Complex alkyl substituents

Basic IUPAC Organic Nomenclature

Complex names for alkyl substituents

The list of common alkyl substituents is quite short and it is therefore quite limited in its scope. In order to deal with other substituents (but it can also be applied to the common branched substituents too), there is a methodology for dealing with more complex substituents. This essentially involves treating the substituent as if it were a molecule on its own, see below.

For the substituent:

determine the point of attachment to the chain that defines the parent root (i.e. the longest continuous chain)
the first carbon in the substituent is regarded as the C1 of the substituent
from C1, find the longest continuous chain originating from C1 - this is the root for the substituent.
identify any substituents off this chain
list these alphabetically with appropriate multipliers
insert locants as required remembering that the point of attachment is defined as C1.

Now include this "complex" substituent in the overall name...

the complex substituent appears in brackets proceeded by its locant
the complex substituent is alphabetised based on the first letter of the name in the bracket : this includes the multiplier e.g. di
this is because the term in brackets is the name of the whole complex substituent and not several substituents.
syntax: locant numbers separated from letters by hyphens, e.g. 1-methyl, locant numbers from locant numbers by commas e.g. 1,1-dimethyl, no spaces.

Consider the following examples:

First work out the name of the complex substituent - this is shown in bold in the figure: The longest chain in the substituent is C3 therefore it's a propyl substitent There is one substituent off this chain, a C1 alkyl group i.e. a methyl group The methyl group is on C2 on the complex substituent Complex substituent name : 2-methylpropyl
Now work on the whole structure: Functional group is an alkane, therefore suffix = -ane The ring structure is C6 therefore root = hex There is one substituent: a complex substituent (see above) = (2-methylpropyl) Note the syntax: no space or hypen to separate the letters of the substituent and parent root. (2-methylpropyl)cyclohexane

First work out the name of the complex substituent - this is shown in bold in the figure: The longest chain in the substituent is C2 therefore it's an ethyl substitent There are two substituents off this chain, both C1 alkyl group i.e. methyl groups Both methyl groups are on C1 on the complex substituent Complex substituent name : 1,1-dimethylethyl
Now work on the whole structure: Functional group is an alkane, therefore suffix = -ane The longest continuous chain is C9 therefore root = non There are two substituents: simple substituent is a C2 alkyl group i.e. an ethyl group a complex substituent (see above) = (1,1-dimethylethyl) The first point of difference rule requires numbering from the left as drawn, the locants are 4- and 5- Alphabetisation requires that the complex substituent be listed first since "d" is before "e". Note the syntax: locant numbers are separated from letters with hyphens, locant numbers are separated from each other with commas. 4-(1,1-dimethylethyl)-5-ethylnonane

Of course, the common alkyl substituents can also be named in this more systematic way. Both methods are used and you should be familiar with both methods. The following table list the common alkyl substituents with both naming methods.

Alkyl group, R-	Common name	Complex name (if different)
CH₃-	methyl
CH₃CH₂-	ethyl
CH₃CH₂CH₂-	propyl
(CH₃)₂CH-	isopropyl	(1-methylethyl)
CH₃CH₂CH₂CH₂-	butyl
CH₃CH₂CHCH₃	sec-butyl or s-butyl	(1-methylpropyl)
(CH₃)₂CHCH₂-	isobutyl	(2-methylpropyl)
(CH₃)₃C-	tert-butyl or t-butyl	(1,1-dimethylethyl)

Remember that if a structure has two chains of equal length, then the choice that gives the simplest branches is chosen, see below for an example.

Functional group is an alkane, therefore suffix = -ane
The longest continuous chain is C6 therefore root = hex
The lower choice is correct - it has more but simpler substituents
There are three simple substituents

a C2 alkyl group i.e. an ethyl group
two C1 alkyl groups i.e. methyl groups

The first point of difference rule requires numbering from the top right as drawn, the locants are 2-, 3- and 4-
Alphabetisation requires that the simple substituents be listed as ethyl before methyl

3-ethyl-2,4-dimethylhexane