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 C_nH_2n+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.

Isomers

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.