At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the axial and equatorial protons, resulting in a single peak at δ 1.4.
Similarly, NMR averages all the conformations of any molecule with rapid conformational equilibria. For example, the CH3 protons in bromoethane have a single resonance and coupling constant for splitting by the CH2 protons due to the rapid internal rotation of the carbon-carbon bond. Lastly, besides conformational equilibria, the time-averaging effect of NMR spectroscopy is also observed in some chemical reactions.
The stable chair conformation of cyclohexane has a staggered arrangement of six pairs of diastereotopic protons, with a total of six axial and six equatorial protons.
At room temperature, the ring of the chair conformer flips to an equivalent chair conformer, where the two conformers are in equilibrium.
The rapid ring flipping - about 105 times per second - results in the interconversion of axial to equatorial protons and equatorial to axial protons.
So, at room temperature, the NMR spectrometer reads all the protons as equivalent, exhibiting a single peak.
As with cyclohexane at room temperature, NMR averages all the conformations of any molecule with rapid conformational equilibria.
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