Indentifying Homotopic, Enantiotopic and Diastereotopic Protons

Homotopic Protons

• Definition:

  • Two protons are homotopic if replacing each one with a different group (e.g., deuterium) leads to the same molecule.
    
    All the protons in ethane CH₃CH₃ are homotopic.
    

  • Homotopic protons are chemically equivalent and have the same chemical shift in NMR (i.e. they appear at same place on the x-axis).

• Symmetry Test:

  • Rotational symmetry: If a molecule can be rotated around an axis to make the protons indistinguishable, they are homotopic.

  • Example: The three protons on the CH₃ group in ethanol (CH₃-CH₂-OH) are homotopic because rotating the molecule around the C-C bond makes them identical.
    

• NMR Implications:

  • Homotopic protons appear as a single peak in the NMR spectrum.

Enantiotopic Protons

• ​Definition:

  • Two protons are enantiotopic if replacing each one with a different group leads to enantiomers (non-superimposable mirror images).
    

  • Enantiotopic protons are chemically equivalent in an achiral environment but may become non-equivalent in a chiral environment.

• Symmetry Test:

  • Reflection symmetry: If replacing each proton with a different group results in enantiomers, the protons are enantiotopic.

  • Example: The two protons on the CH₂ group in ethanol (CH₃-CH₂-OH) are enantiotopic because replacing one with deuterium would create an enantiomer.

• NMR Implications:

  • In an achiral (not chiral) environment, enantiotopic protons appear as a single peak.

  • In a chiral environment (e.g., with a chiral solvent or reagent), they may split into separate peaks.

Diasterotopic Protons

• Definition:

  • Two protons are diastereotopic if replacing each one with a different group leads to diastereomers (non-mirror image stereoisomers).
    

  • Diastereotopic protons are chemically non-equivalent and have different chemical shifts in NMR.

  • Example: The two protons on the CH₂ group in a molecule like 2-butanol (CH₃-CH(OH)-CH₂-CH₃) are diastereotopic because replacing one with deuterium creates diastereomers.

• NMR Implications:

  • Diastereotopic protons appear as separate peaks in the NMR spectrum, even in an achiral environment.

  • They often show complex splitting patterns due to their non-equivalence.

Example - The CH₂ group is diastereotopic, so there are three different couplings to this particular proton: two from the CH₂-O group and one from the methyl group.

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Summary and Key Takeaways

• • Homotopic Protons:

    • Identical in all environments.

    • Appear as a single peak in NMR.

  • Enantiotopic Protons:

    • Equivalent in achiral environments but may split in chiral environments.

    • Appear as a single peak in achiral NMR solvents.

  • Diastereotopic Protons:

    • Non-equivalent in all environments.

    • Appear as separate peaks in NMR, often with complex splitting.