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8.8: 1H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

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Analytical Chemistry

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1H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons
 
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8.8: 1H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

Replacing each alpha-hydrogen in chloroethane by bromine (or a different functional group) yields a pair of enantiomers. Such protons are called prochiral or enantiotopic and are related by a mirror plane. Enantiotopic protons are chemically equivalent in an achiral environment. Because most proton NMR spectra are recorded using achiral solvents, enantiotopic hydrogens yield a single signal.

In chiral compounds such as 2-butanol, replacing the methylene hydrogens at C3 produces a pair of diastereomers. Such protons are called diastereotopic, and they perceive different environments in all conformations. They cannot be interchanged by rotation or reflection and have slightly different chemical shifts.

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