Non-aldehyde protons in the aldehyde region: pyridines
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص282
2025-05-20
713
Two other types of protons resonate in the region around 9–10 ppm: some aromatic protons and some protons attached to heteroatoms like OH and NH. We will deal with NH and OH protons in the next section, but fi rst we must look at some electron-deficient aromatic rings with distinctively large shifts. Protons on double bonds, even very electron-deficient double bonds like those of nitro alkenes, hardly get into the aldehyde region. However, some benzene rings with very electron withdrawing groups do manage it because of the extra downfi eld shift of the ring current, so look out for nitrobenzenes as they may have signals in the 8–9 ppm region.
More important molecules with signals in this region are the aromatic heterocycles such as pyridine, which you saw functioning as a base in Chapters 8 and 10. The NMR shifts clearly show that pyridine is aromatic: one proton is at 7.1 ppm, essentially the same as benzene, but the others are more downfi eld and one, at C2, is in the aldehyde region. This is not because pyridine is ‘more aromatic’ than benzene but because nitrogen is more electronegative than carbon. Position C2 is like an aldehyde—a proton attached to sp2 C bearing a heteroatom— while C4 is electron deficient due to conjugation (the electronegative nitrogen is electron with drawing). Isoquinoline is a pyridine and a benzene ring fused together and has a proton even further downfi eld at 9.1 ppm—this is an imine proton that experiences the ring current of the benzene ring.
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