Nomenclature: Cycloalkanes

Summary

Unsubstituted cycloalkanes are named by adding the prefix cyclo- to the name of the corresponding linear alkane.

The nomenclature of substituted cycloalkanes depends if the cycloalkane is the parent chain, or if it is treated as a substituent (and there are different approaches for deciding this). The names of cycloalkanes as substituents (i.e., cycloalkyl substituents) are given by replacing the suffix -ane with the suffix -yl (e.g., cyclopropane and cyclopropyl).

If the cycloalkane is the parent chain (cycloalkane has either the same number of carbons, or a greater number of carbons than the longest linear alkane chain):

  1. Number the parent chain carbons, giving lower numbers to higher priority substituents.
  2. List the substituents in alphabetical order.
  3. Finish with the name of the cycloalkane parent chain.

If the cycloalkane is a substituent (in a branched alkane; cycloalkane has fewer carbons than longest linear alkane chain):

  1. Number the parent chain carbons, giving lower numbers to higher priority substituents (cycloalkyl substituents are the same priority as other alkyl substituents).
  2. List the substituents in alphabetical order (the prefix cyclo– is considered).
  3. Finish with the name of the alkane parent chain.

Background

Cycloalkanes (or cyclic alkanes) are alkanes (saturated hydrocarbons) that contain one ring (that is, they are monocyclic). These molecules may or may not have substituents on the ring carbons.

Nomenclature

The name of an unsubstituted cycloalkane is based on the number of carbons in the ring. The name starts with the prefix cyclo– followed by the name of the linear alkane made up of the same number of carbons. As an example, the four-carbon cycloalkane is named cyclobutane, as the four-carbon linear alkane is butane. The smallest cycloalkane is cyclopropane, which has a three-membered ring. Here are the structures of the most common cycloalkanes:

Unsubstituted Cycloalkanes

Remember that each corner of these rings represents a carbon. For unsubstituted cycloalkanes, each of these carbons is bonded to two hydrogens.

For assigning the systematic name of alkyl substituted cycloalkanes (that is, cycloalkanes with one or more alkane chains bonded to the ring), the first step is to identify the parent chain. However, there are two different methods for identifying the parent chain.

In one method, the cycloalkane ring is always the parent chain, regardless of the sizes of any alkyl substituents. In the other method, the identity of the parent chain depends on the number of carbons in the cycloalkane ring, as well as the length of the longest linear carbon chain of the alkyl substituents.

Both methods give a systematic name to the cycloalkane, so this is largely up to the discretion of your instructor. The IUPAC prefers the first method (in which the cycloalkane ring is always the parent chain), but the second method is commonly taught in intro organic chemistry (and will be used in this tutorial).

Cycloalkane as the Parent Chain

The parent chain of an alkyl-substituted cycloalkane depends on the number of carbons in the cycloalkane ring, as well as the length of the longest linear carbon chain in the alkyl substituents. If the number of carbons in the cycloalkane ring is equal to or greater than the number of carbons in the longest linear carbon chain of any alkyl substituent, it is the parent chain. However, if there is a linear carbon chain that is made up of more carbons than the cycloalkane, the linear carbon chain is selected as the parent chain. Here are some examples to help clarify this (the parent chain carbons are highlighted in yellow):

Cycloalkane Parent Chains

If the cycloalkane is chosen as the parent chain, the name of the parent chain is the same as that of the unsubstituted cycloalkane (as described above). For example, the three-carbon unsubstituted cycloalkane is cyclopropane, therefore the parent chain name of a substituted three-carbon cycloalkane is also cyclopropane.

With the parent chain identified, the next step is to number the parent chain carbons. For cycloalkanes, as the parent chain is cyclic, you could theoretically start numbering at any position. However, as usual, you need to give the lowest numbers possible to the carbons which are bonded to substituents. For alkyl substituents, alphabetization is once again the tie-breaker. For example, if you have a cycloalkane with an ethyl group on one carbon, and a methyl group on another, the carbon with the ethyl group is higher priority and is labeled as carbon 1; you would then number the other carbons in the ring so as to give the carbon with the methyl group the next lowest number possible. If you have only one substituent? Then you don’t need to worry about numbering.

The full name is assembled by listing the substituents in alphabetical order (including numbers and hyphens as always), and finishing with the name of the cycloalkane parent chain. If there is only one substituent on the cycloalkane ring, there’s no need to include the number of carbon bonded to this substituent. If you have two substituents, which are both on the same carbon, you do include the carbon number (e.g., 1-ethyl-1-methylcyclopropane)

Cycloalkanes as Substituents

If a linear alkyl chain is chosen as the parent chain of a substituted cycloalkane, the name of this parent chain is the same as the name of the corresponding linear alkane. For example, if the linear alkyl chain is four carbons long, the parent chain name is butane.

To refer to a cycloalkane as a substituent (that is, as a cycloalkyl substituent), take the name of the corresponding unsubstituted cycloalkane, and replace the suffix –ane with the suffix –yl. So, if you have a three-carbon cycloalkane as a substituent, this corresponds to the unsubstituted cyclopropane, and the substituent is given the name cyclopropyl. Here are the names and structures of the most common cycloalkyl substituents

Cycloalkyl Substituents

Now that we know how to name cycloalkyl substituents, the nomenclature of cycloalkyl-substituted alkanes is very similar to the nomenclature of branched alkanes. Substituents are listed in alphabetical order (with numbers and hyphens), and the name of the parent chain is added to the end. Note that the prefix cyclo- is considered for alphabetization.

Example

  Cycloalkane Example 1-1

First, identify the parent chain. The longest linear chains of the linear alkyl groups are only one (methyl) or two (ethyl) carbons long, while the cycloalkane ring is made of six carbons. Therefore, the cycloalkane is the parent chain (highlighted in yellow):

Cycloalkane Example 1-2

Next, number the parent chain. We have one ethyl substituent and two methyl substituents. The ethyl substituent has highest priority based on alphabetical order, so the carbon with the ethyl group is numbered carbon 1. There are two different directions that we could number the ring – one will place the methyl groups on carbons 2 and 4, while the other places them on carbons 4 and 6. We want the lowest number possible, which is the first direction:

Cycloalkane Example 1-3

To assemble the name, we first list the substituents. We have a 1-ethyl group, a 2-methyl group, and a 4-methyl group (which are combined to 2,4-dimethyl). In alphabetical order, this becomes: 1-ethyl-2,4-dimethyl (remember that the prefix di- is ignored for alphabetization). We finish with the name of the parent chain. The cycloalkane has six carbons, which is a cyclohexane. Combining these, we get: 1-ethyl-2,4-dimethylcyclohexane.

Cycloalkane Example 1-4 

Example

 Cycloalkane Example 2-1

First, identify the parent chain. The cycloalkane has three carbons, while the longest linear carbon chain is seven carbons – therefore the linear carbon chain is the parent chain, and the cycloalkane is a substituent:

Cycloalkane Example 2-2

Next, number the parent chain. For this molecule, we number from the left, giving lower numbers to the carbons with substituents. Note that cycloalkyl substituents are not given higher priority than other alkyl substituents.

Cycloalkane Example 2-3

As far as substituents, we have a 3-cyclopropyl group and a 4-ethyl group. Listing these in alphabetical order, we get: 3-cyclopropyl-4-ethyl. Note that the prefix cyclo- is not ignored for alphabetization (unlike di-, tri-, etc). The parent chain here is seven carbons, which is a heptane. Thus, the name is: 3-cyclopropyl-4-ethylheptane.

Cycloalkane Example 2-4

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