Low-energy collision-induced dissociation mass spectra of protonated p-toluenesulfonamides derived from aliphatic amines.

Collision-induced fragmentation of protonated N-alkyl-p-toluenesulfonamides primarily undergo either an elimination of the amine to form CH3-(C6H4)-SO2 + cation (m/z 155) or an alkene to form a cation for the protonated p-toluenesulfonamide (m/z 172). To comprehend the fragmentation pathways, several deuterated analogs of N-decyl-p-toluenesulfonamides were prepared and evaluated. Hypothetically, two mechanisms, both of which involve ion-neutral complexes, can be envisaged. In one mechanism, the S–N bond fragments to produce an intermediate [sulfonyl cation/amine] complex, which dissociates to afford the m/z 155 cation (Pathway A). In the other mechanism, the C–N bond dissociates to produce a different intermediate complex. The fragmentation of this [p-toluenesulfonamide/carbocation] complex eliminates p-toluenesulfonamide and releases the carbocation (Pathway B). Computations carried out by the Hartree-Fock method suggested that the Pathway B is more favorable. However, a peak for the carbocation is observed only when the carbocation formed is relatively stable. For example, the spectrum of N-phenylethyl-p-toluenesulfonamide is dominated by the peak at m/z 105 for the incipient phenylethyl cation, which rapidly isomerizes to the remarkably stable methylbenzyl cation. The peaks for the carbocations are weak or absent in the spectra of most of N-alkyl-p-toluenesulfonamides because alkyl carbocations, such as the decyl cation, rearrange to more stable secondary cations by 1,2-hydride and alkyl shifts. The energy freed is not dissipated, but gets internalized, causing the carbocation to dissociate either by transferring a proton to the sulfonamide or by releasing smaller alkenes to form smaller carbocations. The loss of the positional integrity in this way was proven by deuterium labeling experiments.