Nomenclature of Organic Compounds - The IUPAC System of Nomenclature

Topics

IUPAC Nomenclature: Structure and Components
Steps for IUPAC Nomenclature

IUPAC Nomenclature: Structure and Components

IUPAC stands for the International Union of Pure and Applied Chemistry. It was established in 1919 to replace the International Congress of Applied Chemistry. The headquarters of this organisation is in the United States. It is a standardised system for naming chemical compounds. This system provides unique and universally accepted names based on the molecular structure of compounds.

IUPAC names consist of three main components: Prefix - Parent - Suffix

Components of the IUPAC Naming System:

Component	Description	Example
Root (Parent)	The longest continuous carbon chain in the molecule.	"Meth-" (1C), "Eth-" (2C), "Prop-" (3C)
Suffix	Indicates the main functional group in the molecule.	"-ol" (Alcohol), "-al" (Aldehyde), "-oic acid" (Carboxylic Acid)
Prefix	Denotes substituents or side chains attached to the parent chain.	"Chloro-", "Bromo-", "Methyl-"

Steps for IUPAC Nomenclature

Step 1: Identify the Parent Chain

Count the number of carbon atoms in the longest chain.
Assign the corresponding alkane base name:

Number of Carbons	Root Name
1	Meth-
2	Eth-
3	Prop-
4	But-
5	Pent-
6	Hex-
7	Hept-
8	Oct-
9	Non-
10	Dec-

Step 2: Identify the Functional Group

Replace the last ‘e’ from the alkane name with the functional group suffix.
If the functional group is a halogen, it is added as a prefix.

Functional Group	Structure	IUPAC Suffix
Alkene	C=C	-ene
Alkyne	C≡C	-yne
Alcohol	-OH	-ol
Aldehyde	-CHO	-al
Ketone	C=O	-one
Nitrile	C≡N	-nitrile
Carboxylic Acid	-COOH	-oic acid
Carboxylate Ion	-COO⁻	-oate
Ester	-COOR	-oate
Amine	-NH₂	-amine
Amide	-CONH₂	-amide

Step 3: Assign Numbering to the Carbon Chain

Number the chain from the end nearest to the functional group.
The functional group must get the lowest possible number.
If multiple functional groups are present, assign the lowest number to the highest-priority group.
For multiple identical groups, use prefixes like di-, tri-, tetra- (e.g., 2,3-dimethylpentane).

Sr. No.	Structural Formula	Two Numberings of the Carbon Chain	Acceptable Numbering	IUPAC Name of the Compound
1	\[\begin{array}{cc} \phantom{}\ce{CH3 - CH - CH3}\\ \|\phantom{..}\\ \ce{OH}\phantom{}\\ \end{array}\]	\[\begin{array}{cc} \phantom{.}\ce{C^{1} - C^{2} - C^{3}}\\ \phantom{}\|\\ \phantom{..}\ce{OH}\\ \phantom{.}\ce{C^{3} - C^{2} - C^{1}}\\ \phantom{}\|\\ \phantom{..}\ce{OH} \end{array}\]	Both numberings equivalent	propan-2-ol
2	\[\begin{array}{cc} \phantom{}\ce{CH3 - CH2 - CH2 - CH - CH3}\\ \phantom{................}\|\phantom{...}\\ \phantom{..............}\ce{Cl}\phantom{}\\ \end{array}\]	\[\begin{array}{cc} \phantom{.}\ce{C^{1} - C^{2} - C^{3} - C^{4} - C^{5}}\\ \phantom{...........}\|\\ \phantom{...........}\ce{Cl}\\ \phantom{.}\ce{C^{5} - C^{4} - C^{3} - C^{2} - C^{1}}\\ \phantom{...........}\|\\ \phantom{...........}\ce{Cl}\\ \end{array}\]	\[\begin{array}{cc} \phantom{.}\ce{C^{5} - C^{4} - C^{3} - C^{2} - C^{1}}\\ \phantom{}\|\\ \phantom{.}\ce{Cl}\\ \end{array}\]	2-chloropentane
3	\[\begin{array}{cc} \ce{O}\phantom{...............}\\ \|\|\phantom{...............}\\ \phantom{}\ce{CH3 - C - CH2 - CH2 - CH3}\\ \end{array}\]	\[\begin{array}{cc} \ce{O}\phantom{............}\\ \|\|\phantom{............}\\ \phantom{}\ce{C1 - C2 - C3 - C4 - C5}\\ \end{array}\] \[\begin{array}{cc} \ce{O}\phantom{............}\\ \|\|\phantom{............}\\ \phantom{}\ce{C5 - C4 - C3 - C2 - C1}\\ \end{array}\]	\[\begin{array}{cc} \ce{O}\phantom{............}\\ \|\|\phantom{............}\\ \phantom{}\ce{C1 - C2 - C3 - C4 - C5}\\ \end{array}\]	pentan-2-one

Examples of IUPAC Naming

Sr. No.	Common Name	Structural Formula	IUPAC Name
1	Ethylene	CH₂=CH₂	Ethene
2	Acetylene	HC≡CH	Ethyne
3	Acetic acid	CH₃-COOH	Ethanoic acid
4	Methyl alcohol	CH₃-OH	Methanol
5	Ethyl alcohol	CH₃-CH₂-OH	Ethanol
6	Acetaldehyde	CH₃-CHO	Ethanal
7	Acetone	CH₃-CO-CH₃	Propanone
8	Ethyl methyl ketone	CH₃-CO-CH₂-CH₃	Butan-2-one
9	Ethyl amine	CH₃-CH₂-NH₂	Ethanamine
10	n-Propyl chloride	CH₃-CH₂-CH₂-Cl	1-Chloropropane