Alphabetising substituents

Basic IUPAC Organic Nomenclature

Alphabetisation

In constructing the name, substituents are listed in alphabetical order, e.g. an ethyl group proceeds a methyl group.
Only in the event that the first point of difference rule doesn't give a result is alphabetisation used to define the numbering.

For systems named with simple substituents, the following rules apply:

multiplier prefixes (di-, tri- tetra- etc.) are ignored (they refer to multi substituents)
prefixes such as neo-, sec-, tert- (or their abbreviations) are ignored
the prefixes iso- and cyclo- are included (note they are are used without hyphens and are part of the specific substituent name)

For systems named with complex substituents, the following additional guideline applies:

in alphabetising complex substituents, any terms used in the brackets are included.
for example (1,1-dimethylethyl)- is alphabetised as "d"
this is because the term in brackets is the name of the whole complex substituent and not several individual substituents.

Here is an illustrative example of each case :

1-ethyl-2-methylcyclohexane not 2-ethyl-1-methylcyclohexane or 1-ethyl-6-methylcyclohexane		The numbering is determined by the alphabetisation: ethyl preceeds methyl so ethyl gets the lower number. But then we number counter-clockwise to give the methyl the lower number (2- vs 6-)
4-(1,1-dimethylethyl)-5-ethylnonane not 5-ethyl-4-(1,1-dimethylethyl)nonane (the details of naming complex substituents is covered later)		The order is determined by the alphabetisation: the dimethyl of the complex substituent preceeds the simple ethyl substituent.

©Dr. Ian Hunt, Department of Chemistry