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25.3: Branched Alkanes

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    Branched Alkanes

    Beginning with butane, there is an alternate structure possible that is not a straight chain. The structural formula below shows a structure with a three-carbon chain that has a \(\ce{-CH_3}\) group attached to the middle carbon.

    The name of this molecule is 2-methylpropane. The molecular formula is still \(\ce{C_4H_{10}}\), which is the same formula as butane. A structural isomer is one of multiple molecules that have the same molecular formula, but different structural formulas. Butane and 2-methylpropane are structural isomers.

    2-methylpropane is an example of a type of alkane called a branched alkane. The IUPAC system of nomenclature for branched alkanes follows a set of steps which will be applied to the example molecule below.

    1. Find the longest continuous chain of carbon atoms in the molecule. This is called the parent chain. In the example, the longest chain is eight carbon atoms and so the parent hydrocarbon is octane.
    2. Number the carbon atoms in the parent chain. To do this, start at the end that will give the smallest numbers possible to the carbon atoms where the branches originate. In the example above, the branches are on carbons 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.
    3. The atoms attached to the parent chain are called substituents. A substituent that is a hydrocarbon is called an alkyl group. The names of alkyl groups use the same prefixes as the alkanes, but with a -yl suffix. So 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 4-ethyl substituents.
    4. 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-, four is tetra-, etc. 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.
    5. Multiple 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-.
    6. Commas are used to separate multiple numbers. Hyphens come between the number and the name of the substituent. 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.

    Summary

    • Nomenclature rules for branched hydrocarbons are given.

    Contributors

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.