Naming Cycloalkanes

Core Concepts

In this tutorial on naming cycloalkanes, you will learn how to apply the IUPAC nomenclature system. If you enjoy this tutorial, be sure to check out our other naming tutorials listed below.

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What are cycloalkanes?

File:First four cycloalkanes.png - Wikimedia Commons

Similar to alkanes, cycloalkanes are comprised of single-bonded hydrocarbon chains. The only difference is cycloalkanes contain a cyclic structure. Common cycloalkanes are shown in the image above. Notice they all start with the prefix cyclo, which is greek for ‘cycle’ and used in chemistry to describe a round/circular molecule. Moreover, the suffixes tell you how many carbons are in the ring.

How do you name Cycloalkanes?

Naming cycloalkanes using IUPAC Nomenclature

When we are given the condensed formula or skeletal structure of a molecule, we can determine the name by using a system called IUPAC. As mentioned in our naming alkanes tutorial, IUPAC is a set of rules created in 1919 by a group of chemists to guarantee each molecule has a unique name. It is important that each molecule has a unique name because it serves as an instruction manual. It tells you where bonds exactly how the molecule is built and where different substituents are located. On another note, IUPAC nomenclature for cyclic alkanes is very similar to that for linear alkanes. To find the name for a given molecule, you can follow these five easy steps:

1. Identify the parent chain

  • The parent chain is the chain with the cyclic structure. If there is a longer alkyl substituent attached, then the cyclic structure acts as the substituent.
  • Count the number of single-bonded carbons in the structure and then place ‘cyclo’ in front of it. (Remember: 3 carbons= propane, 4=butane, 5=pentane, 6=hexane )
  • In our example above, there are six carbons. This means the parent chain is cyclohexane.

2. Number the parent chain

  • Now that you know the parent chain, you must number it. There are 2 main rules for determining which direction to go:
    • Rule 1: Substituents should be placed on the lowest numbers possible. This means 1-methyl is better than 3-methyl.
    • Rule 2: If a tie occurs in rule one, choose priority based on the alphabet. For example, if there was one methyl substituent and one ethyl substituent, ethyl would win priority.
  • In our example above, we would place a 1 at the ethyl and go counterclockwise. This means we have the following attachments: 1-ethyl, 2-methyl, and 4-methyl.

3. Identify the substituents and how many there are of each.

  • Substituents are attachments. This can be carbon chains like methyl groups, branched groups like sec-butyl, or functional groups. If there are multiple, we will place a numerical prefix in front of it.
  • Since we have 2 methyl groups, we will place a “di” in front.
  • Moreover, we can group the two methyl groups together. Meaning it will be written as 2,4-dimethyl.

4. Put everything together.

  • Substituents should be placed in alphabetical order regardless of the numerical prefixes like “di”.
  • The final product for our example would be 1-ethyl-2,4-dimethylcyclohexane.

Some technical things to consider when naming cycloalkanes:

  1. the parent chain is cycloalkane unless it’s bound to a larger alkyl chain. In this case, it becomes a substituent
  2. If the alkyl chain and the cycloalkane both contain the same number of carbons, then the parent chain will be the cycloalkane/ the ring.
  3. If there’s only one substituent attached to the cycloalkane, it will default to being the 1st carbon.

Do cycloalkanes have benzene rings?

Benzene rings are an aromatic hydrocarbon functional group. In fact, they are the smallest arene with a molecular formula of C6H6. Though similar to cyclohexanes, they are very different. Unlike cyclohexanes, benzene rings contain alternating pi and sigma bonds which makes the compound unsaturated. For reference, an unsaturated compound is a compound that contains double or triple bonds which decrease the amount of hydrogens present. Therefore, cycloalkanes do not have benzene rings.

Further Reading