Basic
Organic Nomenclature
Poly cyclic Alkanes -Spiro Compounds
Multiple rings in a compound are straight forward to name as long as they
are separate rings. It is of course possible for two rings to share one
or more carbon atoms between them. These pages will deal with the simpler
bicyclic systems starting with two rings sharing a single carbon atom.
The naming of these compounds depends on the number of carbon atoms
in each ring. For example compare the three isomers of this bicyclic nonane:
You should note that the rings are not in the same plane as the 2-D
diagrams suggest, but rather intersect at right angles.
The naming of these compounds is as follows:
-
the root name is based on the total number of carbon atoms in both rings,
i.e. above there are nine C atoms, therefore - nonane
-
the prefix spiro is used to indicate the bicyclic
structure with one C atom shared between rings
-
to distinguish between possible isomers the number of carbon atoms in each
ring excluding the common carbon is indicated in square brackets,
with a period between numbers. i.e. for Isomer 1 [4.4], Isomer 2 [3.5],
and Isomer 3 [2.6]. (NOTE: the numbers are given in ascending order.)
-
the numbers appear in the name after spiro
Thus the names for the three isomers above would be:
Isomer 1: spiro[4.4]nonane
Isomer 2: spiro[3.5]nonane
Isomer 3: spiro[2.6]nonane
NOTE: the total of the numbers in brackets will be one less than the
number of carbons in both rings.
The following examples will help illustrate this idea further.
Substituents on the rings are located by numbering the rings starting
on a carbon atom adjacent the the common carbon atom in the small ring,
continuing around the small ring, through the common carbon, and around
the larger ring. The direction for numbering is chosen to give the substituents
the lowest possible number while going around either ring. Numbers for
high priority functional groups which modify the suffix of the compounds
name will appear immediately preceding the suffix.
i.e.