13.9 Uses of ¹H NMR Spectroscopy

13.9 • Uses of ¹H NMR Spectroscopy

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.

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.