Proton chemical shifts tell us about chemistry
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص273
2025-05-19
458
The truth is that shifts and electronegativity are not perfectly correlated. The key property is indeed electron withdrawal but it is the electron-withdrawing power of the whole substituent in comparison with the carbon and hydrogen atoms in the CH skeleton that matters. Methyl groups joined to the same element—nitrogen, say—may have very different shifts if the substituent is an amino group (CH3–NH2 has δH for the CH3 group = 2.41 ppm) or a nitro group (CH3–NO2 has δH 4.33 ppm). A nitro group is much more electron-withdrawing than an amino group. What we need is a quick guide rather than some detailed correlations, and the simplest is this: all functional groups except very electron-withdrawing ones shift methyl groups from 1 ppm (where you find them if they are not attached to a functional group) downfi eld to about 2 ppm. Very electron-withdrawing groups shift methyl groups to about 3 ppm. This is the sort of thing it is worth learning.
● Estimating the chemical shift of a methyl group
Rather than trying to fi t these data to some atomic property, even such a useful one as electronegativity, we should rather see these shifts as a useful measure of the electron withdrawing power of the group in question. The NMR spectra are telling us about the chemistry. The largest shift you are likely to see for a methyl group is that caused by the nitro group, 3.43 ppm, at least twice the size of the shift for a carbonyl group. This gives us our fi rst hint of some important chemistry: one nitro group is worth two carbonyl groups when it come to electron-withdrawing power. You have already seen that electron withdrawal and acidity are related (Chapter 8) and in later chapters you will see that we can correlate the anion-stabilizing power of groups like carbonyl, nitro, and sulfone with proton NMR.
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