Abstract
The olefinic proton resonance spectra of a number of mono- and 1, 2 di-substituted ethylenes have been analysed as AB, ABC or ABX systems to obtain the coupling constants and chemical shifts. The values of J trans (range observed 13·7 to 18·0 c/s in 14 compounds) were significantly larger than J cis in similar molecules. The values of J cis (range observed 6·5 to 12·3 c/s in 8 compounds) are abnormally large when the olefinic bond is conjugated to an aromatic ring. Coupling between adjacent protons in an olefinic methylene group (J gem) is much smaller, and sometimes negative. The shielding of the lone vinyl proton in mono-substituted ethylenes is smaller than that of the methylene group and in two cases (the vinyl group directly bonded to an oxygen atom) the difference is so great as to give an approximation to the ABX condition, so that the methylene protons appear to be shielded to an unusually large extent.
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