The NMR spectrum of iodocyclohexane was taken at 280 °C. At that temperature, the chair interconversion is slow and each chair conformation can be observed separately.

The resonance of proton H^a is well resolved from the rest of the spectrum because of its unique chemical shift. The resonance of proton H^a in one of the conformations at –80 °C is shown in Fig. P13.66.

(a) Which conformation has the H^a resonance shown in Fig. P13.66? Explain.

(b) How would you expect the resonance of H^a in the other conformation (not shown) to differ from the one shown in Fig. P13.66?

(c) The integrals of the H^a resonance in the two conformations at –80 °C are in the ratio 3.39 : 1. What is the DG for the chair interconversion of the two conformations?

From what you know about conformational analysis, which conformation is the major one?

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