Studies in Organic Mass Spectroscopy. Part 15 [1]. Electron Ionization Mass Spectra of Some 5-Nitro-3-thiophencarboxanilides

The study of the electron ionization mass spectra of 5-nitro-3-thiophenecarboxanilides 1-24 has shown peculiar effects induced by the 5-nitro group on the fragmentation of molecular ions M and the thenoyl cation (Scheme 1). A comparison of the abundances of the important fragment ions for 5-nitro-3-thiophenecarboxanilides with those of the corresponding 3-thiophenecarboxanilides shows that the extent of C-N amide bond cleavage decreases in the former series as a consequence of the increased bond strength. The produced thenoyl ion does not eliminate carbon monoxide as 2- and 3-thenoyl cations usually do. Again this behaviour depends on the electronic effects of the nitro group which strongly destabilizes the 5-nitro-3-thienyl cation and consequently strengthens the carbon-3 carbonyl carbon bond. Recalling the behavior of the previously studied 2- and 3-thiophenecarboxanilides most of the 2'-substituted 5-nitro-3-thiophenecarboxanilides give loss of the 'ortho' substituent of the phenyl ring furnishing the cyclic ion (Scheme 2). In the instance of 2'-alkyl substituted derivatives the formation of abundant ions h' by loss of 5-nitrothenoyl radical from M (Scheme 3) was observed, thus confirming the occurrence of a cryptic 'ortho' effect. The results are also discussed in relation to those obtained on some related benzoylanilides.