Nomenclature of Organic Compounds

NCERT Reference:Chapter 8 – Organic Chemistry – Pages 231–235

Quick Notes

IUPAC Name = Prefix + Word root + Suffix

Word Root: Indicates the number of carbon atoms in the longest chain.
Primary Suffix: Indicates saturation/unsaturation (-ane, -ene, -yne).
Secondary Suffix: Denotes functional group (-ol, -al, -one, etc.).

Substituents (alkyl/halogen): Mentioned as prefixes with location number.
Multiple Functional Groups: Highest priority group is given the suffix and others are treated as prefixes.
Special cases: Aromatics like benzene derivatives follow specific naming rules.

Full Notes

The IUPAC System of Nomenclature

The IUPAC (International Union of Pure and Applied Chemistry) system provides a universal and standardized method for naming organic compounds. It follows a set of rules to ensure each name clearly reflects the structure of the compound.

General Structure:
IUPAC Name = Prefix(es) + Word Root + Primary Suffix + Secondary Suffix

Prefix: Includes substituents (e.g., methyl-, bromo-) and locants (position numbers).
Word Root: Indicates the number of carbon atoms in the main (longest continuous) chain.
- 1 = meth, 2 = eth, 3 = prop, 4 = but, 5 = pent, etc.
Primary Suffix: Describes the type of carbon-carbon bond:
- All single: -ane
- One double: -ene
- One triple: -yne
Secondary Suffix: Indicates the functional group, e.g., -ol, -al, -one, -oic acid.
Note: A vowel may be dropped from the primary suffix when it is followed by a secondary suffix starting with a vowel.

IUPAC Nomenclature of Alkanes

Find the longest carbon chain – this becomes the parent chain.
Number the chain from the end nearer to the substituent.
Name and number substituents as prefixes.
Alphabetically order substituents when more than one is present.
Use numerical prefixes for multiple identical groups (di-, tri-, tetra-).

Example 2-methylpentane

CH₃–CH(CH₃)–CH₂–CH₂–CH₃
Longest chain = pentane, substituent = methyl at carbon 2

NCERT 11 Chemistry example diagram for IUPAC naming showing 2-methylpentane with the longest chain numbered and the methyl substituent at carbon 2.

Nomenclature of Organic Compounds Having Functional Group(s)

When a functional group is present, it is given the highest priority (except in a few special cases like ethers and halides).
Locate the functional group and assign it the lowest possible number.

Suffix names for functional groups:

–OH (alcohol): -ol
–CHO (aldehyde): -al
–COOH (carboxylic acid): -oic acid
–NH₂ (amine): -amine
–CO– (ketone): -one

Example Butanoic acid

CH₃–CH₂–CH₂–COOH → Butanoic acid

NCERT 11 Chemistry structure diagram showing naming of CH3CH2CH2COOH as butanoic acid with carboxyl functional group priority.

Nomenclature of Substituted Benzene Compounds

Monosubstituted benzene: The substituent is simply prefixed to "benzene".

NCERT 11 Chemistry diagram of monosubstituted benzene examples such as methylbenzene and chlorobenzene.

Disubstituted benzene:

NCERT 11 Chemistry diagram illustrating ortho 1,2 meta 1,3 and para 1,4 disubstitution patterns on benzene.

For complex substitutions, the benzene ring is treated as a substituent (phenyl group).

NCERT 11 Chemistry examples showing 1,3-dichlorobenzene, 2-bromo-4-methylbenzene, and benzoic acid naming patterns.

Summary

IUPAC naming uses a consistent framework of prefix, word root, and suffixes.
Choose the longest chain and lowest locants for substituents and functional groups.
Functional group priority determines the main suffix; others become prefixes.
Substituted benzene compounds use position descriptors and specific ring rules.