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Chemistry LibreTexts

1.2: Constitutional Isomers

  • Page ID
    222425
  • Learning Objective

    • Introduction to constitutional isomers using alkanes

    Evaluate the two molecules below (Figure \(\PageIndex{1}\)).

    Pentane isopentane.svg

    Figure \(\PageIndex{1}\). Two alkanes for comparison

    How are they the same?

    How are they different?

    Two molecules which have the same molecular formula but different structural formulas, or bonding arrangements, are known as constitutional isomers.

    Pentane is an alkane with five carbon atoms. It has three constitutional isomers, shown below.

    Pentane.svg

    Figure \(\PageIndex{2}\). Unbranched isomer of pentane

    Isopentane.svg

    Figure \(\PageIndex{3}\). Branched constitutional isomer of pentane

    2,2-dimethylpropane.svg

    Figure \(\PageIndex{4}\). Branched constitutional isomer of pentane

    Practice Questions

    1. Draw three constitutional isomers of heptane.

    2. Circle the constitutional isomers of hexane in the figure below. Why are the remaining molecules not considered to be constitutional isomers of hexane?

    Figure \(\PageIndex{5}\).svg

    Figure \(\PageIndex{5}\)

    3. Draw the constitutional isomer of hexane that is missing.

    4. What is the minimum number of carbons in the chain of an alkane to be able to have a constitutional isomer?

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