Basic Organic Nomenclature

An Introduction

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

2. Alkanes
II. Branched Alkanes (iv)

Index


The Branched Branched Alkanes.

One last point about branches: the branches themselves may have branches!

The IUPAC system requires that, if there are two or more choices for the longest continuous chain of carbons, the one that reduces or eliminates the branches on branches is the one to choose. When the branched branch situation cannot be avoided, the branched branch can be numbered (starting at the carbon attached to the main chain as number 1) and the position of the branches on the branch indicated. When constructing the full name, to avoid ambiguity as to which numbers are for the branches on the branch and which for the branches on the main chain, put the whole branched branch name in parentheses, so (....).

For example the following compound is named:
5-(1-ethylpropyl)decane.

Note

  • that putting the branched branch in parentheses clearly designates the '1' as refering to the position of the ethyl group on the propyl branch.
  • that the carbon of the propyl branch that is attached to the main chain is designated '1'.

A second example is the compound
6-(3-methylbutyl)-5-(2-methylpropyl)undecane.

Here the position identifiers in the parentheses obviously refer to the respective branches whilst those outside the parentheses equally obviously apply to the main chain.


Because some small branched branches appear often, the IUPAC system allows the use of names adopted from an older attempt to give compounds systematic names. These name the branched branch and fix the position of the branch on the branch in one go. The root of the name in all instances refers to the total number of carbons in the branched branch.

In the following, the wavy line indicates the point of attachment to the main chain.

Branched Branch Names.
# of carbons
in the branched branch
NameLine Diagram
3isopropyl
4sec-butyl
4isobutyl
4t-butyl
5isoamyl

Notes on the use of these names:

  1. iso is considered to be a part of the name of the alkyl group and so the letter 'i' is used to place the groups in alphabetical order.
  2. other prefixes which are hyphenated are not considered to be a part of the groups name when placing the groups in alphabetical order. E.g. sec-butyl is placed under the letter 'b' (not 's').
  3. iso means a one carbon branch on the second to last carbon in the branch.

Examples:

1. 4-isopropyl-3-methylheptane:

2. 6-sec-butyl-5-isobutyldecane:

Self assessment questions
Next page : Cycloalkanes

Last page: Branched Alkanes (iii)

Index


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