Branched Chain Alkane Definition

Isopentane is an example of a branched chain alkane.

Anne Marie Helmenstine, Ph.D.

An alkane is a saturated hydrocarbon. Alkanes may be linear, branched, or cyclic. Here's what you need to know about the branched alkanes.

Branched Alkane Definition

A branched chain alkane or branched alkane is an alkane which has alkyl groups bonded to its central carbon chain. Branched alkanes contain only carbon and hydrogen (C and H) atoms, with carbons connected to other carbons by single bonds only, but the molecules contain branches (methyl, ethyl, etc.) so they are not linear. 

How to Name Simple Branched Chain Alkanes

There are two parts to each name of a branched alkane. You may consider these parts as prefix and suffix, branch name and stem name, or alkyl and alkane. The alkyl groups or substituents are named in the same way as the parent alkanes, except each contains the suffix -yl. When not named, alkyl groups are represented as "R-".

Here is a table of common substituents:

Substituent Name
CH3- methyl
CH3CH2- ethyl
CH3CH2CH2- propyl
CH3CH2CH2CH2- butyl
CH3CH2CH2CH2CH2- pentyl

Names are constructed in the form locant + substituent prefix + root name according to these rules:

  1. Name the longest alkane chain. This is the longest string of carbons.
  2. Identify the side chains or branches.
  3. Name each side chain.
  4. Number the stem carbons such that the side chains will have the lowest numbers.
  5. Use a hyphen (-) to separate the number of the stem carbon from the name of the side chain.
  6. The prefixes di-, tri-, tetra-, penta-, etc. are used when there is more than one alkyl group attached to the main carbon chain, indicating how many times the specific alkyl group occurs.
  7. Write the names of different types of alkyl groups in alphabetical order.
  8. Branched alkanes may have the prefix "iso".

Examples of Branched Chain Alkane Names

  • 2-methylpropane (This is the smallest branched chain alkane.)
  • 2-methylheptane
  • 2,3-dimethylhexane
  • 2,3,4-trimethylpentane

Different Methods of Representing Branched Alkanes

Linear and branched alkanes may be represented using a:

  • skeletal formula, showing only bonds between carbon atoms
  • shortened structural formula, showing atoms, but no bonds
  • full structural formula, with all atoms and bonds depicted
  • 3-D model, showing atoms and bonds in three dimensions

Importance and Uses of Branched Alkanes

Alkanes do not readily react because they are saturated hydrocarbons. However, they can be made to react to yield energy or to make useful products. Branched alkanes are of particular importance in the petroleum industry.

  • When provided sufficient activation energy, alkanes react with oxygen to produce carbon dioxide, water, and energy, thus alkanes are valuable fuels.
  • The process of cracking breaks longer chains alkanes into smaller alkanes and alkenes to increase octane number and to make polymers.
  • C4-C6 alkanes may be heated with platinum or aluminum oxide catalysts to cause isomerism to produce branched chain alkanes. This is used to improve octane number.
  • Reforming increases the number of cycloalkanes and benzene ring-containing hydrocarbons to improve octane number.
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Helmenstine, Anne Marie, Ph.D. "Branched Chain Alkane Definition." ThoughtCo, Aug. 27, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 27). Branched Chain Alkane Definition. Retrieved from Helmenstine, Anne Marie, Ph.D. "Branched Chain Alkane Definition." ThoughtCo. (accessed February 2, 2023).