- Simple branched substituents are usually referred to by their common names
- IUPAC nomenclature systematically names more complex branched substituents
The next step in complexity in hydrocarbon nomenclature is when the branching substituent itself has a branching group. These specific types of branching substituents are always named using common nomenclature. Note: while IUPAC has a series of rules regarding how to name complex branched substituents, they are not frequently utilized. Below is a table listing the common name for the simplest branched hydrocarbon substituents.
These common names can readily be incorporated into standard IUPAC nomenclature (repeated below) as branching substituents.
Review: Hydrocarbon nomenclature rules:
- Find the longest continuous chain of carbon atoms. This chain determines the parent name (root) of the alkane.
- Identify the suffix for the root.
- Number the chain beginning at the end of the chain nearest any branching, thus obtaining the use of lower numbers in the name.
- The root of each substituent is the longest chain of carbons in the branch. Substituent alkyl groups are referred to with suffix “-yl”.
- If an alkyl group appears more than once, use the prefixes: di, tri, tetra, penta, hexa, etc.
- Complete the name. Use these numbers to designate the location of the substituents groups. Numbers are separated from letters by dash (-) and two numbers are separated by a comma (,). If two or more substituents are present on the same carbon atom, use the number twice. List the groups alphabetically (alphabetize based on the root group from step 4, not the prefixes from step 5).
In the molecule below, the longest continuous chain of carbons is 9, making it a nonane.
As you can see, there is a branching substituent off of the side chain. To name this group using common nomenclature, count the total number of carbons in the branch, which gives you 4 (i.e. a butyl group). If you compare the branching subsitutent to the table above, you will see this is a "secbutyl" group. Combined, the name of this molecule is 5-secbutylnonane.
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