four and five rings are named as derivatives of phenanthrene.

Six rings and above are named with a root for the number of rings and the suffix -helicene.

	benzo[c]phenanthrene		dibenzo[c,g]phenanthrene
hexahelicene		tetradecahelicene

Note how the molecules with four and more rings in an arc cannot remain flat and become like a spiral. With six and more rings, the overlap of the rings is such that the molecules exist as enantiomers (see next section) in left and right spiral forms stable enough to be isolated.

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(vi) Molecules with benzene or naphthalene units in a ring are named with a prefix giving the number of units followed by phenylene (for benzene) or naphthalene.

	triphenylene		trinaphthalene

(v) Other C-6 fused systems with common names.

	pyrene		chrysene
	picene		perylene
	coronene		ovalene

(vi) Finally a couple of molecules containing rings other than C-6:

	acenaphthene		azulene

Next page : Stereochemistry. I

Index