It is often necessary to determine if two protons or two groups of protons are equivalent or nonequivalent. The simplest way to distinguish protons as being equivalent or not is by using substitution tests. Substituting for one proton with deuterium and then substituting for the other proton with deuterium will lead to two new structures. The relationship between the two structures will determine the equivalency or nonequivalency of the two original protons.
Homotopic protons are those that, when substituted for by deuterium, lead to the same structure. Homotopic protons are always equivalent, and will give one signal in the NMR.
Figure 33. Homotopic protons.
Enantiotopic protons are those that, when substituted for by deuterium, lead to a pair of enantiomeric structures. Enantiotopic protons appear to be equivalent (and will usually give one signal in the NMR), but can be made nonequivalent by placing the molecule in a chiral environment (chiral resolving reagent).
Figure 34. Enantiotopic protons.
Diastereotopic protons are those that, when substituted for by deuterium, lead to a pair of diastereomeric structures. Diastereotopic protons are not equivalent and will usually give different signals in the NMR.
Figure 35. Diastereotopic protons.
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