Ring currents pyridine

Compound Thiol ring current Pyridine ring current... 

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

Some values of RRC are as follows Me, 2 p-anisyl, 92 phenyl, 100 2-pyridyl, 98 3-pyridyl, 96. These figures support previous estimates of ring current which suggest pyridine is about as aromatic as benzene. 

The arsabenzene ring system has been actively studied for just over a decade. Its bond delocalization, diamagnetic ring current and electronic structure demonstrate that arsabenzene has a high degree of aromatic character. While its lack of basicity strongly differentiate it from pyridine, arsabenzene has a rich organic chemistry quite similar to that of normal benzocyclic aromatics. 

In theory then, pyridine is aromatic. But is it in real life The most important evidence comes from the proton NMR spectrum. The six protons of benzene resonate at 8h 7.27 p.p.m., some 2 p.p.m. downfield from the alkene region, clear evidence for a ring current (Chapter 11). Pyridine is not as symmetrical as benzene but the three types of proton all resonate in the same region. 

How many types of pyridine ring are there Look at a model to predict possible ring current effects. 

The NMR spectra of pyridine, phosphabenzene, arsabenzene, and stibabenzene are quite similar in appearance (11,12). The y protons appear at lowest field partially as a result of remoteness from the heteroatom. The effect of the magnetic anisotropy of the heteroatom should decrease for the )3 and y protons. The chemical shift differences for the y protons in pyridine and arsabenzene are almost negligible. The low-field chemical shift value was interpreted in terms of the presence of ring current in the heteroatomic derivatives. 

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