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2.10: Uses of ¹H NMR Spectroscopy

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    NMR is used to help identify the product of nearly every reaction run in the laboratory. For example, we said in Section 8.5 that hydroboration–oxidation of alkenes occurs with non-Markovnikov regiochemistry to yield the less highly substituted alcohol. With the help of NMR, we can now prove this statement.

    Does hydroboration–oxidation of methylenecyclohexane yield cyclohexylmethanol or 1-methylcyclohexanol?

    Methylenecyclohexane reacts first with borane and tetrahydrofuran, then hydrogen peroxide and hydroxide to form cyclohexylmethanol or 1-methylcyclohexanol; question mark near structures indicates product is unknown, could be either.

    The 1H NMR spectrum of the reaction product is shown in Figure 13.16a. The spectrum shows a two-proton peak at 3.40 δ, indicating that the product has a –CH2– group bonded to an electronegative oxygen atom (–CH2OH). Furthermore, the spectrum shows no large three-proton singlet absorption near 1 δ, where we would expect the signal of a quaternary –CH3 group to appear. (Figure 13.16b) gives the spectrum of 1-methylcyclohexanol, the alternative product.) Thus, it’s clear that cyclohexylmethanol is the reaction product.

    The 1 H N M R spectrum of cyclohexylmethanol (peaks at 0, 0.9, 1.2, 1.4, 1.7, 2.8. and 3.4) and 1-methylcyclohexanol (peaks at 0, 1.1, and 1.4).
    Figure 13.16: (a) The 1H NMR spectrum of cyclohexylmethanol, the product from hydroboration–oxidation of methylenecyclohexane, and (b) the 1H NMR spectrum of 1-methylcyclohexanol, the possible alternative reaction product.

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