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Heterocyclic Compounds: Nomenclature and Classification

Early organic chemists named heterocyclic organic compounds according to when they arose, how they were prepared, and a few other characteristics.

Heterocyclic Compounds


Nomenclature Based on a Trivial Method

Early organic chemists named heterocyclic organic compounds according to when they arose, how they were prepared, and a few other characteristics.

The sources of heterocyclic compounds are normally identified by their names. So, the name of the compound was determined by its source. Examples include picoline, a product derived from coaltar. The word pictus, translated as tarry in English, is derived from Latin.

As with heterocyclic compounds, heterocyclic compounds were also named based on the properties they possessed.

In the same way, Furfural's name is derived from its source. Furfural refers to barn oil. The compound is extracted from distilling barn oil.

In the history of heterocyclic chemistry, trivial nomenclature is considered to be the first nomenclature method. Nevertheless, some disadvantages exist with the system. A trivial system will only reveal the formula and not the structure of a compound. There are approximately 60 trivial names recognized by the IUPAC system of nomenclature. This recognition is important because it can be used to create alternative compounds that have more systematic names or derivatives. The following examples illustrate how trivial names may be recognized for heterocyclic compounds.

Nomenclature that Follows a Systematic Process

For monocyclic heterocyclic compounds with three to ten members, this is the most commonly used nomenclature. Each of these members contains one or more heteroatoms at various degrees of unsaturation. Structures of molecules are described using systematic nomenclature. These nomenclature systems serve to specify the heteroatom types, positions, ring sizes, and number in heterocyclic compounds. Following is the syntax used to derive the nomenclature from this systematic method;

Name = prefix + stem + suffix

The prefix

Heteroatoms are described by their type, number, and position.
Multiple heteroatoms of the same kind are designated with prefixes di, tri, etc.
When heteroatoms differ, the order of citations starts with a heteroatom of the lowest atomic number and the highest group in the periodic table. For example:

O > S > Se > N > P > Si > B > Hg

The stem

From 3 to 10 members, it determines the ring size.

The suffix

Based on these factors, it determines whether a system is saturated or unsaturated

Whether N is present or absent

It is the size of the ring.


Heterocyclic compounds can be divided into two categories based on their structural and electronic arrangements
  • Aliphatic heterocyclic compounds
  • Aromatic heterocyclic compounds
Cycles amines, amides, ethers, and thioethers are examples of aliphatic heterocyclic compounds. A polycyclic heterocycle containing only single bonds is called a saturated heterocycle. Aliphatic heterocycles are mainly affected by ring strain. In the figure above, you can see examples of such compounds.

An aromatic heterocyclic compound has an analog, unlike benzene. As with aromatic heterocycles, Huckel's rule applies. The aromatic compounds of Huckel's rule are cyclic and have a planar geometry due to their conjugate double bonds; they must also have (4n+2) π electrons. An example of an aromatic heterocyclic compound is illustrated in Fig.

In a heterocyclic ring, three or more atoms may be included, whether they are saturated or unsaturated. Additionally, heterocyclic rings may contain two or more heteroatoms that are either similar or different.

A heterocyclic compound can be classified into three groups based on the variety of its structure.
  • The heterocyclic compounds containing five atoms can be considered to be derived from benzene through the replacement of a single C=C bond with a lone electron pair. These heterocyclic compounds can be further classified into the following categories according to the number of hetero atoms present in the cyclic ring.
  • Furans, thiophenes, and pyrroles are heterocyclic compounds containing one heteroatom.

  • More than one heteroatom can be present in heterocyclic compounds. Pyrazine, imidazole, thiazole, oxazole, triazole and tetrazole are examples of heterocyclic compounds of this type.

  • Six membered heterocyclic compounds - These compounds can have an isoelectronic atom replace the carbon atom of benzene. Compounds that contain six members may also be classified in the same manner as compounds that contain five members.
  • Heterocyclic compound with one hetero atom - These compounds usually come in the form of pyridine, pyran, or thiopyran.

  • A class of heterocyclic compounds that contain more than one heteroatom includes compounds like pyridazine, pyrimidine, and pyrazine, for example.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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