Basic Organic Nomenclature

Mono-Cyclic Alkanes

The bonding of carbon atoms is not limited to simple chains. Carbon atoms can form cyclic structures as well. The general formula for a linear or branched chain is C_nH_2n+2 where n is the number of carbon atoms. In a mono cyclic ring the general formula is C_nH_{2n .} The loss of the two hydrogen atoms represents the additional carbon-carbon bond in the ring.

Naming cyclic rings follows the same rules as for linear chains, with the addition of the prefix cyclo to the name of the compound.

Names of the simple cyclic hydrocarbons

Formula	Name	Line Diagram	Molecular Model
C₃H₆	cyclopropane
C₄H₈	cyclobutane
C₅H₁₀	cyclopentane
C₆H₁₂	cyclohexane

Substitution on the ring is also possible. When the ring is larger than the longest continuous chain of carbon atoms in a substituent, the ring will be the parent compound.

Example 1.)

The ring is larger than the ethyl group so the name is: ethylcyclopentane
NOTE: no number is needed as there is only a single substituent.

Multiple substituents are located on the ring by numbering clockwise or counter-clockwise from one substituent to the other(s), thus there will always be a substituent at carbon number 1. The substituent, and direction of numbering, are chosen to give the lowest total for all substituents, i.e. starting at one substituent what is the shortest distance arround the ring, in either direction, to the next substituent.. If the numbers for the substituents will be the same in either direction, then the substituent that comes first alphabetically should have the lower number.

Example 2.)

The ring is larger than both groups so the name is: 1-ethyl-2-methylcyclobutane
NOTE: the ethyl group is first alphabetically so it gets the lower number..

Example 3.)

The name is: 4-isopropyl-1,2-dimethylcyclohexane
NOTE:

this gives to lowest total number for the three substituents, i.e. the shortest distance from the two methyl groups to the isopropyl group.
the isopropyl comes before the dimethyl

When the chain is longer than the ring, the ring becomes a substituent on the chain. As a substituent, the ring is named following the rules for alkanes, drop the -ane and replace it with -yl. For example a cyclopropane ring as a substituent becomes a cyclopropyl group.

Example 4.)

The chain is larger so the name is: 4-cyclobutyl-5-cyclopropyl-2-methylheptane

Next Page: Cycloalkanes - Stereochemistry

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