Basic Organic Nomenclature

Polycyclic Alkanes - Bicyclic Compounds

The other bonding pattern seen in bicyclic compounds is when the two rings share a pair of carbon atoms. For example;
 

The carbons labeled "a" & "b" are common to both rings and are referred to as bridgehead carbons . There are three different paths between carbons "a" & "b" containing 1, 2, and 4 intervening carbon atoms respectively.

This type of compound is named as a bicyclo[x.y.z]alkane where x, y, and z are the number of intervening carbon atoms in each path, listed in descending order, and the alkane is the name for the total number of carbon atoms in the rings.

            Thus the example above would be named:        bicyclo[4.2.1]nonane.

            NOTE: the total of the three numbers will be two less than indicated in the root alkane name.

More Named Examples:
 
bicyclo[2.2.1]heptane
or
norbornane
bicyclo[3.2.2]nonane
bicyclo[4.1.0]heptane
bicyclo[4.4.0]decane
or decalin

Numbering the ring systems for substituent positioning

The rules for numbering the rings in these bicyclic compounds differ from those for the spiro compounds and are as follows:
 
  1. Start with one of the bridgehead carbons and number it 1.
  2. Proceed around the longest chain of carbons to the second bridgehead.
  3. Number the second bridgehead carbon and continue on around the next longest chain of carbons back towards the first bridgehead carbon.
  4. Pass over the first bridgehead carbon (it already has the number 1) and along the shortest chain of carbons to the second bridgehead carbon again.
NOTE: Pick the direction around the ring that will produce the smallest numbers for the substituents in either ring. Numbers for high priority functional groups which modify the suffix of the compounds name will appear immediately preceeding the suffix.

For Example the previous compounds are numbered as follows:
 
bicyclo[2.2.1]heptane
or
norbornane
bicyclo[3.2.2]nonane
bicyclo[4.1.0]heptane
bicyclo[4.4.0]decane
or decalin

Test yourself with the following examples:
 
Compound
Line Diagram
Molecular Model
7,7-dimethylbicyclo[2.2.1]heptane
6-ethyl-2,2,9-trimethylbicyclo[3.2.2]nonane
3-isopropyl-1-methylbicyclo[4.1.0]heptane
10-bromo-2,2-dimethylbicyclo[4.4.0]decane
or
10-bromo-2,2-dimethyldecalin
4-chloro-1,7-dimethylbicyclo[4.3.0]nonane

There are of course more complex variations of multiple ring structures, ones have three, four, five, or more rings. These are beyond the scope of this course and will be ignored.

Next Page: Alkenes and Alkynes

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