Question

Describe how H-1 NMR spectroscopy can be used to differentiate cis- from trans-4-tert-butylcyclohexanol.

Answer 1

The hydoxyl proton will have different ppm in both  cis-4-tert-butylcyclohexanol and trans-4-tert-butylcyclohexanol.

These two compounds are diastereomers, and hence they will have different signals.

First record the 1H NMR and figure out, which proton signals in the 1H NMR correspond to the stereotopic centers. The t-butyl group will likely always be equatorial, but the hydroxyl will be either axial or equatorial for the cis and trans respectively.

Its only the proton on the hydroxylated carbon will thus be in different orientations for both.

Record the 1H NMR in low temperature, which will have the effect of blocking the chair-chair flipping of the cyclohexane and locking the compound with the t-butyl guaranteed in the equatorial position. since your flipping will be locked, the axial protons will produce one signal and the equatorial protons will produce a different signal (since they are each in electronically unique environments).

The cis isomer has a resonance at 3.94 ppm, while the trans isomer has a resonance at 3.52 ppm.