7. INTRODUCTION TO NOMENCLATURE OF ORGANIC CHEMICAL SYSTEMS WITH PARTICULAR REFERENCE TO HETEROCYCLIC COMPOUNDS CONTAINING UP TO THREE RINGS

INTRODUCTION TO NOMENCLATURE OF ORGANIC CHEMICAL SYSTEMS WITH PARTICULAR REFERENCE TO HETEROCYCLIC COMPOUNDS CONTAINING UP TO THREE RINGS: A TEACHER’S COMPREHENSIVE GUIDE

Welcome, future organic chemists and pharmaceutical scientists!

Organic chemistry is the branch of chemistry that mainly studies compounds made from carbon. In the earlier days, the word “organic” was used because most chemical compounds being studied were obtained only from living things such as plants and animals. But today, organic chemistry includes all carbon compounds, no matter whether they come from living sources or are made artificially in the laboratory or in industries.

Carbon has a special ability: it can join with other carbon atoms to form very long chains and rings of many sizes. This chain-forming ability of carbon is called catenation. Besides carbon, organic compounds usually also contain other elements such as hydrogen, halogens, oxygen, sulfur, nitrogen, and phosphorus. Because of the presence of these atoms, organic molecules can become very large and complicated.

In organic chemistry, we study important concepts like structure, homology, isomerism, and functional groups, which help us understand organic compounds in more detail.

Dpharmguru’s exam insights:

Organic chemistry nomenclature is a fundamental topic that appears in almost every pharmacy exam. Remember: The IUPAC system is the universal language of chemistry—it allows scientists worldwide to communicate clearly about compounds. Pay special attention to the word roots, prefixes, and suffixes, as well as the rules for naming heterocyclic compounds. These topics are frequently tested in both theory and practical exams!

CLASSIFICATION OF ORGANIC COMPOUNDS

Organic compounds can be grouped in two main ways:

• Based on structure
• Based on functional group

1. ON THE BASIS OF STRUCTURE

A) ACYCLIC OR OPEN CHAIN COMPOUNDS

These compounds have an open chain skeleton, meaning they do not have rings.

• Saturated Compounds: These organic compounds contain only single bonds between carbon atoms.
• Unsaturated Compounds: These compounds contain double or triple bonds, which show that some carbons are not fully bonded to other atoms.

Examples: Methane (with single bond), 2,5-dimethyl-2-heptene (with double bond)

B) CYCLIC OR CLOSED CHAIN COMPOUNDS

These organic compounds have rings instead of open chains.

1) HOMOCYCLIC OR CARBOCYCLIC COMPOUNDS

These compounds have rings made only of carbon atoms.

• Alicyclic Compounds: These homocyclic compounds are similar to open chain compounds and do not contain benzene rings. Examples: Cyclopropane, Cyclobutane
• Aromatic Compounds: These compounds have a benzene ring in their structure.

2) HETEROCYCLIC COMPOUNDS

These organic compounds have one or more heteroatoms (atoms other than carbon) such as O, N, S in the ring.

• Alicyclic Heterocyclic Compounds: These compounds resemble aliphatic (non-aromatic) compounds in properties.
• Aromatic Heterocyclic Compounds: These compounds behave like aromatic benzene compounds.

2. ON THE BASIS OF FUNCTIONAL GROUP

A functional group is a specific atom or group of atoms present inside a molecule that controls its chemical behavior. It is responsible for the characteristic properties of an organic compound.

Examples:

• In alcohols, the functional group is –OH (hydroxy group)
• In aldehydes, the functional group is –CHO (aldehydic group)
• In carboxylic acids, the functional group is –COOH (carboxylic acid group)

a) Functional Groups Containing Carbon–Carbon Multiple Bonds

• Alkenes: Contain carbon–carbon double bond (C=C)
• Alkynes: Contain carbon–carbon triple bond (C≡C)
• Arenes: Contain alternate single and double bonds in a six-membered ring of carbon atoms (aromatic ring)

b) Functional Groups Containing Carbon–Oxygen Double Bond

Organic compounds having C=O (carbonyl group) form different families including aldehydes, ketones, carboxylic acids, and their derivatives.

c) Functional Groups with Carbon Bonded to N, O, S or P

Here, carbon is connected to other atoms through single or double bonds. In amines, carbon is bonded to nitrogen, and in thiols, carbon is bonded to sulfur.

NOMENCLATURE OF ORGANIC CHEMICAL SYSTEMS

As the number of new organic compounds discovered is increasing every day, many of these compounds have similar molecular formulas but different structures (they are called isomers). Because of this, a proper and systematic naming method is required for naming organic compounds.

