Mass spectral evidence for the conversion of benzenephosphonic acid to its cyclic, trimeric anhydride and of benzenephosphonous acid to phenylphosphine

Abstract

AbstractThe mass spectra of benzenephosphonic and benzenephosphonous acids are readily observable under normal operating conditions and, at low sample temperatures (50 to 85°C), the 70 eV electron‐impact degradations are characteristic. The phosphonic acid shows peaks for the molecular ion at 158 amu and for characteristic degradation fragments at 141 (C6H5PO2H), 94 (C6H6O), 78 to 7 (C6H6, C6H5), 65 (H3PO2, C5H5), 51 (C4H3), 47 (PO) and 39 (C3H3) amu. Above 120°, however, and after a pressure surge indicative of a thermal dehydration in the sample, a peak at 420 amu associated with the cyclic, trimeric anhydride, (C6H5PO2)3, is observed along with a characteristic set of fragment peaks at 373, 357, 343, 327, 280, 262, 233, 216 and 199 amu, whose interrelations are summarized in Scheme 1. The phosphonous acid, at 75°C and at 70 eV, shows peaks for the molecular ion at 142 amu and for characteristic degradation fragments at 124 (C6H5PO), 111 to 107 (C6H5PH ± 2H), 94 (C6H6O), 78 to 7 (C6H6, C6H5), 65 (H3PO2, C5H5), 51 (C4H3), 47 (PO) and 39 (C3H3) amu. The phenylphosphine is apparently formed by rearrangement in the excited state after electron‐impact rather than by thermal disproportionation in the sample as the latter requires the formation of relatively more of the phosphonic acid than is observed. At higher sample temperatures (120°C) somewhat increased amounts of the phosphonic acid, presumably formed by partial disproportionation in the sample, are observed. Accurate mass assignments and broad metastables confirm the postulated fragmentation processes.