How to Name Cycloalkanes

When learning how to name cycloalkanes, remember that the process is very similar to naming non-cyclic alkanes, but there are a few differences that make naming cycloalkanes unique.

  • The prefix “cyclo-” is placed before the typical name of the chain. In the first example below, the parent chain has 3 carbons, so it would normally be called “propane,” but because the 3 carbons form a ring, it is called “cyclopropane.”

3 examples of cycloalkanes

 

  • When choosing a parent chain when naming cycloalkanes, the ring is given preference, even if it is not the longest chain of carbons. The example below illustrates this idea in more detail.

 

Example

Name the following cycloalkane:

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Answer

Heptylcyclopentane

As the first step in how to name cycloalkanes, we must choose the parent chain. To do this, find the longest uninterrupted chain of carbons. We have two choices:

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We choose the ring as the parent chain even though it contains fewer carbons than the straight chain.

We then name the substituent coming off the ring. Since it has 7 carbons, it is named as a heptyl substituent.

As our final step, we put everything in the name together. This molecule is named heptylcyclopentane.

This figure shows a step-by-step guide on how to name cycloalkanes

With this kind of molecule, students frequently ask why the name isn’t 1-heptylcyclopentane (note the “1” has been added). The reason is because there is no need to specify the location of the heptyl substituent. If you add the heptyl to any carbon on the cyclopentane, the molecule stays the exact same, so there’s no need to specify its position. We only add the number to a name when it’s needed to specify the position of the substituent. There is no need to specify the “1-” at the beginning of the name in all mono-substituted rings.

 

  • In naming cycloalkanes, numbering the carbons is done by assigning priority of the functional groups off the ring. In the molecule below, the halogen is higher priority than the alkyl methyl group, so we know the numbering will begin with either the chlorine or the fluorine below.

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We next turn to alphabetical order. Therefore, chlorine is off carbon 1 as “c” comes before “f” in the alphabet. We then decide we want to give the next lowest priority to the fluorine because it is a halogen and has higher priority. We therefore number the ring going counterclockwise.

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We know our parent chain is “cyclohexane” as it is a 6-membered ring.

So in naming this cycloalkane, the answer is: 1-chloro-3-fluoro-5-methylcyclohexane.

 

If the substituents are all the same when naming cycloalkanes, then number the parent chain to minimize the number of each substituent as much as possible.

  • It is common for chemists to specify a bond’s location in 3-D space through the use of dashes and wedges. The bond consisting of small lines  is called a dash, and it indicates that the atom on the bond is going into the page in 3-D space.  The filled-in bond is called a wedge, and it indicates that the atom on the bond is coming out of the page in 3-D space.

 

This has important consequences for naming cycloalkanes. If 2 substituents in a di-substituted ring are on the same side of the ring (meaning both on wedges or both on dashes), then the molecule is named as cis (pronounced as “sis”; think cis same side to remember this). If the 2 substituents in a di-substituted ring are on opposite sides of the ring (one dash and one wedge), then the molecule is named as trans.

In naming cycloalkanes, you must distinguish between cis and trans.

There will be much, much more on the use of dashes and wedges in the next chapter.