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Structure of Alkanes

Author: Hans Lohninger

An alkane is a saturated acyclic hydrocarbon having the maximum possible number of hydrogen atoms. This implies that alkanes have only single bonds. Each carbon atom in an alkane has sp3 hybridization. Alkanes are aliphatic compounds.

The general formula for alkanes is CnH2n+2; except for the first four alkanes (n=1..4), their chemical names can be derived from the number of C atoms by using Greek numerical prefixes denoting the number of carbons and the suffix "-ane":

Formula Name No. of Isomers
CH4 methane 1
C2H6 ethane 1
C3H8 propane 1
C4H10 butane 2
C5H12 pentane 3
C6H14 hexane 5
C7H16 heptane 9
C8H18 octane 18
C9H20 nonane 35
C10H22 decane 75
....a more complete list can be found here.


The five isomers of hexane: (a) n-hexane, (b) 2-methyl-pentane, (c) 3-methyl-pentane, (d) 2,3-dimethyl-butane, (d) 2,2-dimethyl-butane.

The atoms of higher alkanes (n > 3) can be arranged in multiple ways, forming different isomers. The unbranched configurations are called n-alkanes (n = "normal"). The number of isomers increases rapidly with the number of carbon atoms, a list of the number of isomers can be found in the List of Alkanes.

The isomers of alkanes are named according to the following IUPAC convention:

  1. Identify the longest continuous chain of carbon atoms
  2. Name this longest root chain using standard naming rules
  3. Name each side chain by changing the suffix of the name of the alkane from "-ane" to "-yl"
  4. Number the root chain so that the sum of the numbers assigned to each side group will be as low as possible
  5. Number and name the side chains before the name of the root chain
  6. If there are multiple side chains of the same type, use prefixes such as "di-" and "tri-" to indicate it as such, and number each one.
  7. Add side chain names in alphabetical (disregarding "di-" etc. prefixes) order in front of the name of the root chain

A few nonsystematic ("trivial") names are retained in the IUPAC system: isobutane for 2-methyl-propane, isopentane for 2-methyl-butane, and neopentane for 2,2-dimethyl-propane; further the name isooctane (2,2,4-trimethyl-pentane) is very widely used in the petrochemical industry.

Last Update: 2011-02-21