¹H NMR Chemical Shift Equivalence: Enantiotopic and Diastereotopic Protons

Enantiotopic protons are related by a mirror plane and produce an enantiomeric pair when replaced by another group. In chloroethane, the alpha-hydrogens are enantiotopic. Because proton NMR spectra use achiral solvents, enantiotopic protons perceive identical environments and yield the same chemical shift, appearing as a single signal.

Diastereotopic protons produce diastereomers when replaced and perceive different environments in all conformations. Unlike enantiotopic protons, they cannot be interchanged by rotation or reflection and exhibit slightly different chemical shifts. In 2-butanol, the methylene hydrogens at C3 are diastereotopic and appear as separate signals.

Geminal vinyl hydrogens in unsymmetrical alkenes are diastereotopic because they perceive different chemical environments. These protons cannot be interchanged by rotation or reflection, resulting in slightly different chemical shifts. This diastereotopic relationship causes them to appear as distinct signals in the NMR spectrum.

Prochiral hydrogens are another term for enantiotopic protons. Replacing each prochiral hydrogen by a different functional group yields a pair of enantiomers. These protons are related by a mirror plane and are chemically equivalent in an achiral environment, producing a single NMR signal.

In chiral compounds like 2-butanol, protons near the chiral center perceive different environments and may exhibit different chemical shifts. The chiral center creates distinct stereochemical environments that diastereotopic protons cannot interconvert between, resulting in separate NMR signals for protons that would be equivalent in achiral molecules.

Enantiotopic protons are related by a mirror plane symmetry and are distinguishable only when replaced by a chiral group. In typical NMR experiments using achiral solvents, the mirror plane symmetry remains valid, making the protons magnetically equivalent. This symmetry relationship causes them to experience identical magnetic environments and produce one signal.

No, diastereotopic protons remain distinguishable in all conformations and environments because they cannot be interchanged by rotation or reflection. Unlike enantiotopic protons, which become equivalent in achiral solvents, diastereotopic protons maintain their different chemical shifts regardless of solvent choice, always appearing as separate signals.