Basic Organic Nomenclature

Branched Alkanes

Since the Carbon atom is capable of forming 4 bonds, it is possible for the atom to form bonds to several other carbon atoms, to a maximum of four. In general carbon atoms are referred to as primary  (1o) when bonded to one other carbon group, secondary (2o) when bonded to two other carbon atoms, tertiary (3o) when bonded to three, and quaternary (4o) when bonded to four. The fact that carbon atoms readily form bonds to other carbon atoms can lead to a number of different possible structures containing the same numbers of hydrogen and carbon atoms. These different molecules with the same chemical formula are referred to as structural isomers. The number of structural isomers increases rapidly as the number of carbon atoms increase.

Number of structural isomers of alkanes

Number of Carbon Atoms
Number of Isomers
4
2
5
3
6
5
7
9
8
18
9
35
10
75
12
355
15
4347

IUPAC deals with these isomers in a very simple way which will be explained using several examples.

Example 1) C4H10 isomers 

Isomer I is a linear chain of 4 carbons atoms and is named butane. 

Isomer II has a 3 carbon chain with one carbon attached to the middle carbon of the chain. Thus the longest continuous chain is 3 carbon atoms long, with a substituent, the single carbon atom, on the second carbon of the chain. IUPAC rules for naming these branched structures is as follows:

1.) choose a root name to indicate the longest continuous chain of carbons, in this case prop for the three carbons.

2.) name the branch or branches by:

  • using the root name for the longest continuous chain of carbons in the branch (in this case meth for the one carbon)
  • followed by -yl to indicate that this is a substituent on the main carbon chain, i.e. methyl in this case.
  • the location of the substituent is given by the atom number of the carbon atom to which the substituent is attached. The numbering scheme is determined by examining the parent chain from each end, looking for the first point of difference. The first point of difference will be when you can not directly match all of the atoms attached on the carbon. Normally the end of the parent chain which has a substituent closest to it will be the starting end for numbering the chain. (note: in this case it does not matter which way the chain is numbered, the methyl group will be on atom 2 of the propane chain)
    • 3.) put the three parts of the name (substituent + root + family) together to form the compound's name, with the branch name(s) prefixing the root, and the family name taking its usual form.
    Applying these three steps to isomer II we obtain the name: 2-methylpropane

    NOTE: the hyphen between the number and the prefix, and there is no space between the prefix and family name.
    NOTE: the number is generally left out if there is no ambiguity about the location of the substituent. In this example the methyl group can only be located on carbon 2 of the propane chain, otherwise it would be a butane molecule. Therefore, methylpropane is an acceptable name.

    Work through these steps for the three isomers of C5H12.
     

    pentane
    2-methylbutane
    (Numbered right-to-left)
    2,2-dimethylpropane

    You should note that there are usually several ways of drawing 2-D structural representations of the same molecule, for example:

    are actually the same molecule in different orientations. If you can not see this use the dynamic structure below the convince yourself it is true.
     
     





    Finally, notice that the two methyl groups in the third isomer of pentane is not named methylmethylpropane, but 2,2-dimethylpropane. When there are several substituents that are the same group, i.e. methyl groups, the total number of occurrences of the group is indicated by adding di- (2 groups the same), tri- (3 groups the same), tetra- (4), penta- (5), hexa- (6),... as a prefix to the substituent name. Individual substituents are located by the number indicating where they are attached to the parent chain. (Note: there should be as many numbers locating substituents on the parent chain, as the prefix indicates, i.e. di- implies two substituents so there needs to be two numbers locating the two groups) Consecutive numbers in the name of an organic compound are separated by a comma.

    If this has been insufficient as an introduction continue on with the C6H14 isomers.

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