Diastereotopic Protons in 1H NMR Spectroscopy: Examples
Last updated: February 8th, 2022 |
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Diastereotopic protons are chemically non equivalent protons that each produce distinct chemical shifts. Typically, this type of proton is of a CH2 group found in a chiral molecule, but it may also be found in the more subtly unique chemical environments of achiral compounds. Let’s take a look at an alkene to observe the type of environment this proton resides in.
Below, we have 1,1-dimethylethylene. Note the plane of symmetry. The two olefinic protons (Ha, Hb) have identical chemical shifts due to their equivalence from the mirror plane present in the molecule.
If we tweak one of the methyl groups to replace it’s protons with different atoms or another functional group (e.g. CD3), the molecule loses its symmetry. The olefinic protons are no longer identical and now produce different chemical shifts.
Within this new asymmetrical compound, the two olefinic protons are now what we call diastereotopic. To test and confirm that these protons are in fact diastereotopic, we can replace one of each of the protons with another atom (e.g. D). Replacing Ha with D leads to the Z-alkene, while replacing Hb with D leads to the E-alkene. In comparing these two new compounds, we can see their relationship between each other is that they are diastereomers. They are stereoisomers, but not mirror images of each other.
Diastereotopic protons will produce different chemical shifts in a 1H NMR spectrum due to the difference in their chemical environments.
This type of proton can also be seen in cyclic compounds. For example, we know that cyclopropane produces only one singlet in a 1H NMR spectrum, as its protons are equivalent, but how would the addition of a substituent affect its remaining protons? Methylcyclopropane produces 4 separate signals. This is because the addition of the methyl group changes the chemical environment of the remaining protons present in the molecule.
This can be seen just the same in larger cyclic molecules. Each methylene group of the cyclic compounds in the presence of a methyl group will become a diasteretopic pair with different chemical shifts.
So far, we’ve covered diastereotopic protons found in achiral molecules, but the same type of protons can be found in molecules containing a chiral center, too. Let’s take a look at the example below. 2-butanone produces 3 signals in a 1H spectrum, while 2-butanol produces 6 (including the OH proton). What causes the extra chemical shifts?
The extra chemical shifts come from the methylene protons adjacent to the chiral center of the alcohol. As we can see above, just like the alkene example, we can swap out protons with different atoms to confirm the protons’ diastereotopic relationships. Replacing Ha with D leads to the (R,R) compound, while replacing Hb with D leads to the (R,S) compound. The two new compounds are diastereomers, confirming the CH2 protons adjacent to the chiral center are diastereotopic, These protons produce two distinct chemical shifts.
This post was contributed by Nick Tiedemann. Thanks Nick!