Basic Organic Nomenclature
We have seen that if two (or more) carbon rings in a molecule are
completely separate from each other (i.e. they have no carbons common to
the two rings), then the naming is straight forward. However it is
possible for the two rings to be sharing one or two carbons, and special
ways have been formulated to name such compounds.
Compounds with two rings which share one or two carbons are called bicycloalkanes. They are named differently depending whether the two rings are connected at one carbon or at two.
In this first page, we look at rings sharing only one carbon, as illustrated by the three examples:
Compound A :
Compound B :
Compound C :
Note: Look carefully at the pdb images: they show that the two rings are not in the same plane, in fact they are at right angles to each other at the common carbon. This is often not shown when the structures are put into two dimensions.
Instead of naming these compounds as bicycloalkanes as might be supposed, the whole two-ring structure is given the prefix spiro after the way the two rings connect at the single carbon (in a sort of spiral!).
To name these compounds, the root name is taken from the total number of carbons in both rings.
Thus for compound A above, there are 9 (nine) carbons in both rings together leading to the name spirononane. However, the size of the two rings may vary, for if you look closely at compounds B and C above, you will count 9 (nine) carbon atoms in both rings for each of these compounds as well. So all are spirononanes, but quite obviously they are constitutional isomers (differing in atom connectivity) of each other.
To complete the naming of these compounds, the size of the two rings is
used to differentiate them: a count is made of the number of non-
common carbons in each ring, and the two numbers obtained in this
count are used in the middle of the name, in square brackets [..].
compound A above has counts 4 and 4 in the two rings and so is spiro[4.4]nonane.
compound B is spiro[3.5]nonane,
compound C is spiro[2.6]nonane.
Hint: the sum of the two numbers plus 1 equals the number of carbons in the two rings giving the root name: 4 + 4 + 1 = 9; 3 + 5 + 1 = 9; 2 + 6 + 1 = 9.
Some further examples for you to examine:
Numbering for positioning ring substituents.
If substituents are present on the ring, numbering of the rings starts in the small ring next to the ring junction, passes round this ring and through the ring junction into the larger ring. The numbering goes round the two rings in the direction which produces substituents with the lowest numbers in each ring.
Review Cycloalkanes (ii)
Next page : Bicyclic Alkanes (ii)
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