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Equivalency

Distinguishing Chemically Equivalent and Non-Equivalent Protons in Proton NMR Spectroscopy

A critical factor influencing the response of protons within a molecule to the applied magnetic field is their chemical equivalence.

Chemical Equivalence and Its Impact on NMR Spectra

Chemical equivalence refers to protons residing in identical electronic environments within a molecule. These protons experience the same neighboring atoms and bond angles. Consequently, they exhibit identical resonance frequencies in an NMR experiment, manifesting as a single peak in the spectrum. Conversely, chemically non-equivalent protons occupy distinct electronic environments due to variations in neighboring atoms or bond arrangements. This disparity results in different resonance frequencies, leading to the appearance of separate peaks in the NMR spectrum.

For instance, in ethane (CH₃CH₃), all six protons are chemically equivalent. They are each bonded to three identical hydrogen atoms, experiencing a uniform electronic environment. The NMR spectrum of ethane will, therefore, display a single peak for all six protons. In contrast, ethanol (CH₃CH₂OH) possesses three chemically equivalent protons on the methyl group (CH₃) that are non-equivalent to the two protons on the methylene group (CH₂). The methyl protons experience a distinct electronic environment due to their proximity to the oxygen atom compared to the methylene protons bonded to two hydrogens and a carbon. This difference translates to two separate peaks in the ethanol NMR spectrum.

Refining Equivalence: Homotopic, Enantiotopic, and Diasterotopic Protons

These classifications further delineate the concept of equivalence and introduce the notion of theoretical proton exchange within a molecule.

  • Homotopic Protons: These are chemically equivalent protons that can be interchanged by a simple rotation around a single bond. In ethanol, the three protons on the methyl group (blue) are homotopic.


  • Enantiotopic Protons: These are chemically equivalent protons that are related by a plane of symmetry. They cannot be interchanged through rotation but require bond breaking and reformation.  In ethanol, the two methylene protons (red) are enantioptopic.  To determine if protons are enantiotopic, imagine you replacing each proton separately with an X.  If the resulting two structures are enantiomers, then they are enantiotopic.


     

  • Diastereotopic Protons: These are chemically non-equivalent protons that are not mirror images but have distinct spatial arrangements.  To determine if protons are diastereotopic, imagine replacing each proton separately with an X.  If the resulting two structures are diastereomers, then they are diastereotopic.  Consider the two red protons in alanine below, replacing these protons with X atoms results in the formation of diastereomers, therefore they are diastereotopic and chemically non-equivalent.


 

Understanding these classifications is essential for predicting the number of peaks in an NMR spectrum. Homotopic and enantiotopic protons will always be chemically equivalent, while diasterotopic protons are chemically non-equivalent.