Proton NMR distinguishes axial and equatorial protons in cyclohexanes
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص415
2025-05-31
536
Coupling is a through-bond phenomenon, as we know from the couplings in cis and trans alkenes, where trans alkenes have much larger coupling constants as their orbitals are perfectly parallel. Another case of perfectly parallel orbitals occurs with trans-diaxial protons in cyclohexanes. Typical coupling constants are 10–12 Hz for trans-diaxial protons, but much smaller (2–5 Hz) for axial/equatorial and equatorial/equatorial protons.
This makes assignment of conformation easy. The simple ester below has a triple triplet for the black H, with two large coupling constants (8.8 Hz) that must be to axial protons (green) and two small coupling constants (3.8 Hz) that must be to equatorial Hs (brown). This is possible only if the black H is axial and the ester group must therefore be equatorial. The acetal ester on the right is very different: it is a simple triplet with two small coupling constants (3.2 Hz), which is too small for an axial/axial coupling. The only possibility therefore is that the black proton is equatorial, and one of the 3.2 Hz couplings is to its equatorial neighbour, and the other to its axial neighbour. The ester group must be axial in this compound.
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