1. Heterocyclic compounds
Nomenclature of
Heterocyclic compounds

2. HETEROCYCLIC COMPOUNDS
Cyclic compounds with at least two different atoms in the ring are known as heterocyclic compounds. The ring itself is called a heterocycle
A hetero atom is an atom other than carbon. The name comes from the Greek word heteros, which means "different."
The N-atom is the most common heteroatom. Next in the importance are O- and S-atoms. Heterocycles with Se-, Te-, P-, As-, Sb-, Bi-, Si-, Ge-, Sn-, Pb- or B-atoms are less common.

3. It has been found that the introduction of hetero-atom in carbocyclic molecules makes predictable changes in their physical and chemical characteristics.
These changes are not only in the behavior of the parent ring but also in the chemical behavior of the functional groups attached to these rings

4. NOMENCLATURE OF HETEROCYCLIC
COMPOUNDS
There are several methods to nominate heterocycles which can be summarized as follow:
Trivial names
These names are given to heterocycles and usually arising from the origin of compounds or of their utility and they are approved by the IUPAC

5. Five membered rings containing one heteroatom

6. Five membered rings containing two heteroatoms

7. Six membered rings

8. Hantzsch-widman system
The basis for this system were introduced independently by A. Hantzsch and O. Widman.
This system was suggested at the beginning for six and five membered rings and then was extended to include rings from 3 to 10 atoms.
The names of the rings is derived by combining a prefix and a suffix
( i.e Name = prefix + suffix )
According to this system a prefix is used to indicate the nature of heteroatom in the ring (O, N, S, P,……)
a suffix is used to indicate the size and the degree of saturation in the ring.

9. Prefix: is used to indicate the nature of heteroatom in the ring (O, N, S, P,……)
When different heteroatom is present the highest priority will be
as follow:
Prefix
Valence
Elements
oxa 2
5) Oxygen (O)
thia
6) Sulphur (S)
aza 3
9) Nitrogen (N)

10. O > S > N For a ring has O + N ► Oxaza
The Latin numbers (di, tri, tetra,….) are used to indicate the number of heteroatoms.
Ex: Tetra + aza ► tetraza
When different heteroatom is present the highst periority will be as follow:
O > S > N
For a ring has O + N ► Oxaza
For a ring has S + N ► thiaza
For a ring has O + S ► oxathia

11. Suffix is used to indicate the size and the degree of saturation in the ring
Suffix = Stem + end
Stem
Ring size ir 3 et 4 ol 5 in 6* ep 7
The end refers to the degree of unsaturation in the ring ( as shown in the next table)

12. 2: System for Hantzsch-Widman names
Rings containing no Nitrogen
Rings containing Nitrogen
Ring size
Saturated
Unsaturated
Irane
Irene
iridine
irine 3
etane
ete
etidine 4
olane
ole
olidine 5
ane***
ine
ane** 6*
epane
epine
**** 7
ocane
ocine 8
onane
onine
onin 9
ecane
ecine
ecin 10

13. Irane Irene Iridine Irine etane Ete etidine ete
Rings containing no Nitrogen
Unsaturated Saturated
Rings containing Nitrogen
Ring size
Irane
Irene
Iridine
Irine 3
oxirane
thiirane
Oxirene
thiirene
Aziridine
azirine
etane
Ete
etidine
ete 4
Thietane
oxitane
Thiete
oxete
Azetidine
Azete

14. Olane
Ole
Olidine 5
Oxolane
thiolane
oxole
thiole
ane***
ine
ane** 6*
Oxane
thiane
Oxine
thiine

16. The rings that have one heteroatom, the rings are numbered so that the heteroatom is given number 1.
When different heteroatom is present the highest priority will be
O > S > N and move in the direction that gives the other heteroatoms the nearest possible numbers.

17. For partially saturated 5-membered rings containing Nitrogen
For partially saturated rings we must designate the location of the nearest
unsaturated atom by Δn (where n = the number of the first unsaturated atom).

18. For full unsaturated rings which contain extra hydrogens, the positions of extra hydrogens are indicated as nH in the prefix

19. When a heterocycle contains two similar atoms, the one carrying the heavier substituent is numbered first.
When a heterocycle contains two or more N atoms, we begin numbering from SP3 nitrogen without deviation from the priority

20. (iii) Replacement Nomenclature
Heterocyclic systems are named by considering them to be derived from cyclic hydrocarbons by replacing a carbon corner by heteroatom.
The heteroatoms are indicated by the same prefixes used in the Hantzsch-Widman nomenclature.
When more than one heteroatom is present citation numbers are used (O>S>N).

