Basic Organic Nomenclature

An Introduction

Dave Woodcock,
Associate Professor Emeritus UBC (Okanagan)
©1996,2000, 2008

2. Alkanes
IV Bicycloalkanes (ii)
Two Ring Carbons Common

Index


When the two rings of the bicyclic compound have two carbons common there are three different paths between the two common carbons following carbon, carbon bonds. Take for example the following molecule:
,

In this example, the carbon atoms labeled a and b are the carbons common to the two rings, often refered to as bridgehead carbons, and the three different paths between a and b contain 1,2 and 4 intervening carbon atoms.

The generic name for compounds such as this is

bicyclo[x.y.z]alkane

where x, y, and z are the numbers of intervening carbons on the three paths between the two bridgehead carbons cited in decreasing numerical order, and alk refers to the total number of carbons in the ring systems. (For a check, this number is the sum of the three numbers, x, y, and z, plus 2 for the two bridgehead carbons.)

The compound illustrated above is thus named:

bicyclo[4.2.1]nonane

Note carefully the similarities and the differences in naming these compounds and in naming the spiro compounds.


Other named examples:


bicyclo[2.2.1]heptane (aka norbornane)


bicyclo[4.3.2]undecane


bicyclo[4.1.0]heptane


bicyclo[4.4.0]decane, aka decalin

Before proceding further, make sure that you have this basic naming system understood and learned.


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 round the longest chain of carbons to the second bridgehead.
  3. Number the second bridgehead carbon and continue on round 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.

Examples.
Follow the numbering of the following two compounds:


bicyclo[2.2.1]heptane (aka norbornane)

br> bicyclo[4.1.0]heptane

Follow the naming of the following substituted bicycloalkanes:


7,7-dimethylbicyclo[2.2.1]heptane


4-ethyl-2-isopropylbicyclo[4.1.0]heptane


1-methyl-8-propylbicyclo[4.3.0]nonane


1,9-dimethylbicyclo[4.2.1]nonane

2,6,6-trimethylbicyclo[3.1.1]heptane

Self-Assessment Problems.
Review Bicycloalkanes (i)

Next page : Functional Groups with Prefixes Only.

Index


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