# Nomenclature of Alkanes and Cycloalkanes

IUPAC rules are used to give each organic molecule a unique name. Rules involve determining the name of the parent structure and then adding a modifying suffix to the name based on functional groups. Then substituents' names and locations are added as prefixes with a number, or locant, that indicates their location on the parent structure.
An alkane is a hydrocarbon containing only ${\rm {C{-}C}}$ single bonds and hydrogen with CnH2n+2 stoichiometry. An alkene is a hydrocarbon containing at least one carbon-carbon double bond (${\rm {C{=}C}}$) with CnH2n stoichiometry. An alkyne is a hydrocarbon that contains at least one carbon-carbon triple bond (${\rm {C{\equiv}C}}$) with CnH2n-2 stoichiometry. Alkanes are the foundation of the IUPAC (International Union of Pure and Applied Chemistry) system of organic nomenclature, the system used to name organic compounds.

### IUPAC Names of Hydrocarbons

Name Number of Carbons
Methane 1
Ethane 2
Propane 3
Butane 4
Pentane 5
Hexane 6
Heptane 7
Octane 8
Nonane 9
Decane 10

The names of the first ten hydrocarbons are indicated with a prefix followed by -ane, such as methane and ethane.

When naming compounds, start with the longest continuous chain of carbon atoms, and write the name of the parent compound. Next, name the substituents by their locant using hyphens to separate the words. A substituent is an atom or group of atoms (functional group) that replaces a ${\rm {C{-}H}}$ bond in an organic compound. The locant is the position or location of a substituent in an organic molecule. For example, a compound that consists of five carbons would have the base name of pentane (pent- is the prefix for five carbons). If that pentane had a one-carbon group (substituent) coming off of the second carbon (locant), the name of that compound would be 2-methylpentane.

When a functional group is added as a substituent, the suffix -yl is added to its name. For example, methane becomes methyl when added as a substituent. When determining the locant of a substituent, the lowest numeric location on the parent chain is chosen. For example, 2-methylpentane is the correct name, not 4-methylpentane.

#### Naming Alkanes Using IUPAC

If the same substituent appears more than once in a compound, use the prefixes di-, tri-, tetra-, etc.

#### Alkane Constituents

Di-, tri-, tetra-, and other prefixes indicating multiple substituents are not considered in alphabetization. Likewise, sec- and tert-, such as in sec-butyl, are not considered in alphabetization.

#### Alkane with Two Substituents

A compound that is cyclic (or a ring or a cycloalkane) describes a molecule containing a ring of at least three carbons.

#### Examples of Cycloalkanes

Monosubstituted cycloalkanes are cyclic rings with only one substituent and will always have the substituent on the first carbon. In monosubstituted cycloalkanes, it is not necessary to include a locant, as it is understood that the substituent has to be on carbon 1. If there is more than one substituent, the carbons are numbered in order to give the lowest set of numbers. For example, 1,2,4-trimethylcyclohexane is a correct name, while 1,4,6-trimethylcyclohexane would be incorrect. Both represent the same compound, but the correct name always has the smaller combination of numbers (1,2,4 versus 1,4,6). If a double bond or alcohol (OH) is present, those groups have priority and will have the lowest number. For example, 3,4-dimethylcyclohexene is correct, while 1,2-dimethy-3-cyclohexene is incorrect.

#### Numbering Carbons in Cycloalkanes with Substituents

A bicyclic compound is a molecule containing two rings. Bicyclic compounds are named with the basic formula bicyclo[X.Y.Z]alkane, where X, Y, and Z are the carbons in the bridges. In a bicyclic compound, the carbon where three bonds from the two cyclic rings meet are considered the bridgehead carbons. There are always two and only two bridgehead carbons. One of the two bridgehead carbons must always be the number 1 carbon, and the numbering proceeds through the longest bridge. In order to correctly name a bicyclic compound, use the following steps:

1. Identify the two bridgehead carbons.

2. Number from one bridgehead carbon around to the other bridgehead carbon.

3. Identify the different pathways between the bridgehead carbons.

4. Tally up the number of carbons along the paths.

5. Arrange the path lengths in descending numerical order inside the brackets [X.Y.Z], where X is the longest path and Z is the shortest path.

6. Add up the total number of carbons in the bicyclic structure (not counting substituents) to get the alkane name.

7. Place the alkane name after the brackets.

8. Add bicyclo before the brackets to get the final name, bicyclo[X.Y.Z]alkane.