23. (iv) Naming substituted monocyclic compounds
In case of heterocycles with one heteroatom the hetero atom is given No. 1. Numbers are then given in the way that gives substituents least possible numbers.
(When more than one heteroatom is present the heteroatom of highest priority is numbered 1. Numbering is then made in the way that gives other heteroatoms least possible numbers. This means least difference between the first two atoms and not least algebraic sum of numbers.
According to IUPAC rules the organic compounds are arranged in order of decreasing priority as follow:

24. a. Cation and anions
b. Acids: Carboxylic, peroxycarboxylic, thiocarboxylic, sulfonic, sulfinic,… Etc.
c.Derivatives of acids: anhydrides, esters, acyl halides, amides, hydrazides,
imides, amidines, …… etc.
d. Nitriles: cyanides, isocyanides.
e. Aldehydes, thioaldehydes and their derivatives
f.Ketones, thioketones and their derivatives. g. Alcohols, phenols, thioles, and their esters derivatives with inorganic acids.
h. hydroperoxides.
i. Amines, imines and hydrazines.
j. Ethers, thioethers.
k. Peroxides.
All other functional groups, which are included in one of these classes, are then written as prefixes in form of the name in alphabetic order.

26. B) Polycycles (A) Trivial names
Trivial names approved to be utilized are given below. These were drawn from the IUPAC (Blue Book).

29. Fusion names
When a heterocyclic ring is fused to a carbocyclic ring, the hetero ring is chosen as the parent or base component. Then the systematic name is formulated from the prefix benzo- and the trivial name of the heterocyclic component as follows:
The bonds between the ring atoms are denoted according to the successive numbers of the ring atoms by the letters a, b, c, etc.
The letter must be as early as possible and hence benzo[d]thiophene is incorrect.

31. •Polycyclic systems with two or more heterocycles
The base component is chosen as that in the Hantzsch-Widman nomenclature except that of nitrogen is given the highest priority.
Prefixes are formed by changing the terminal (e) of the trivial or the Hantzsch-Widman names of the components into (o)
Chose of the base component.
i- Is there only one component which contains nitrogen?
No Yes: choose this as base component

32. ii- Is nitrogen absent from all the rings?
NO Yes: choose the ring containing a heteroatom Which occurs
highest?

33. iii- Are there more than two rings presents?
No Yes: choose the component containing the greater
number of rings

34. iv- Are the two rings of different size?
No Yes: choose the larger one

35. V- Do the rings contain different numbers oh heteroatoms?
No Yes: chose the one with the greater number

36. Vi- Are there differences between the numbers of each kind of heteroatom in the two rings?
No Yes: choose the ring with the greater number of atoms
listed first

37. Vii- In case of the two rings are of the same size and contains the same heteroatoms, how to select the base component?
Rule is to select the base ring to be the one in which heteroatoms has less separation between the heteroatoms. Thus:

38. Viii- The ring containing the greatest Varity of heteroatom
arranged according to their order priority

39. How to Choose of the base component (Summary)
1- When a heterocyclic ring is fused to a carbocyclic ring (choose the hetero ring as the parent or base component
2- One ring only contains N (choose the ring containing N)
3- No, Nitrogen ( choose according to the priority O ˃ S )

40. 4- One consists of two or more rings
(Choose one with the greater number of rings)
5- Two rings of different size (choose the larger)
6- Choose the one with more heteroatoms

41. 7- Same number of heteroatoms (choose O > S > N)
8- Same type and number of heteroatoms, and same ring size (choose lower separation between atoms)
9- Choose the ring containing the greatest Varity of heteroatom.

42. Bridge Specification Acenaphthylene Acenaphtho Naphthalene Naphtho
Adding prefixes to the name of a base component
The base component is chosen as before
Prefixes are formed by changing the terminal e of the trivial name into <O>
(e.g. pyrazole is converted to pyrazolo)
Some prefixes are formed as in the following table
Acenaphthylene
Acenaphtho
Naphthalene
Naphtho
Anthracene
Anthra
Perylene
Perylo
Benzene
Benzo
Pyridine
Pyrido
Furan
Furo
Pyrimidine
Pyrimido
Imidazole
Imidazo
Quinoline
Quino
Isoquinoline
Isoquino
Thiophene
Thieno

43. Name = prefix[number, letter]base
The position of fusion between the prefix and the base is indicated by inserting (in square brackets) number for the prefix and italic letter for the base component.
Name = prefix[number, letter]base

46. If a position of fusion is occupied by a heteroatom, the names of the two component rings are chosen in a way that both contain this heteroatom.

47. Symmetric and Asymmetric Heterocycle

49. Other examples for Bridge Specification

52. a)- Give low numbers to the heteroatoms

53. b) Give low number to heteroatoms in the order O> S> N

55. c) Give hydrogen atom the lowest numbers

56. A carbon atom common to two or more rings is given a lower case letter after the numeral of the atom immediately proceeding. A heteroatom common to two or more rings receives an individual locant.