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Introductory 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:
Now include this "complex" substituent in the overall name...
Consider the following example:

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

 

complex substituent


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"
 4-(1,1-dimethylethyl)-5-ethylnonane
complex substituent


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

parent chain selection

or
parent chain selection



organic chemistry ©Dr. Ian Hunt, Department of Chemistry University of Calgary