Chemistry · General chemistry II · Concept
Naming alkanes and haloalkanes
An alkane’s name is built from its longest continuous chain of carbons, which gives the root, such as hexane for six carbons. Each branch is an alkyl group, such as methyl or ethyl, and a halogen atom is a halo group, such as chloro. Number the chain from the end nearer the first branch, give each group its number, list the groups alphabetically and use di-, tri- or tetra- for identical groups.
Find the longest chain
The parent chain is the longest continuous chain of carbon atoms, even when it bends around corners in the drawing. Its length gives the root: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non- and dec- for 1 to 10 carbons, with the ending -ane for an alkane.
Name the branches
Every group attached to the parent chain is a substituent. Carbon branches are alkyl groups: methyl (one carbon), ethyl (two), propyl or isopropyl (three) and the four butyl groups. A halogen atom is named as fluoro, chloro, bromo or iodo.
Number the chain
Number the carbons from the end nearer the first branch. If the first branches are the same distance from each end, compare the next branch, and so on: the first point of difference decides.
Write the name
Write each group’s number, a hyphen and its name, in alphabetical order, and finish with the parent name. Identical groups share one name with di-, tri- or tetra- and their numbers separated by commas; those prefixes don’t count when alphabetizing.
Methane and ethane
A single group on methane or ethane needs no number, because every position gives the same compound: chloromethane, bromoethane. With two groups, numbers are needed again: 1,2-dichloroethane.
Common mistakes
- Naming a shorter chain that runs straight across the drawing instead of the longest chain, as in 2-ethylpentane for 3-methylhexane.
- Numbering from the end farther from the first branch, as in 4-methylhexane.
- Listing groups by number instead of alphabetically.
- Counting di- or tri- when alphabetizing.
Key terms
- Alkane
- An acyclic saturated hydrocarbon containing only single carbon–carbon bonds. Its general formula is CₙH₂ₙ₊₂.
- Parent chain
- The chain of carbons a systematic name is built on: the longest continuous chain, which for an alkene, alkyne or alcohol must contain the double bond, triple bond or –OH carbon. Its length gives the root of the name, such as hex- for six carbons.
- Substituent
- An atom or group attached to the parent chain in place of a hydrogen, such as a methyl group or a chlorine atom. In a name it becomes a prefix with a number for where it sits: 3-methylhexane.
- Alkyl group
- An alkane missing one hydrogen, attached through that carbon: methyl, ethyl, propyl, isopropyl, and the four butyl groups (butyl, isobutyl, sec-butyl, tert-butyl).
- Locant
- The number in a name that says which carbon of the parent chain carries a group or a multiple bond, as the 2 in 2-propanol. The chain is numbered from the end that gives the lower numbers.
Work through an example
Name the alkane (CH₃)₂CHCH₂CH(C₂H₅)CH₂CH₂CH(CH₃)₂.
Name a branched alkane →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
Try in the workspace
Open the example inputs, change a value and keep a useful result on your board.
Draw it and check the name Check the name 2-ethylpentane on 3-methylhexane See 3-methylhexane in every representation Open worked example on a board Organic nomenclature in Chemistry ReferenceYour existing work stays on this device. Examples open as editable copies.