IUPAC Nomenclature of Cycloalkanes & Cycloalkenes | Organic Chemistry

Cycloalkanes & Cycloalkenes

IUPAC Nomenclature Rules | Organic Chemistry

1. Nomenclature of Cycloalkanes

Cycloalkanes are saturated hydrocarbons arranged in a ring. General Formula: $C_nH_{2n}$ (Isomeric with Alkenes).

Rule: Use the prefix 'cyclo' before the name of the corresponding straight-chain alkane.
Example: Cyclopropane (3 carbons), Cyclohexane (6 carbons).

2. Rule: Ring vs Side Chain

When an alkyl chain is attached to a ring, we must decide which is the Parent and which is the Substituent.

If Number of C in Ring $\ge$ Number of C in Chain:
Ring is Parent (Name: Alkylcycloalkane).

If Number of C in Chain > Number of C in Ring:
Chain is Parent (Name: Cycloalkylalkane).

Examples:

• Propylcyclohexane (Ring has 6, Chain has 3 $\to$ Ring is Parent).
• 1-Cyclobutylpentane (Ring has 4, Chain has 5 $\to$ Chain is Parent, Ring is 'Cyclobutyl' substituent).

3. Numbering Substituted Cycloalkanes

1. Monosubstituted: No numbering required (e.g., Methylcyclohexane).
2. Polysubstituted: Follow the Lowest Locant Rule. Number the carbons such that the set of locants is minimized at the first point of difference.
3. Tie-Breaker: If locants are identical (e.g., 1,3 vs 1,3), prioritize Alphabetical Order. The group coming first alphabetically gets the lower number (usually 1).

Example: 1-Ethyl-3-methylcyclohexane (not 3-Ethyl-1-methyl...).

4. Nomenclature of Cycloalkenes

Cyclic compounds containing a double bond.

Critical Numbering Rule: The double bond carbons are ALWAYS numbered C1 and C2.

Direction of Numbering

You must number through the double bond ($1 \to 2$) in the direction that gives the substituents the lowest possible numbers.

Example: 3-Methylcyclohexene

• Double bond is at 1-2.
• We number towards the methyl group to give it locant 3 (path 1-2-3).
• Going the other way (1-6-5-4-3) would give it locant 6, which is incorrect.

5. Priority of Functional Groups

If a principal functional group (like $-OH, -COOH, -CHO$) is present:

• On the Ring: The carbon attached to the FG gets position 1. Double bonds get lower priority than the FG.
• On the Chain: The chain becomes the parent, and the ring becomes a substituent, regardless of size.

Practice Quiz

Test your knowledge on Cyclic Compounds.

Your Score: 0 / 10

© 2026 chemca-chemistry made easy

Quiz Revision Notes Downloads Class Notes Video Lectures