8.8:
¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons
In chloroethane, the alpha-hydrogens produce an enantiomeric pair upon replacement.
These enantiotopic protons are related by a mirror plane and produce identical chemical shifts in an achiral environment. Because proton NMR spectra are typically recorded using achiral solvents, enantiotopic protons yield the same signal as each other in an achiral solvent.
However, in 2-butanol, which has a chiral center, the methylene hydrogens at C3 produce a pair of diastereomers when replaced.
These protons are called diastereotopic and perceive different environments in all conformations.
They cannot be interchanged by rotation or reflection and exhibit slightly different chemical shifts.
Similarly, geminal vinyl hydrogens in unsymmetrical alkenes are diastereotopic.
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.