Module 5: Structure and Nomenclature of Multi-cyclic Compounds

Saturated, alicyclic hydrocarbon systems consisting of two rings only, having two or more atoms in common, take the name of an open chain hydrocarbon containing the same total number of carbon atoms preceded by the prefix bicyclo. The number of carbon atoms in each of the three bridges (a bridge is a valence bond or an atom or an unbranched chain of atoms connecting two different parts of a molecule; the two tertiary carbon atoms connected through the bridge are termed bridge heads) connecting the two tertiary carbon atoms is indicated in descending order in brackets.

Numbering of bicyclic rings starts with the bridgehead and proceeds by the longest path to the next bridgehead. Unsaturated carbons are given the lowest possible numbers.

Monospiro compounds (spiro compounds are bicyclic compounds with only one atom common to both rings) are named by placing spiro before the name of the aliphatic hydrocarbon of the same number of carbons. The number of carbons linked to the spiro atom in each ring is indicated in ascending order in brackets. Numbering starts next to the spiro atom, through the smaller ring first.

Assemblies of two identical cyclic hydrocarbon systems are named by
a) placing the prefix bi before the name of the corresponding hydrocarbon, or
b) placing bi before the name of the corresponding radical.

For assemblies of two unlike cyclic hydrocarbon systems, one ring system is the base component, substituted by the other. The base is:
a) the system containing the larger number of rings,
b) the system containing the larger ring,
c) the system with the lowest state of hydrogenation (most unsaturated ring).

Heterocyclic rings have names as given below.