There are two naming systems used for organic compounds:

• Trivial or common system
• IUPAC (International Union of Pure and Applied Chemistry) system

TRIVIAL OR COMMON SYSTEM

The trivial naming system was used long ago. But this system had many problems, and because of those issues, the IUPAC system was officially introduced in 1993.

1) Naming Based on Source

Organic compounds were often named according to the source from which they were obtained.

Example: Formic acid got its name from the Latin word “Formicus”, meaning red ant, because it was obtained from ants.

2) Naming Based on Properties

• Glucose (sweet taste)
• Glycol (sweet but poisonous)
• Glycerol (sweet)

3) Naming Based on Discoverer

• RMgX (Grignard Reagent) – named after Victor Grignard
• R₂Zn (Frankland Reagent) – named after Edward Frankland

4) Naming Based on Structure

Some names were simply based on how the compound looked structurally (shape, type of chain, etc.)

Drawbacks of the Trivial System

• It was not systematic, so naming a large number of compounds became impossible
• The names were not scientific
• The same compound discovered in different places often got different names

Example: Acetic acid came from Latin word Acetum (vinegar). Methyl alcohol was earlier called wood spirit, as it was first isolated from wood.

IUPAC SYSTEM OF NOMENCLATURE

The International Union of Pure and Applied Chemistry (IUPAC) introduced a scientific and systematic method of naming organic compounds. This system makes it easy to identify compounds without confusion.

Each organic compound must be named in such a way that its structure can be drawn just by reading its name.

The IUPAC name is formed by combining:

Prefix + Primary Prefix + Word Root + Primary Suffix + Secondary Suffix

Meaning of Terms

• Prefix: Shows side chains or branches
• Primary Prefix (Cyclo): Indicates cyclic structure
• Word Root: Shows number of carbon atoms in the longest chain
• Primary Suffix: Shows type of bond (ane, ene, yne)
• Secondary Suffix: Shows functional group

Word Roots for Chain Length

Different Types of Carbon Atoms

• Primary (1°) Carbon: Attached to one carbon
• Secondary (2°) Carbon: Attached to two carbons
• Tertiary (3°) Carbon: Attached to three carbons
• Quaternary (4°) Carbon: Attached to four carbons

Rules for IUPAC Naming of Branched Alkanes

• Identify the longest continuous carbon chain → this becomes the parent chain
• Number the carbon atoms in the parent chain from the end closest to the substituent (branch)
• Write the position number + name of substituent + parent name
• If more than one substituent is present, arrange their names alphabetically
• If two substituents appear at equal positions from both ends, give the lower number to the substituent that comes first alphabetically
• If the same substituent repeats, use prefixes di, tri, tetra, etc., and separate numbers by commas (these prefixes do not affect alphabetical order)

Dpharmguru’s exam insights:

IUPAC naming rules are frequently tested. Remember: The longest continuous chain determines the parent name, and numbering should always give the lowest possible number to the substituents or functional groups. A common exam question is: “What is the IUPAC name of a compound with a methyl group at position 2 and an ethyl group at position 4 on a hexane chain?” (Answer: 2-methyl-4-ethylhexane).

NOMENCLATURE OF HETEROCYCLIC COMPOUNDS

IUPAC rules provide three methods to name heterocyclic compounds.

1) HANTZSCH–WIDMAN NOMENCLATURE

This naming system depends on three main factors:

• Type of heteroatom (Z) present in the ring
• Size of the ring (number of atoms, n)
• Whether the ring is saturated or unsaturated

This method is used for heterocyclic compounds containing 3–10 atoms in a ring.

i) Type of Heteroatom

A prefix is used to indicate which heteroatom is present:

ii) Ring Size

Ring size is indicated by using suffixes. Many suffixes are taken from Latin numbers:

iii) Saturation and Unsaturation

Important Notes:

• Unsaturated suffix is used when the ring has maximum non-cumulated double bonds
• Saturated suffix is used only when the ring has no double bond
• When the ring is partially unsaturated, words like dihydro, tetrahydro are added
• Saturated 3, 4, 5-membered nitrogen rings use: iridine, etidine, olidine

Numbering Rule

When substituents are present, numbering must start from the heteroatom, which always gets position 1. Other substituents should get the lowest possible numbers.

Rings with More Than One Heteroatom

When multiple heteroatoms are present, the prefixes are combined and the ring is numbered to give the heteroatoms the lowest possible numbers. The order of priority for heteroatoms is: O > S > N > P.

2) COMMON NAMES

Many heterocyclic compounds are known widely by their non-IUPAC (common) names.

