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Exchangeable Protons and Deuterium Exchange

In 1H NMR spectroscopy, certain protons, such as those in hydroxyl (-OH), amine (-NH), and carboxylic acid (-COOH) groups, are considered exchangeable protons. These protons can readily exchange with deuterium (D) from deuterated solvents like D2O (deuterium oxide) or other deuterium sources. This exchange occurs because these protons are labile and participate in hydrogen bonding or acid-base equilibrium. When deuterium replaces a proton, the corresponding 1H NMR signal disappears, as deuterium (2H) is NMR "silent" in a 1H NMR experiment. This phenomenon is particularly useful for identifying and confirming the presence of exchangeable protons in a molecule.

Example: Deuterium Exchange in an Alcohol

Below are two 1H NMR spectra of an alcohol:

  1. Spectrum A: The 1H NMR spectrum of the alcohol in a deuterated solvent (e.g., CDCl3) shows a broad singlet for the hydroxyl proton (-OH) typically in the range of 0.5–5.0 ppm, along with signals for the aliphatic protons.
  2. Spectrum B: After adding a drop of D2O to the same sample, the -OH proton signal disappears due to deuterium exchange, while the signals for the non-exchangeable protons (e.g., alkyl group protons) remain unchanged.

This dramatic change in the spectrum confirms the presence of the exchangeable hydroxyl proton and demonstrates the utility of deuterium exchange in 1H NMR analysis.

Alcohol HNMR

Alcohol Treated with D2O HNMR