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1.3: Alkyl Substituents

  • Page ID
    222784
  • Learning Objective

    • How to name organic molecules with alkyl substituents

    An alkane can be appended onto an existing chain to create a branched molecule. This branched piece of the molecule is called a substituent. A substituent made from an alkane is called an alkyl group. Alkyl groups are named similarly to unbranched alkane chains.

    Alkane Condensed Structure Alkyl Group Condensed Structure
    Methane CH4 Methyl CH3-
    Ethane CH3CH3 Ethyl CH3CH2-
    Propane CH3CH2CH3 Propyl CH3CH2CH2-
    Butane CH3CH2CH2CH3 Butyl CH3CH2CH2CH2-

    Alkyl groups can also be branched. For example, there are three constitutional isomers of the butyl substituent. In these diagrams, the negative charge on the carbon indicates the site of the bond from the substituent to the rest of the molecule.

    Figure \(\PageIndex{1}\).svg

    Figure \(\PageIndex{1}\). Isobutyl

    Figure \(\PageIndex{2}\).svg

    Figure \(\PageIndex{2}\). Sec-butyl

    Figure \(\PageIndex{3}\).svg

    Figure \(\PageIndex{3}\). Tert-butyl

    When naming molecules according to the IUPAC system of substitutive nomenclature, remember prefix-parent-suffix (like un-believe-able).

    1. prefix: what are the substituents?
    2. parent: how many carbons in the parent chain?
    3. suffix: what is the family of compounds?

    In the case of an alkane, the suffix is -ane.

    Practice Questions

    1. The name of this molecule is 2-methylhexane. Identify the alkyl group. Label the parent chain carbons from 1-6.

    Figure \(\PageIndex{4}\).svg

    Figure \(\PageIndex{4}\): 2-methylhexane

    2. The name of this molecule is 3-methylheptane. Identify the alkyl group. Label the parent chain carbons from 1-7.

    Figure \(\PageIndex{5}\).svg
    Figure \(\PageIndex{5}\): 3-methylheptane

    3. Identify the alkyl group in Molecule A. Label the parent chain carbons from 1-8. What is the name of this molecule?

    Figure \(\PageIndex{6}\).svg
    Figure \(\PageIndex{6}\): Molecule A

    4. Identify the alkyl group in Molecule B. Label the parent chain carbons. What is the name of this molecule?

    Figure \(\PageIndex{7}\).svg
    Figure \(\PageIndex{7}\): Molecule B

    5. The name of this molecule is 2,3-dimethylpentane.

    Figure \(\PageIndex{8}\).svg
    Figure \(\PageIndex{8}\): 2,3-dimethylpentane

    What is the name of Molecule C?

    Figure \(\PageIndex{9}\).svg
    Figure \(\PageIndex{9}\): Molecule C

    6. The name of this molecule is 4-ethyl-2-methyloctane.

    Figure \(\PageIndex{10}\).svg
    Figure \(\PageIndex{10}\): 4-ethyl-2-methyloctane

    What is the name of Molecule D?

    Figure \(\PageIndex{11}\).svg
    Figure \(\PageIndex{11}\): Molecule D

    7. The name of this molecule is 4-isopropylnonane.

    Figure \(\PageIndex{12}\).svg
    Figure \(\PageIndex{12}\): 4-isopropylnonane

    What is the name of Molecule E?

    Figure \(\PageIndex{13}\).svg
    Figure \(\PageIndex{13}\): Molecule E

    8. The name of this molecule is cyclobutane.

    Figure \(\PageIndex{14}\).svg
    Figure \(\PageIndex{14}\): cyclobutane

    What is the name of Molecule F?

    Figure \(\PageIndex{15}\).svg
    Figure \(\PageIndex{15}\): Molecule F

    9. The name of this molecule is methylcyclobutane.

    Figure \(\PageIndex{16}\).svg
    Figure \(\PageIndex{16}\): methylcyclobutane

    The name of this molecule is 1-ethyl-2-methylcyclohexane.

    Figure \(\PageIndex{17}\).svg
    Figure \(\PageIndex{17}\): 1-ethyl-2-methylcyclohexane

    What is the name of Molecule G?

    Figure \(\PageIndex{18}\).svg
    Figure \(\PageIndex{18}\): Molecule G

    10. Write the steps that you use to name an alkane, in order, as instructions for a student who doesn't know how to do it.

    11. Draw any alkane and go through the steps in naming your molecule.

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