Club Chemistry: Naming Cyclic Alkanes

Rules of Organic Nomenclature For Single-Bonded Carbon Rings

Alkanes are compounds consisting of linear carbon atoms connected by single bonds. They can exist as straight chains or with their carbon atoms linked to form rings.

Organic chemistry is the science concerned with the structure and reactions of carbon compounds. The name "organic" derives from the initial belief of scientists that such compounds were strictly associated with living systems. Of course, this is now known to be untrue, but the name stuck.

The Alkanes

The alkanes contain only single carbon-to-carbon bonds; e.g., C-C-C-C-C. It is their single-bonded nature that distinguishes alkanes from the alkenes, which posses at least one carbon double bond; e.g., C-C=C-C, and the alkynes, which have a triple bond; e.g., C≡C. Remember, to save time in drawing carbon compounds, hydrogen atoms are not shown and are assumed to be there.

Naming Cyclic Alkanes

Cyclic alkanes, like their straight chain relatives, are hydrocarbons. Unlike linear alkanes, cyclic alkane molecules have no free ends. The carbon atoms are connected in a closed loop. Just as in naming linear alkanes the first step in naming cyclic alkanes is to count the number of carbons. The name of any alkane is based on using standard prefixes corresponding to the number of carbons, with an "-ane" stuck on the end. In the case of a cyclic compound, the prefix "cyclo-" is attached to the front of the name.

If one knows the naming rules for linear alkanes, it's easy to name cyclic ones. A four-carbon ring is cyclobutane; a six-carbon ring is cyclohexane, etc.

Naming Cyclic Alkanes Containing One Functional Group

Just like straight-chain compounds, cyclic compounds can have functional groups (sometimes called side groups) bonded to one of more of the main chain carbons. To name such compounds, simply prefix the name of the cycloalkane with a prefix based on the group name. Take for example a cyclohexane that has a single nitro (ONO) group attached to one of its carbons in place of a hydrogen. The name of this compound is nitrocyclohexane.

This naming method works for most functional groups. The alcohols and some others are exceptions. If a hydroxyl group (OH) is substituted in place of a hydrogen on cyclohexane, it forms cyclohexanol (which is the official IUPAC name rather than hydroxycyclohexane). Other example substituted cyclic alkanes are methylcyclopentane, aminocycloheptane (cycloheptylamine) and chlorocyclopropane.

Naming Cyclic Alkanes Containing More Than One Functional Group

Cycloalkanes with one functional group require no numbering system. However, a numbering scheme similar to that used for linear alkanes comes into use if there is more than one substitution.

Start by assigning the number 1 to the carbon so that the second group (and third group, etc.) has as low a number as possible.
If two groups could have the same number, number them in alphabetical order.
After assigning the numbers, name the compound so that the side groups occur in alphabetical order (NOT numerical order).

Consider a cyclopentane with three groups attached. It would be numbered 1, 2, 4 and not 1, 3, 5. (Draw it and prove it.) Now further consider an ethyl at C-1, a fluorine at C-2 and a butyl at C-4. The full name would be 4-butyl,1-ethyl,2-fluorocyclopentane, not 1-ethyl,2-fluoro,4-butylcyclopentane.

Cyclic Alkanes as Functional Groups

Cyclic alkanes can be functional groups themselves. A couple of examples should be sufficient to get across the idea of how they are named. If a cyclohexane group is attached to the carbon-2 position of a linear octane, this would be 2-cyclohexyloctane.

If the number of carbons in the ring is equal to or greater the number of carbons in a linear alkane group bonded to it, use the cyclic group as the main chain. An example of this is propylcyclobutane (not cyclobutylpropane).

References

IUPAC Nomenclature of Organic Chemistry, Monocyclic Hydrocarbons

Jim Hollister, Rolf Unterleitner, Doug Kent, "Naming Cycloalkanes". Personal academic website of Jim Hollister, Learning Skills Center, University of California, Davis, undated.

The copyright of the article Naming Cyclic Alkanes in Chemistry is owned by Philip McIntosh. Permission to republish Naming Cyclic Alkanes in print or online must be granted by the author in writing.