Chemistry Guide


An alkane is a saturated hydrocarbon. Alkanes consist only of hydrogen and carbon atoms and all bonds are single bonds. Alkanes have the general chemical formula: CnH2n+2. For example, methane is CH4, in which n = 1 (n being the number of carbon atoms).There are two main commercial sources of alkanes: petroleum and natural gas.

The IUPAC nomenclature (systematic way of naming compounds) for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane".

Linear Alkanes

Straight-chain alkanes are sometimes indicated by the prefix "n-" (for normal) where a non-linear isomer exists. Although this is not strictly necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g., n-hexane or 2- or 3-methylpentane. Alternative names for this group are: linear paraffins or n-paraffins.

The members of the series (in terms of number of carbon atoms) are named as follows:

Methane, CH4 – one carbon and four hydrogen
Ethane, C2H6 – two carbon and six hydrogen
Propane, C3H8 – three carbon and 8 hydrogen
Butane, C4H10 – four carbon and 10 hydrogen
Pentane, C5H12 – five carbon and 12 hydrogen
Hexane, C6H14 – six carbon and 14 hydrogen
Branched Alkanes
Simple branched alkanes often have a common name using a prefix to distinguish them from linear alkanes, for example n-pentane, isopentane, and neopentane. For example:
Isobutane (IUPAC: 2-methylpropane)


IUPAC naming conventions can be used to produce a systematic name. The key steps in the naming of more complicated branched alkanes are as follows:
  • Identify the longest continuous chain of carbon atoms
  • Name this longest root chain using standard naming rules
  • Name each side chain by changing the suffix of the name of the alkane from "-ane" to "-yl"
  • Number the root chain so that the sum of the numbers assigned to each side group will be as low as possible
  • Number and name the side chains before the name of the root chain
  • 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.
  • Add side chain names in alphabetical (disregarding "di-" etc. prefixes) order in front of the name of the root chain
Cyclic Alkanes
So-called cyclic alkanes are, in the technical sense, not a subset of alkanes, but are cycloalkanes instead. They are hydrocarbons just like alkanes, but contain one or more rings.

Simple cycloalkanes have a prefix "cyclo-" to distinguish them from alkanes. Cycloalkanes are named as per their acyclic counterparts with respect to the number of carbon atoms, e.g., cyclopropane (C3H6) is a cycloalkane with 3 carbon atoms just like propane (C3H8), but they are joined up in a three-membered ring. In a similar manner, butane and cyclobutane, etc.
Cyclopropane (C3H6)

Substituted cycloalkanes are named similarly to substituted alkanes — the cycloalkane ring is stated, and the substituents are according to their position on the ring, with the numbering decided by the Cahn–Ingold–Prelog priority rules.
*       *       *
There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is saturated, and is a hydrocarbon. Waxes include examples of larger alkanes where the number of carbons in the carbon backbone is greater than about 17, above which the compounds are solids at standard ambient temperature and pressure (SATP).

Alkanes are not very reactive and have little biological activity. Alkanes can be viewed as a molecular tree upon which can be hung the more active/reactive functional groups of biological molecules.





This article uses material from the Wikipedia article "Alkanes", which is released under the Creative Commons Attribution-Share-Alike License 3.0.