Extension and Revision of the von Baeyer System for Naming Polycyclic Compounds (Including Bicyclic Compounds)
(IUPAC Recommendations 1999)

Synopsis, Preamble, VB-1 to VB-5

Contents of Section

VB-1 Definitions and Terminology
VB-2 Main Ring
VB-3 Naming Hydrocarbon Bicylic Systems
VB-4 Numbering of Bicyclic Systems
VB-5 Main Bridge
References for this Section


These recommendations document the von Baeyer system for naming polycyclic ring systems described in Rules A-31, A-32 and B-14 of the Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F and H, 1979 and R-2.4.2 of A Guide to IUPAC Nomenclature of Organic Compounds, 1993 and extend the system to cover more complex cases. It provides guidance on the naming of ring systems which previously the rules did not cover. The method is to identify the main ring and main bridge which provide the basic bicyclic system and to number these atoms. Then all further bridges, whether or not they include additional atoms, are identified by indicating not only the number of atoms but also the two atoms to which the bridge is attached. The final name also indicates the number of rings and the total number of skeletal atoms in the ring system. Heteroatoms, unsaturation and substituents are indicated in the usual way.


This system of naming polycyclic compounds was first developed for bicyclic compounds by von Baeyer (ref 1) and was extended to tricyclic systems by Buchner and Weigand (ref 2). Von Baeyer nomenclature was adopted by Patterson (ref 3) in his work for IUPAC on ring nomenclature which was used in the Ring Index (ref 4) and by the Chemical Abstracts Service Index Guide, Appendix IV, ¶ 155, 161, 184 (ref 5). IUPAC extended von Baeyer nomenclature in the current rules, see rules A-31, A-32 (hydrocarbons), B-14 (heterocyclic compounds) plus examples in B-6.1, C-514.4, C-551.2; see also D-6.24 (siloxanes), D-6.33 (silathianes), D-6.43 (silazanes), D-6.51 (silazane radicals), D-6.71 (organosilicon compounds), D-7.51 (organoboron compounds), and D-7.54 (borazane) (ref 6). See also a summary in recommendation R-2.4.2 and examples in R-, R-, R-5.8.2 and R.7.1.1 (ref 7); the lambda convention, rule Lm-2.0 (ref 8); and examples in the radicals and ions document e.g. RC- (ref 9). Deficiencies in the rules were pointed out by Eckroth (ref 10) and Rücker and Rücker (ref 11). This document sets out to rectify such deficiencies and replaces the existing rules A-31, A-32 and B-14 (ref 6).

In the examples below the numbering shown refers to the preferred name unless specifically indicated as corresponding to an alternative incorrect name.

Polycyclic ring systems which are to be treated by the von Baeyer system are named by the following rules applied in order until a decision is made.

VB-1 Definitions and Terminology

A bridgehead is any skeletal atom of the ring system which is bonded to three or more skeletal atoms (excluding hydrogen).

A bridge is an unbranched chain of atoms or an atom or a valence bond connecting two bridgeheads.

Two bridgeheads are selected as main bridgeheads. These two bridgeheads must be linked by at least three bridges (which may include bridgehead atoms required for secondary bridges). No atom is present in more than one bridge.

The main ring of the ring system is the ring identified by VB-2 and includes the two main bridgeheads.

The main bridge is a bridge which connects the two main bridgeheads.

A secondary bridge is any bridge not included in the main ring or the main bridge.

An independent secondary bridge links bridgeheads which are part of the main ring or main bridge.

A dependent secondary bridge links at least one bridgehead which is part of a secondary bridge.

A polycyclic system is regarded as containing a number of rings equal to the minimum number of scissions required to convert the system into an open-chain compound. The number of rings is indicated by the appropriate prefix, bicyclo- (not dicyclo-), tricyclo-, tetracyclo-, etc.

VB-2 Main Ring

The main ring is selected so as to include as many skeletal atoms of the polycyclic compound as possible. The main ring is shown in bold in examples for VB-2 to VB-4.


not bicyclo[3.1.2]octane (7-membered main ring larger than 6)

not tricyclo[,8]dodecane (10-membered main ring larger than 9)

VB-3 Naming Hydrocarbon Bicyclic Systems

A bicyclic system (which comprises the main ring and main bridge only) is named by:

the prefix bicyclo- (indicating the number of rings);

numbers indicating the bridge lengths (i.e. number of skeletal atoms excluding the bridgehead atoms) separated by full stops and placed in square brackets. The three numbers are cited in decreasing order of size (e.g. [3.2.1]);

the name of the hydrocarbon indicating the total number of skeletal atoms.



VB-4 Numbering of Bicyclic Systems

The atoms of a bicyclic system are numbered from a bridgehead atom via the longest path to the second bridgehead atom; numbering of atoms continues round the main ring; and then the main bridge atoms are numbered starting from the lower numbered bridgehead atom.



VB-5 Main Bridge

If there is more than one bridge the main bridge is selected to include as many as possible of the atoms not in the main ring.

The main ring and main bridge are shown in bold in examples for VB-5 to VB-7.


not tricyclo[,5]octane (one-atom main bridge larger than no-atom bridge)

References for this Section

1. A. Baeyer, Systematik und Nomenclatur bicyclischer Kohlenwasserstoffe, Ber. Dtsch. Chem. Ges., 33, 3771-3775 (1900). (Although he always published his research as just Baeyer he was always referred to with the honorific as von Baeyer)

2. E. Buchner and W. Weigand, Bornylen und Diazoessigester [Nebst einer Nomenklatur tricyclischer Kohlenstoff-Ringsysteme nach Adolf von Baeyer], Ber. Dtsch. Chem. Ges., 46, 2108-2117 (1913).

3. A.M. Patterson, Proposed international rules for numbering organic ring systems, J. Am. Chem. Soc., 47, 543-561 (1925); A.M. Patterson, The nomenclature of parent ring systems, J. Am. Chem. Soc., 50, 3074-3087 (1928).

4. A.M. Patterson and L.T. Capell, The Ring Index, Reinhold, New York, 1940; A.M. Patterson, L.T. Capell and D.F. Walker, The Ring Index, 2nd edition, American Chemical Society, Washington, DC, 1960 (Supplements 1963, 1964 and 1965). This was continued as the Parent Compound Handbook and then the Ring Systems Handbook 1993 ed., Chemical Abstracts Service, Columbus, Ohio, 1993 (and November 1997 cumulative supplement).

5. Chemical Abstracts Service, Index Guide, Appendix IV, 1997.

6. International Union of Pure and Applied Chemistry, Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F and H, 1979 edition, Pergamon Press, Oxford, 1979.

7. International Union of Pure and Applied Chemistry, A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993, Blackwell Scientific Publications, Oxford, 1993.

8. IUPAC Commission on the Nomenclature of Organic Chemistry, Treatment of variable valence in organic nomenclature (Lambda convention) (Recommendations 1983), Pure Appl. Chem., 56, 769-778 (1984).

9. IUPAC Commission on the Nomenclature of Organic Chemistry, Revised nomenclature for radicals, ions, radical ions and related species (IUPAC recommendations 1993), Pure Appl. Chem., 65, 1357-1455 (1993).

10. D.R. Eckroth, A method for manual generation of correct von Baeyer names of polycyclic hydrocarbons, J. Org. Chem., 32, 3362-3365 (1967).

11. G. Rücker and C. Rücker, Nomenclature of organic polycycles out of the computer - how to escape the jungle of the secondary bridges, Chimia, 44, 116-120 (1990).

Continued with VB-6
Return to von Baeyer Nomenclature home page.