This document discusses the nomenclature of heterocyclic compounds. It begins by defining heterocyclic compounds as carbocyclic compounds where one or more carbon atoms are replaced by a heteroatom such as nitrogen, oxygen, or sulfur. The International Union of Pure and Applied Chemistry has worked to systematize the nomenclature of these compounds. Single three to ten-membered rings are named by combining prefixes with the name of the parent ring structure. Numbering and naming becomes more complicated for fused ring systems, though many are known by common names like indole or isoquinoline. Spiro heterocycles contain two rings fused at a common point and are named based on the number of spiro atoms and heteroatoms

1. Nomenclature of
Heterocyclic Compounds
Mr. Shriram Bairagi
Assistant Professor, HOD
Dept. of Pharmaceutical Chemistry
Ideal College of Pharmacy and Research,
Kalyan

2. What are heterocyclic compounds?
 These are considered to be derived from
the carbocyclic compounds by the
replacement of one or more carbon
atoms by the heteroatom(s) such as N, O,
S etc.

3. In recent years the International Union of Pure and
Applied Chemistry (IUPAC) has made efforts to
systematize the nomenclature of heterocyclic
compounds.
According to this system single three-to-ten-membered
rings are named by combining the appropriate prefix or
prefixes

5. Accordingly some examples of compounds
named on the basis of above two tables are
cited below:

8. •Numbering of the heterocyclic rings becomes essential when
substituents are placed on the ring. Conventionally, the hetero
atom is assigned position 1 and the substituents are then
counted around the ring in a manner so as to give them the
lowest possible numbers.
•While writing the name of the compound, the substituents are
placed in an alphabetical order. In case the heterocyclic ring
contains more than one hetero atom, the order of preference for
numbering is O, S and N.
•The ring is numbered from the atom of preference in such a
way so as to give the smallest possible number to the other
hetero atoms in the ring.
•As a result the position of the substituent plays no part in
determining how the ring is numbered in such compounds.

10. The numbering and nomenclature of heterocyclic rings
become more complicated for condensed or fused ring
systems, i.e., when a part of one ring is also a part of another
ring. Such ring systems, however, are known by
nonsystematic or common names, such as indole, isatin,
isoquinoline, etc

11. •There is yet another system of nomenclature for fused rings
that is commonly employed. According to this system, the side
of the heterocyclic ring is labelled by the letters a, b, c, etc.,
starting from the atom numbered 1.
•Therefore side ‘a’ being between atoms 1 and 2, side ‘b’
between atoms 2 and 3, and so on as shown below for pyridine.

17. Spiro Heterocycles
 The compounds in which two rings are fused at a common point
are known as spiro compounds and the common atom which is
quaternary in nature is designated as spiro atom. They are
classified according to the number of spiro atoms; monospiro-,
dispiro-, trispiro- ring systems.
 Spiro heterocycles with one spiro atom consisting of one or both
heterocyclic rings are named by prefixing spiro to the name of
normal alkane with same number of carbon atoms.
 The number of atoms in each ring are indicated by arabic numbers
separated by a full stop and enclosed in a square bracket in
ascending order and are placed between spiro prefix and the name
of hydrocarbon.

18.  The heteroatoms are indicated by the prefixes and are prefixed
with their positions to the name of spiro hydrocarbon.

19. •The numbering starts from the ring atom of the smaller ring (if rings
are of unequal size) attached to the spiro atom and proceeds first
around the smaller ring and then around the larger ring through the
spiro atom.
•The heteroatoms are assigned the lowest possible number locants.
If there is a choice between two different heteroatoms, the
preferential numbering is decided according to the appearance of the
heteroatoms in the table.

20.  The heterocyclic ring is preferred over the carbocyclic ring of the
same size. If both the rings are heterocyclic, the preference is given
to the heterocyclic ring with the heteroatom appearing in the table.
• If unsaturation is present in a ring, the pattern of numbering remains
the same but the direction around the ring remains in such a way that
the multiple bond is given number as low as possible. However, the
heteroatom is preferred over the multiple bond.

21. Gonane is the simplest possible steroid and is composed of
seventeen carbon atoms, bonded together to form four fused rings.
The three cyclohexane rings (designated as rings A, B, and C) form
the skeleton of phenanthrene; ring D has a cyclopentane structure.
Hence, together they are called cyclopentaphenanthrene
Steroids:

25. REFERENCES
 1. “Advances in Heterocyclic Chemistry”, Vols. 1 to 27, A. R. Katritzky
and J. A. Boulton, (Eds. ), Academic Press, New York (1963-1980).
 “The Chemistry of Heterocyclic Compounds”, Vols. 1 to 29, A.
Weissberger, (Ed. ), Wiley Interscience, New York (1950 to 1975).
 “Heterocyclic Chemistry”, Vols. 1 to 9, R. C. Elderfield, (Ed. ), Wiley,
New York (1950 to 1967).
 J. A. Joule and G. F. Smith, Heterocyclic Chemistry, Van Nostrand
Reinhold Co., 2nd ed., London (1978).
 O. Büchardt, (Ed. ), Photochemistry of Heterocyclic Compounds, John
Wiley, New York (1976).
 R. S. Chan, Introduction to Chemical Nomenclature, Butterworths,
London (1974).

26. THANK
YOU