16.3 : ¹H NMR of Labile Protons: Temporal Resolution

Protons bonded to heteroatoms such as nitrogen and oxygen exhibit a range of chemical shift values. This is due to the varying degree of hydrogen bonding between the proton and the heteroatom in other molecules. The extent of hydrogen bonding affects the electron density around the proton, thereby giving different chemical shift values for the protons in the proton NMR spectrum.

The –OH proton in alcohols typically appears in the range of δ 2 to 5 ppm but can vary depending on the specific alcohol and conditions. This –OH proton undergoes rapid proton exchange with other molecules and is referred to as a labile proton. So, the exact δ value of the –OH proton and the spin-spin coupling involving the –OH proton and the other protons in the alcohol depends on the rate of the –OH proton exchange. The proton exchange rate, in turn, depends on multiple factors, including the purity of the alcohol sample, temperature, and solvent.

Figure 1. The triplet of a –OH proton in the proton NMR spectrum of neat ethanol.

For example, the –OH proton exchange in a pure dry ethanol sample is very slow on the NMR time scale, giving enough time for the spin-spin coupling. Consequently, as depicted in Figure 1, the proton NMR spectrum of pure dry ethanol shows a triplet for the –OH proton due to the spin-spin coupling with adjacent methylene protons. The methylene protons are seen as a multiplet due to the coupling with the –OH proton and the methyl protons.

The presence of trace amounts of an acid or base increases the proton exchange rate. The proton exchange becomes rapid, and the NMR spectrometer only records an average of all the possible environments for the –OH proton.

Figure 2. The broad singlet of a –OH proton in the proton NMR spectrum of neat ethanol.

As a result, as recorded in Figure 2, the –OH proton exhibits a broad singlet. The protons from the adjacent methylene group present a quartet where the splitting is caused only by the adjoining methyl group.