Radical and non-radical mechanisms for alkane oxidations by hydrogen peroxide-trifluoroacetic acid

The oxidation of cyclohexane by the H2O2-trifluoroacetic acid system is rev isited. Consistent with a previous report (Deno, N.; Messer, L. A. Chem. Co mm. 1976, 1051), cyclohexanol forms initially but then esterifies to cycloh exyl trifluoroacetate. Small amounts of trans-1,2-cyclohexadiyl bis-(triflu oroacetate) also form. Although these products form irrespective of the pre sence or absence of O-2, dual mechanisms are shown to operate. In the absen ce of Oz, the dominant mechanism is a radical chain reaction that is propag ated by CF (3) over circle abstracting H from C6H12 and S(H)2 displacement of C6H ((11)) over circle on CF3CO2OH. The intermediacy of C6H ((11)) over circle and CF ((3)) over circle is inferred from production of CHF3 and CO2 along with cyclohexyl trifluoroacetate, or CDF3 when cyclohexane-d(12) is used. In the presence of O-2, fluoroform and CO2 are suppressed, the reacti on rate slows, and the rate law approaches second order (first order in per acid and in C6H12). Trapping of cyclohexyl radicals by quinoxaline is ineff icient except at elevated (similar to 75 degreesC) temperatures. Fluoroform and CO2, telltale evidence for the chain pathway, were not produced when q uinoxaline was present in room temperature reactions. These observations su ggest that a parallel, nonfree radical,oxenoid insertion mechanism dominate s when O-2 is present. A pathway is discussed in which a biradicaloid-zwite rionic transition state is attained by hydrogen transfer from alkane to per oxide oxygen with synchronous O-O bond scission.