Examples: Furan, Pyridine, Pyrrole, Thiophene, Imidazole, Pyrimidine, Purine

3) REPLACEMENT NOMENCLATURE

In this method:

• The name is based on the parent carbocycle
• A prefix is added to show replacement by heteroatoms:
  • Aza (for nitrogen)
  • Oxa (for oxygen)
  • Thia (for sulfur)

Numbering Rule: Number the ring so that the heteroatom gets the lowest possible number.

Dpharmguru’s exam insights:

Heterocyclic compounds are frequently tested in pharmacy exams. Remember: The Hantzsch-Widman system is the primary method for naming heterocycles. Know the prefixes for different heteroatoms (Oxa, Aza, Thia) and the suffixes for different ring sizes. Common names like furan, pyridine, and pyrrole are also important to remember. A common exam question is: “What is the IUPAC name of a five-membered ring containing nitrogen?” (Answer: Azole or pyrrole depending on unsaturation).

COMPARISON: TRIVIAL VS IUPAC NOMENCLATURE

FREQUENTLY ASKED QUESTIONS (FAQs)

1. What is the difference between acyclic and cyclic compounds?

Acyclic compounds have open chains of carbon atoms with no rings. Cyclic compounds have closed rings of carbon atoms.

2. What is catenation?

Catenation is the ability of carbon atoms to join with other carbon atoms to form very long chains and rings of many sizes. This property is unique to carbon and is the reason for the vast number of organic compounds.

3. What is a functional group?

A functional group is a specific atom or group of atoms present inside a molecule that controls its chemical behavior. It is responsible for the characteristic properties of an organic compound.

4. What is the difference between homocyclic and heterocyclic compounds?

Homocyclic (or carbocyclic) compounds have rings made only of carbon atoms. Heterocyclic compounds have rings containing one or more heteroatoms (atoms other than carbon) such as O, N, or S.

5. What is the priority order of prefixes in the Hantzsch-Widman system?

In the Hantzsch-Widman system, when multiple heteroatoms are present, the prefixes are arranged in the following priority order: O > S > N > P.

6. What is the difference between alicyclic and aromatic compounds?

Alicyclic compounds are cyclic compounds that do not contain a benzene ring and behave like aliphatic compounds. Aromatic compounds contain a benzene ring and have special stability and properties due to delocalized electrons.

SUMMARY

Organic chemistry is the study of carbon compounds, and its nomenclature is essential for clear communication among scientists worldwide. This guide covered:

• Classification of organic compounds based on structure (acyclic, cyclic, homocyclic, heterocyclic) and functional groups
• Trivial (common) system—its advantages and drawbacks
• IUPAC system—the systematic method for naming organic compounds
• Hantzsch-Widman nomenclature for heterocyclic compounds—including prefixes for heteroatoms, suffixes for ring size, and rules for numbering
• Common names of important heterocyclic compounds
• Replacement nomenclature as an alternative method

As I always tell my students: “Nomenclature is the language of chemistry. Learn the rules, and you will be able to read, write, and understand the entire library of organic compounds!”

REFERENCES AND FURTHER READING

• Pharmacy Council of India (PCI). (2022). Pharmaceutical Chemistry Syllabus. New Delhi: PCI.
• Morrison, R. T., & Boyd, R. N. (2019). Organic Chemistry (7th ed.). Pearson Education.
• Finar, I. L. (2020). Organic Chemistry (Vol. 1 & 2). Pearson Education.
• IUPAC. (2020). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. Royal Society of Chemistry.
• Williams, D. A., & Lemke, T. L. (2019). Foye’s Principles of Medicinal Chemistry (8th ed.). Wolters Kluwer.

Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns. Pharmaceutical regulations and guidelines may vary by region—always refer to your local regulatory authorities for specific requirements.

written by:
Dr. N. Sujith Kumar

Dr. N. Sujith Kumar is a pharmacy educator, Pharm.D graduate, and academic content creator from Jawaharlal Nehru Technological University Kakinada (JNTUK), where he completed his Doctor of Pharmacy (Pharm.D) degree between 2015 and 2021. He has more than 7 years of experience creating pharmacy educational content, writing study materials, and reviewing academic articles for pharmacy students. He has also contributed guest articles to pharmacy education platforms, including PharmD Guru. At D.PharmGuru, his work focuses on simplifying complex Diploma in Pharmacy (D.Pharmacy) subjects into easy-to-understand notes, practical explanations, and exam-oriented educational resources for students across India. His areas of focus include Human Anatomy and Physiology, Pharmaceutics, Pharmacology, Pharmaceutical Chemistry, Hospital and Clinical Pharmacy, and other core D.Pharmacy subjects.

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