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?
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.