alkanes-5

Basic Organic Nomenclature

Branched Alkanes - Branched Branches or Complex Substituents:

To complicate the naming of organic molecules, the branches on the parent chain can have branches themselves. To try to simplify this problem IUPAC requires that when there are two or more choices for the longest chain of carbon atoms, the one that eliminates or reduces the number of complex substituents is the one of choice. When this can not be avoided, i.e. there is only one choice for the longest chain, then the branched branch is numbered (starting at the carbon attached to the main chain as number 1) and the position of the branches on the branch indicated using this numbering scheme. The complex substituent is then placed in parentheses to remove any confusion over numbering.

Example 1.)

The compound is named: 5-(1-ethylpropyl)nonane

Note :

that putting the branched branch in parentheses clearly designates the '1' as referring to the position of the ethyl group on the propyl branch (numbered in the above diagram).

that the carbon of the propyl branch that is attached to the main chain is designated '1'.

Example 2.)

This compound is named: 5-(1-methylethyl)-8-(2-methylpropyl)dodecane

Note :

that the methylethyl proceeds methylpropyl alphabetically.

that when there is no preference in numbering right-to-left or left-to-right on the parent chain, the convention is to give the substituent that comes first alphabetically the lower number.

Example 3.)

This compound is named: 5-(1,2-dimethylpropyl)-2-methyldecane

Note:

that the dimethylpropyl substituent comes first alphabetically, in complex substituents the first letter of the group is used in alphabetizing the order, this is because the entire substituent is a dimethylpropyl group.

There are a number of common names used for smaller complex substituents. This has arisen from past attempts to provide a systematic method for naming complex organic molecules. These common names identify the branched branches and attachment points of the branch(es) within this complex substituent. In all cases the root name for the complex substituent is based on the total number of carbon atoms in the complex substituent, the prefix tells you how the branched substituent is attached to the main chain in the complex substituent. (NOTE: in the following examples the parent chain of the entire molecule is referred to as "R", a common shorthand in organic chemistry).

Number of C Atoms	Line Drawing	Name
3		isopropyl
4		isobutyl
4		sec-butyl
4		tert-butyl
5		isopentyl
5		sec-pentyl

Notes on the use of these names:

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.
the other prefixes, which are hyphenated, are not considered to be a part of the group's name when placing the groups in alphabetical order. E.g. sec-butyl is placed in order by the letter "b" (not "s").
iso means a methyl group attached one carbon from the end of the chain.
sec- means the branched chain is attached to the main chain at carbon number two (i.e. a 1-methyl_____ group)
tert- means two methyl groups attached to the second last carbon in the chain.
the sec, and tert are commonly italicized, while iso is not

Examples:
5-sec-butyl-4-isopropylnonane

6-tert-butyl-3-methylunodecane

Tutorial Test 1

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