FTIR Study of the Mechanism of the Cl and Br Atom Initiated Oxidation of Acetylene

Article abstract of DOI:10.1021/j100172a042

FTIR analysis of the products of the Cl and Br atom initiated oxidation of C2H2 at 700 Torr total pressure of N2/O2 diluent and 296 +/- 2 K has shown that the (halovinyl)peroxy radical (XCH=CHOO; X = Cl or Br) formed primarily undergoes isomerization followed by unimolecular dissociation to (a) HC(O)X + HCO, (b) HC(O)CHO + X, and (c) HX + CO + HCO.The branching ratios determined were a:b:c = 0.26:0.21:0.53, for X = Cl, and 0.17:0.09:0.74, for X = Br, independent of O2 pressure in the range 10-700 Torr.The formation of O3 in ca. 10percent yield, depending on O2 pressure, was observed in both the Cl and Br atom initiated oxidations, and it is proposed that a small fraction of the (halovinyl)peroxy radicals reacts with O2 to form O3 and a (halovinyl)oxy radical (XCH=CHO).It is noted that addition of a halogen atom to acetylene can produce both cis and trans isomers of both the halovinyl and the (halovinyl)peroxy radicals.On steric grounds, the isomerization rate may be slower for the cis form of the (halovinyl)oxy radical than for the trans form, thereby leading to the formation of O3 preferentially via (cis-halovinyl)oxy radical.Evidence for the formation of HO radical was also obtained in both the Cl and Br atom initiated oxidations, and this has been attributed to the O2 reaction of the (halovinyl)oxy radical: XCHCHO + O2 -> >XCH(OO)CHO> -> HC(O)X + CO + HO.