IUPAC Alkane Nomenclature Rules in a Nutshell
For some excellent examples, see
the exact IUPAC wording.
2.1, 2.6 The longest continuous chain of carbon atoms is the parent chain.
If there is no longest chain because two or more chains are the same longest length,
then the parent chain is defined as the one with the most branches. (The idea here is
to keep the name simpler. More branches numbered from the parent chain means fewer parentheses needed later.)
2.2 Carbon atoms connected to the parent chain but not part of it are parts of branches.
To avoid ambiguity, branches are numbered based on the carbon number of the parent chain at the point of
attachment to the parent chain.
2.2 The general idea in naming organic compounds is to always aim for the smallest numbers possible.
Whenever two or more possibilities exist, which is usually the case, because there are two ends you can start numbering from on all acyclic chains,
"smallest numbers" means smallest at the first difference. Thus:
- Between 3-ethyl-4,8-dimethylnonane
and
7-ethyl-2,6-dimethylnonane,
the second results in the
lower first-different number.
- Between 2,3,8-trimethylnonane
and
2,7,8-trimethylnonane,
the first results in the
lower first-different number.
2.4 Basically, what this means is that if you start checking
from both ends of the chain, stepping toward the center of the
chain one carbon at a time counting 1, 2, 3,...,
then the branch you get to first sets all the other numbers for the name.
If two branches are reached at the same time, then the "winning" branch is the one that
is first aphabetically. If the branches are the same, then you have to keep
stepping in toward the center until a difference is found. If no difference is found, then it
doesn't matter which end you number from.
2.3 Once all the numbers for the branches are determined, the branches
are named using -yl, and ordered alphabetically. If branches themselves are branched, then
the complete name of the branch (with numbers) must be determined at this time.
It is the complete name of the branch which is alphabetized. Thus, for example:
- 4-(2,2-dimethylbutyl) comes before 3-ethyl
When alphabetizing, hyphenated prefixes such as sec- are not included.
- sec-butyl comes before isopropyl
2.5 Finally, when more than one of the same branch is present,
the prefixes di, tri, tetra, etc. for simple branches and
bis, tris, tetrakis, etc. for branches containing numbers,
are added just after the numbers locating the branch on the parent chain.
The name is constructed by separating numbers with commas and adding hyphens before and after sets of numbers so that they don't run into words.
Branches with numbers are set off with parentheses so that it is clear that the numbers only refer to that branch.
Other than that, there is no punctuation and there are no spaces in the names.
These prefixes do NOT COUNT FOR ALPHABETIZING PURPOSES. For example:
- 3-ethyl comes before 2,2-dimethyl
- 4-hexyl comes before 2,3-diisopropyl
- 3-ethyl comes before 2,4-bis(2-methylbutyl)