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1.6: Benzene and Conjugation

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  • Learning Objective

    • Understanding and naming benzene derivatives.

    Aromatic hydrocarbons, like this benzene ring, have unexpected chemistry.

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

    We would expect to name this molecule 1,3,5-cyclohexatriene and see its double bonds react like other double bonds. However, these double bonds do not react in the same way as double bonds in a standard alkene. Observe the result of the below experiment under the appropriate reaction conditions.

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

    Figure \(\PageIndex{2}\).

    This surprising stability of the double bonds in the benzene ring is not due to the cyclic arrangement of the carbon chain. Under the appropriate reaction conditions, we see the following results for adding hydrogen across the double bonds of the molecules below.

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

    Figure \(\PageIndex{3}\).

    Measuring the bond lengths gives the following result.

    Bond Type Bond Length (Angstroms)
    Single 1.54
    Double 1.34
    Benzene 1.4

    Therefore, the three “double bonds” of the benzene ring are not true double bonds. The electrons are shared across all of the carbons in the ring, an arrangement called conjugation which is better represented by the below structure.

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

    Figure \(\PageIndex{4}\).

    When depicting benzene rings using the double bond drawing, remember that the ring is a hybrid between two equally likely resonance structures.

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

    Figure \(\PageIndex{5}\).

    Therefore both structures below would be named 1-chlorobenzene.

    Figure \(\PageIndex{6}\).svg
    Figure \(\PageIndex{6}\): 1-chlorobenzene

    When a benzene ring has two substituents, they are named based upon their position to each other rather than by numbers.

    Figure \(\PageIndex{7}\).svg
    Figure \(\PageIndex{7}\). o is for ortho, with substituents at 1,2:o-dichlorobenzene
    Figure \(\PageIndex{8}\).svg
    Figure \(\PageIndex{8}\). m is for meta, with substituents at 1,3:m-chloromethylbenzene
    Figure \(\PageIndex{9}\).svg
    Figure \(\PageIndex{9}\). p is for para, with substituents at 1,4:p-chlorofluorobenzene

    Practice Questions

    1. What is the name of Molecule A?

    Figure \(\PageIndex{10}\).svg
    Figure \(\PageIndex{10}\): Molecule A

    2. A benzene ring can be a substituent.

    The name of this molecule is 2-phenyldecane.

    Figure \(\PageIndex{11}\).svg
    Figure \(\PageIndex{11}\): 2-phenyldecane

    What is the name of Molecule B?

    Figure \(\PageIndex{12}\).svg
    Figure \(\PageIndex{12}\): Molecule B

    Conjugation is not limited to cyclic structures. One of the two molecules below does not react with halogen halides under standard reaction conditions. Which molecule do you predict is the more stable one? Why? What prediction would you make about the lengths of its carbon-carbon bonds?

    Figure \(\PageIndex{13}\).svg

    Figure \(\PageIndex{13}\).

    Practice Questions

    1. Write the steps that you use to name a benzene derivative or a molecule containing a benzene ring as a substituent, in order, as instructions for a student who doesn't know how to do it.
    2. Draw any benzene-containing molecule and go through the steps in naming your molecule.
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