Introduction to Organic Chemical Classification and Nomenclature
Organic chemistry is the branch of chemistry that focuses on carbon-based compounds. The name organic was originally used because most carbon compounds studied were obtained from plants or animals. Today, organic chemistry studies all carbon compounds, regardless of their source.
Carbon atoms have a special ability to join together to form long chains or different-sized rings. This property is called catenation. Along with carbon, other elements like hydrogen, oxygen, nitrogen, sulphur, halogens, and phosphorus are also found in organic compounds. Because of these combinations, organic molecules can be complex in structure.
To understand organic chemistry, we study concepts like structure, functional groups, homology, and isomerism.
Classification of Organic Compounds
Organic compounds are classified in two major ways:
- On the basis of structure
- On the basis of functional groups
1. Structural Classification of Organic Compounds
Organic compounds are grouped into open-chain (acyclic) compounds and closed-chain (cyclic) compounds.
Acyclic (Open-Chain) Compounds
These compounds have straight or branched chains:
- Saturated compounds → carbon atoms joined only by single bonds
Example: Methane - Unsaturated compounds → contain double or triple bonds
Example: Alkenes and alkynes
Cyclic (Closed-Chain) Compounds
These compounds contain atoms arranged in a ring.
Their types include:
A. Homocyclic (Carbocyclic) Compounds:
Rings made only of carbon atoms.
- Alicyclic compounds → ring resembles aliphatic chains (e.g., Cyclohexane)
- Aromatic compounds → contain benzene ring systems (e.g., Benzene, Naphthalene)
B. Heterocyclic Compounds
Rings contain one or more heteroatoms (O, N, S, etc.).
- Alicyclic heterocycles (e.g., Oxirane)
- Aromatic heterocycles (e.g., Pyrrole, Furan, Thiophene)
Functional Groups and Their Importance
A functional group is a specific atom or group of atoms responsible for the characteristic properties of an organic compound. Examples include:
- –OH (Alcohol group)
- –CHO (Aldehyde group)
- –COOH (Carboxyl group)
All compounds having the same functional group belong to the same family and show similar chemical behaviour.
Nomenclature of Organic Chemical Systems
Due to the increasing number of organic compounds and many compounds having same molecular formula but different structures, systematic naming became necessary.
There are two naming systems:
- Trivial or common system
- IUPAC system
Trivial or Common Naming System
Earlier organic compounds were named based on:
- Source (e.g., Formic acid from ants)
- Property (e.g., Glucose means sweet)
- Discoverer (e.g., Grignard reagent)
- Structure (e.g., based on number of carbons)
Limitations of trivial names:
- Not scientific
- Same compound could have multiple names
- Cannot name large number of compounds systematically
IUPAC System of Naming Organic Compounds
The International Union of Pure and Applied Chemistry (IUPAC) introduced systematic rules for naming.
An IUPAC name consists of:Prefix + Word Root + Primary Suffix + Secondary Suffix
- Word root → number of carbon atoms in the main chain
- Primary suffix → saturation type (–ane, –ene, –yne)
- Secondary suffix → functional group (–ol, –al, –oic acid, etc.)
- Prefix → substituents (alkyl groups, halogens)
Nomenclature of Heterocyclic Organic Chemical Compounds
Heterocyclic compounds contain a ring having carbon atoms and at least one different atom (N, O, S).
Naming Rules (Hantzsch-Widman System)
- Type of heteroatom defines prefix:
- O → oxa
- N → aza
- S → thia
- Ring size determines suffix:
- 3-membered → irane
- 4-membered → etane
- 5-membered → olane/ole
- 6-membered → inane/ine
Examples:
- Oxirane (3-membered ring with oxygen)
- Thiophene (5-membered aromatic ring with sulphur)
- Pyridine (6-membered aromatic ring with nitrogen)
Multiple Heteroatoms in Ring
Priority order for naming:
O > S > N
If two atoms are present, prefix is repeated, e.g.:
- 1,3,5-Triazine (three nitrogen atoms)
Fused Heterocyclic Rings
When two rings share one or more sides, the fused system is named using letter positions like [a], [b], [c].
Examples:
- Benzo[b]furan
- Benzo[c]thiophene
