Mass Spectrometric Investigations of Phenylacetic Acid Derivatives, III: Fragmentations of meta‐ and para‐substituted Phenylacetamides after Electron Impact

Abstract

The electron impact induced fragmentations of m-and p-substituted phenylacetamides and N,N-dimethyl-phenylacetamides 1-14 were investigated and compared with the o-analogues. All m-and p-substituted amides yield molecular ions with high relative intensities which do not lose their meta-and para-substituents. Loss of HNCO from M+. is dominant in the prim. amides, whilst for the tert. amides the classical benzyl cleavage is the most favourable fragmentation pathway. We have previously reported on the fragmentations of ortho-substituted phenylacetamides,) and phenylacetate~~) after electron impact. Of the ortho-substituted phenylacetamides studied (substituent X = CI, Br, F, NO,, OCH,, CH,, and CN) only the 0-0, o-Br-, and o-NO,-amides lose the ortho substituents directly from their M+. giving rise to large (M-'XPsignals, and do not yield detectable M+'(<O.l % rel. int.) at high and low ionizing energies. The loss of HNCO from M+' of all ortho-substituted primary amides occurs to various degrees (12-100 % rel. int.) depending on the nature of the substituent (see2)). In o-substituted tert. amides, however, the "classical" benzyl cleavage is a main fragmentation. The loss of o-substituents was explained assuming an intramolecular aromatic substit~tion,)~), which is a typical example of an ortho effect in aromatic sys- Loss of HNCOAs shown in scheme 1 (as examplified for a p-substituted amide) and Tab. 1 the molecular ions of the prim. amides la-14a generally lose HNCO, induced by H-migration from the amide N-atom, being the main fragmentation reaction in many cases. The resulting radical cation A subsequently eliminates a hydrogen atom yielding the (M-'CONH2)+ ion (this ion is also produced directly from M+' by benzyl cleavage as indicated by Holmes and Benoit')), or alternatively the substituent X to give the C7H7+ ion (m/z 91) in all cases at a R=H tend). In parallel with these studies the behaviour of meta-and para-substituted phenylacetamides 1-14 was investigated in order to clarify how the posi-tion of these substituents influences the fragmentation reactions.The EIMS of the amides 1-14 show molecular ions of va-1 rn-NO2 6 p-Br 11 rn-OCH3 rying intensities without exception (7-95 % at 70 eV and 2 p-NO2 1 m-F 12 p-OCH313-100 % at nom. 12 eV). A direct loss of the substituent X from M+'was not detected for any compound (see Tab. 1 and 9 m-CL

Experiments on the Chaperon Effect in the Nitration of Aromatics

“…N , N -Dimethyl-4-nitrobenzeneacetamide (12c). Pale yellow crystals: mp 87 °C;33c IR (pellet) 2933w, 1649s, 1604w, 1513s, 1396w, 1350s, 1264w, 1135m, 1110w, 858w, 818m, 799w, 740s, 694w cm -1 ; 1 H NMR δ 2.99 (s, 3H), 3.07 (s, 3H), 3.82 (s, 2H), 7.38−7.50 (m, 2H), 8.12−8.23 (m, 2H); 13 C NMR δ 169.35, 146.79, 142.74, 130.02, 123.56, 40.15, 37.51, 35.60; MS …”