Proximity Effects in Mass Spectra of Benzanilides

Abstract

The analytical value of peaks arising by a proximity effect in the electron ionization mass spectra of benzanilides has been established by examining the spectra of numerous examples of general structure XC6H4NHCOC6H4Y. Significant [M-X]+ signals are observed only when X = Cl, Br, I or CH3O in the 2-position. The presence of strong [M-X]+ signals, but negligibly weak [M-Y]+ peaks, even when the C-Y bond would be expected to break more readily than the C-X bond, indicates that these diagnostically useful signals do not arise by simple cleavage. Similarly, the presence of an appreciable [M-Cl]+ signal, but no [M-Br]+ signal, in the spectra of representative examples of 4-Br-2ClC6H3NHCOC6H4Y, reveals that loss of a substituent from the 2-position occurs much more rapidly than fission of a weaker bond to a substituent in the 4-position. These trends are interpreted in terms of cyclization of the ionized 2-substituted benzanilide, followed by elimination of the substituent originally in the 2-position, to form a protonated 2-arylbenzoxazole.

Hammett Correlation in Competing Fragmentations of Ionized 2‐Substituted Benzanilides Generated by Electron Ionization Mass Spectrometry

“…In contrast, the corresponding 2‐Br‐4‐ClC 6 H 3 NHCOC 6 H 4 Y +. isomers expel a bromine atom, but not a chlorine atom [28] …”

“…Ionized 2-substituted benzanilides, 1 + * , of general structure XC 6 H 4 NHCOC 6 H 4 Y + * [X = CH 3 O, Cl, Br or I; Y = H, F, Cl, Br, I, CH 3 , OCH 3 , CF 3 or NO 2 in the 3 or 4 position] have recently been shown to fragment by two main routes, one of which had previously been undetected. [28] Simple cleavage gives the substituted benzoyl cation, [YC 6 H 4 CO] + , 2. An alternative reaction involves cyclisation of 1 + * to form 3, which may eliminate X * to give the very stable protonated 2-substituted benzoxazole, 4, Scheme 2.…”

“…Ionized 2‐substituted benzanilides, 1 + ⋅, of general structure XC 6 H 4 NHCOC 6 H 4 Y +. [X=CH 3 O, Cl, Br or I; Y=H, F, Cl, Br, I, CH 3 , OCH 3 , CF 3 or NO 2 in the 3 or 4 position] have recently been shown to fragment by two main routes, one of which had previously been undetected [28] . Simple cleavage gives the substituted benzoyl cation, [YC 6 H 4 CO] + , 2 .…”

“…In contrast, the corresponding 2-Br-4-ClC 6 H 3 NHCOC 6 H 4 Y + * isomers expel a bromine atom, but not a chlorine atom. [28] An abbreviation system is used to describe the benzanilides in order to facilitate the presentation and discussion of results: "BZ" denotes "benzanilide"; a prefix defines the substituent 1.…”

“…Full details of this procedure, including an illustrative example, have been reported. [28] All compounds were fully characterised by a combination of High-Resolution Mass Spectrometry (HRMS, to determine the molecular formula), 1 H NMR and IR spectroscopy. Details are provided in the supplementary information.…”