Ch_322a_9.013 - 5 per second This is sufficiently fast for...

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Equivalency of Protons: A Closer Look Chemical and magnetic equivalence depends on fast molecular changes that average over time the environments of individual hydrogens. Rotations Around Single Bonds In a single conformation, the three H of the methyl group in ethyl chloride are not equivalent. C C H H H Cl H H H H H Cl H H Fast rotation around the carbon-carbon single bond (~10 12 per second) exchanges the positions of the 3 H so that they experience the same average environment in the nmr experiment. The two gauche H are in a different environment than the single anti H .
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Other Fast Molecular Processes Chair-Chair Interconversion in Cyclohexane The axial and equatorial H in cyclohexane are nonequivalent in any single chair conformation. H H H H H H fast At 25 o C, the rate of chair-chair interconversion is ~10
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Unformatted text preview: 5 per second. This is sufficiently fast for the environments of the axial and equatorial H to be time-averaged in the nmr experiment. At low temperature, the rate slows down and the two types of H become magnetically nonequivalent. Inversions at Nitrogen in Amines N H X : : A B fast A and B are diastereomers. Although the H in A and B are in different environments, they are not distinguished in the nmr experiment because of fast interconversion that time-averages the environments. Protic H Exchange The protic H in alcohols are generally exchanged rapidlybetween molecules . RO-H + RO-H RO-H + RO-H fast The environment experienced by the protic H depends on the rate of this exchange....
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