25.1. Hydrocarbons http://www.ck12.org
3 and 5 when the chain is numbered left-to-right. If it were to be numbered right-to-left, the branches would
be on carbons 4 and 6, so the left-to-right order is preferable.- The atoms attached to the parent chain are called substituents. A substituent that is itself a hydrocarbon is
called analkyl group. The names of alkyl groups use the same prefixes as the alkanes, but with a –yl suffix.
A 1-carbon alkyl group is a methyl group, a 2-carbon alkyl group is an ethyl group, and so on. The substituents
are named by placing the number from the parent carbon chain in front of the name of the substituent. In the
current example, we have 3-methyl and 5-ethyl substituents. - Use a prefix to indicate the appearance of more than one of the same substituent in the structural formula. Two
of the same group is di-, three is tri-, and four is tetra-. Larger numbers use the same prefixes as those used to
name straight-chain alkanes. For example, if methyl groups were attached to both carbons 2 and 3, that part
of the name would be 2,3-dimethyl. This rule does not apply to the current structure above. - Different substituents are listed in alphabetical order. Ignore any of the prefixes from rule 4. In the current
example, the 5-ethyl- comes before the 3-methyl. - Commas are used to separate multiple numbers. Hyphens are used to separate numbers from the names of
various substituents. The parent name comes immediately after the last substituent. There are no blank spaces
in the name.
The correct name for the above structure according to the IUPAC system is 5-ethyl-3-methyloctane.
If the name of a branched alkane is known, it is relatively easy to draw the structural formula. First, use the parent
hydrocarbon name to draw the longest continuous carbon chain. Then, number the carbons and add the substituents
onto the correct carbon atoms. Finally, add in hydrogen atoms so that each carbon has four bonds.
Sample Problem 25.1: Branched Alkanes
Name the following branched alkane.
Step 1: Plan the problem.
Follow the steps in the text. The parent chain is the longest continuous carbon chain. Number the carbons in the
parent chain and label each substituent with the number of the carbon to which it is attached. Alphabetize the
substituents, and string together the full name of the compound.
Step 2: Solve.
Note that there is not a substituent on the left-most carbon. Instead, the longest continuous chain happens to contain
a bend when drawn in this form. This is most often done either to save space or to trick students on an exam. Because
these molecules are not shown three-dimensionally, this “bend” is not really a necessary part of the structure. The
longest continuous parent chain is ten carbons in length. It should be numbered right-to-left because that will give
the substituents smaller numerical values. There are two methyl groups, both on carbon 4, and a propyl group on
carbon 5. The IUPAC name is 4,4-dimethyl-5-propyldecane.