Cl-Initiated oxidation of N-ethyl-perfluoroalkanesulfonamides: A theoretical insight into the experimentally observed products

Theoretical calculations at the B3LYP level of theory were performed to investigate the Cl-initiated oxidation of the N-ethyl-perfluorobutanesulfonamide (C4F9SO2-NH-CH2CH3; NEtFBSA). We studied in particular the thermodynamic aspects of the possible oxidation reactions so to obtain a first indication on the conceivably preferred pathways and on the structure and stability of the observed products. The reactions of Cl with the model compound CF3SO2-NH-CH2CH3 produce the three primary radicals CF3SO2-N-CH2CH3 (1), CF3SO2-NH-CHCH3 (2), and CF3SO2-NH-CH2CH2 (3). Both 2 and 3 react exoergically with O2 and eventually form the ketone CF3SO2-NH-C(O)-CH3 and the aldehyde CF3SO2-NH-CH2-C(O)-H, respectively. The addition of O2 to the N atom of 1 is instead endoergic, but the radical can exoergically react with RO2 with formation of CF3SO2-N(O)-CH2CH3, CF3SO2-O-N-CH2CH3, and/or CF3SO2-N-O-CH2CH3. These three intermediates may in turn react with HO2 to produce, at least in principle, the four neutral products CF3SO2-N(OH)-CH2CH3, CF3SO2-N(O)H-CH2CH3, CF3SO2-O-NH-CH2CH3, and CF3SO2-NH-O-CH2CH3. This explains the proposed formation of a mixture of isomeric products of general formula C4F9SO2N(C2H5OH) from the Cl-initiated oxidation of NEtFBSA. CF3SO2-N(O)-CH2CH3 and CF3SO2-O-N-CH2CH3 may also dissociate into CH3CH2-NO and CF3SO2 and/or CF3. This suggests that the sulfonamide radicals CnF2n+1SO2-N-CH2CH3, once oxidized, could be direct precursors of perfluoroalkanecarboxylic acids via CnF2n+1SO2 and CnF2n+1O2. The formation of CnF2n+1SO3-O-N-CH2CH3 by reaction of CnF2n+1SO2-N-CH2CH3 with O2, while exoergic, is not supported by the experiments on the Cl-initiated oxidation of NEtFBSA.