Basic Organic Nomenclature

An Introduction

An Introduction

Dave Woodcock,
Associate Professor Emeritus UBC (Okanagan)
©1996,2000, 2008

2. Alkanes
II. Branched (i)

Index
Since carbon requires four bonds to other atoms, it is possible for compounds to form in which more than two other carbon atoms are bonded to a carbon: up to four other carbons, of course.

Note: It is sometimes convenient to refer to a carbon atom as primary (1�), secondary (2�), tertiary (3�) or quaternary (4�) according to the number of other carbon atoms directly bonded to it; further, this designation is extended to the non-carbon atoms (or groups) attached to that carbon. Thus a 3� carbon has three other carbons bonded to it, and, for an alkane, one hydrogen which by extension is termed a 3� hydrogen.

This possibility of having the same number and type of atoms bonded together in different ways is called isomerism: isomers are compounds having the same molecular formula but different atomic arrangements. In the alkane series, the number of possible isomers increases dramatically with number of carbons as shown:

Number of 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 4,347
20 366,319

The IUPAC system of nomenclature deals with the presence of isomers in a very easy way.

The smallest alkane to exhibit isomerism is butane with 2 isomers:

Isomer A has the four carbons in a chain and so is butane.


Isomer B has three carbons in a chain and the fourth carbon branching off the centre. To name compounds such as this one, with branches, the IUPAC system:
  1. chooses a root name to indicate the longest continuous chain of carbons, in this case prop for the three carbons.
  2. names the branch:
    • 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 number of carbons is in a branch.
  3. puts the three parts of the name (branch + root + family) together to form the compounds name, with the branch names prefixing the root, and the family name taking its usual form.
Following these steps, compound B is named : methylpropane.

Work through the steps to come up with the following names for the three isomers of pentane:

Pentane :

Methylbutane :

and

Dimethylpropane :

Note that close inspection of the static line forms:

using the dynamic model:

shows that the two forms represent the same molecule (methylbutane) turned through 180�.

    A point to note in these names:

  • the two methyl branches are not written separately (methylmethylpropane), instead the prefix di- is used.
    Similarly the prefixes tri (3), tetra (4), penta (5), hexa (6), and so on are used. (Did you note that Greek was being used here!)
This topic continues with the names of the 5 isomers of hexane and introduces the numbering of the main chain.
Self-study questions
Next page: Branched Alkanes (ii)

Index

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