Alkane Nomenclature

Section IIa: Alkane Nomenclature

Alkanes represent the simplest of the functional groups which are common in organic chemistry. An alkane contains only carbon and hydrogen (a hydrocarbon) and contains only single bonds (termed a saturated hydrocarbon. Alkanes have the general formula CnH2n+2, thus, an alkane with 10 carbons (n = 10) will have 2(10) + 2 = 22 hydrogens, or the molecular formula C10H22.

The root, or parent name for an unbranched alkane is taken directly from the number of carbons in the chain according to a scheme of nomenclature established by the International Union of Pure and Applied Chemistry (IUPAC), as shown below:

Name Condensed Structure
methane CH4
ethane CH3CH3
propane CH3CH2CH3
butane CH3(CH2)2CH3
pentane CH3(CH2)3CH3
hexane CH3(CH2)4CH3
heptane CH3(CH2)5CH3
octane CH3(CH2)6CH3
nonane CH3(CH2)7CH3
decane CH3(CH2)8CH3
undecane CH3(CH2)9CH3
dodecane CH3(CH2)10CH3

Hydrocarbons, however, are not restricted to linear, unbranched chains and there are often many possible orders in which a hydrocarbon with a given molecular formula can be constructed. Compounds having the same molecular formula, which differ in the order of attachment of the individual atoms, are called constitutional isomers. An example of the three possible constitutional isomers of pentane (C5H12) are shown below.

Although there are only three constitutional isomers for pentane, for alkanes having larger numbers of carbons, the number of isomers is staggering; for C30H62, there are over 4 billion possible constitutional isomers.

In order to be able to communicate chemical information, it is essential to have a systematic set of rule defining nomenclature for organic compounds. As mentioned previously, the IUPAC system of nomenclature accomplishes this and the rules for naming linear and branched alkanes are given below:
For complex molecules, the IUPAC system of nomenclature can generate somewhat bewildering, and equally complex nomenclature. This is further complicated by the fact that many common molecules are routinely referred to using simple names which are descriptive of the molecule, or have arisen historically. The most common examples of these are the substituent names for side-chains containing three to five carbons, where the prefixes iso..., sec-..., tert-..., and neo are commonly used. The structures corresponding to these substituents are shown below:

Common Substituent Names:

propyl -CH2CH2CH3
isopropyl -CHCH(CH3)2
butyl -CH2CH2CH3
isobutyl -CH2CHCH(CH3)2
sec-butyl -CH(CH3)CH2CH3
tert-butyl -C(CH3)3

When these descriptors are used in an IUPAC name, iso is alphabetized normally; the hyphenated prefixes, however (sec- and tert-) are disregarded when alphabetizing.

A more systematic method for the nomenclature of side-chains involves identifying the longest chain in the substituent, numbering the substituent from the point of attachment to the parent, and indicating side-chains on the substituent using the standard method described for simple alkanes. The name is enclosed in parenthesis to indicate that the numbering corresponds to the local side-chain, not the parent chain. Thus:

As a student of organic chemistry, you will encounter a variety of nomenclature conventions, and many non-standard names in common usage must simply be learned. Systematic nomenclature, however, is important to clearly understand as these methods are utilized in the cataloging of chemical information in print and computerized databases and effective information retrieval requires a good working knowledge of these methods.

The origin of the prefixes sec- and tert-, given above, rests with an attempt to describe the nature of the branched carbon unit. By definition, a primary carbon is one which is attached to one other carbon atom, a secondary carbon is one which is attached to two, a tertiary carbon is attached to three, and a quaternary carbon is attached to four other carbon atoms; these are often abbreviated as 1, 2, 3 and 4 carbons.

Copyright 1996, Paul R. Young, University of Illinois at Chicago, All Rights Reserved