NMR Spectroscopy and Proton Equivalence

The H NMR spectrum of 2,3-dimethyi-2-butene. Since all 12 protons in the molecule are chemically equivalent, there is only one peak in the spectrum. 

The H NMR spectrum of 2-methyl-2-butene. There are four kinds of protons and four different signals. 

Problem 13.13 How many kinds of nonequivalent protons are present in each of the folkra compounds  

Problem 13.14 How many signals would you expect the following compound to have in its ] spectrum  

We said previously that differences in chemical shifts are caused by small local magnetic fields of electrons surrounding the different nucl Nuclei that are more strongly shielded by electrons require a higher applied field to bring them into resonance and therefore absorb on the right side ( 

For relatively small molecules, a quick look at a structure is often enough to decide how many kinds of protons are present and thus how many NMR absorptions might appear. If in doubt, though, the equivalence or nonequiva-lence of two protons can be determined by comparing the structures that would be formed if each hydrogen were replaced by an X group. There are four possibilities. 

I One possibility is that the protons are chemically unrelated and thus nonequivalent. If so, the products formed on replacement of Ii by X would be different constitutional isomers. In butane, for instance, the -CH3 protons are different from the -CH2- protons, would give different products on replacement by X, and would likely show different NMR absorptions. 

B The fourth possibility arises in chiral molecules, such as (R)-2-butanol. The two — CH2- hydrogens at C3 are neither homotopic nor enantiotopic. Since replacement of a hydrogen at C3 would form a second chirality center, different diastereomers (Section 9.6) would result depending on whether the pro-R or pro-S hydrogen were replaced. Such hydrogens, whose replacement by X leads to different diastereomers, are said to be diastereotopic. Diastereotopic hydrogens are neither chemically nor electronically equivalent. They are completely different and would likely show different NMR absorptions. 

Identify the indicated sets of protons as unrelated, homotopic, enantiotopic, or diastereotopic  

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