The oxidation of bromide ion by [(FeTPP)(2)O]+SbF6- in dichloromethane

The oxidation of cetyltrimethylammonium bromide (CTAB) by mono-oxidised iro n(III) tetraphenylporphyrin mu-oxo-dimer, [(FeTPP)(2)O]+SbF6-, has been fou nd to be much slower than the corresponding oxidation of iodide ion, but ca n still be studied by stopped-flow techniques. The stoichiometry of the rea ction has been established as 2 [Fe(TPP)(2)O](+)+3Br(-) --> 2(FeTPP)(2)O+Br -3(-). The rate law for the reaction is d[(FeTPP)(2)O]/dt=[Fe(TPP)(2)O+].(k (f)[CTAB]-k(r)). The rate constant for the first term has been identified w ith the rate determining forward process, the formation of the unstable Br- 2(-), and the second term with the attack of Br-2(-) on the uncharged mu-ox o-dimer. A subsequent process involves what appears to be the rapid oxidati on of Br-2(-) in the presence of Br- to give Br-3(-). At 298 K k(f) is 737( +/- 30) M-1 s(-1) and k(r) is 0.80(+/- 0.16) s(-1). For k(f), Delta H doubl e dagger=58.8(+/- 1.6) kJ mol(-1), Delta S double dagger=8.62(+/- 0.45) J K -1 mol(-1); for k(r), Delta H double dagger=43.6(+/- 10.8) kJ mol(-1), Delt a S double dagger=-97.8(+/- 36.0) J K-1 mol(-1). The addition of cetyltrime thylammonium perchlorate (inert to reaction with the oxidised mu-oxo-dimer) slowed the reaction in a manner which indicated competition between perchl orate and bromide for the formation of ion pairs with the oxidised iron(III ) dimer, the bromide ion pair being the reactive one.