Basic Organic Nomenclature

An Introduction

Dave Woodcock,
Associate Professor Emeritus UBC (Okanagan)
©1996,2000, 2008

2. Alkanes
III. Cycloalkanes (i)

Index
Alkanes in which at least one of the continuous carbon chains is linked back on to itself in the form of a ring are known as cycloalkanes.

To name such rings, the number of carbons in the ring forms the basis of the name and the prefix cyclo is added to indicate that the ring exists.

Molecular formula note:

To form a ring, two hydrogens on the end carbons of a carbon chain are lost when the new C,C bond is formed closing the ring. Thus, the molecular formula of a cycloalkane differs from that of an acyclic alkane by two hydrogens for each ring.

General formulae:

  • alkanes: CnH2n+2
  • cycloalkanes, one ring: CnH2n
  • cycloalkanes, two rings: CnH2n-2
  • etc.
Case 1.

If the ring contains the longest continuous chain of carbons then the root name indicates this ring.

Simple Non-substituted Cycloalkanes.
FormulaNameLine DiagramMolecular Model
C3H6cyclopropane
C4H8cyclobutane
C5H10cyclopentane
C6H12cyclohexane

Examples of substituted cycloalkanes in which the longest continuous chain is the ring:

ethylcyclobutane 1-ethyl-2-methylcyclopentane

Note: the ring carbons are numbered in the same way that the main chain carbons of acyclic (i.e. non-cyclic) alkanes are numbered.
Case 2.

When the ring carbon chain is not the longest carbon chain in the molecule (and at other times to simplify the naming process), the ring forms the branch and is named in a prefix to the root name (cyclo is still needed, this time in the prefix):

4-cyclopentyloctane 3-cyclobutyl-1-cyclopropyl-4-methylpentane
Self Assessment Problems
Review Last Page: branched branches

Next page : Cycloalkanes (ii)

Index

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