Kinetics and mechanism of the addition of triphenylphosphoniocyclopentadienide to tetrahalogeno-p-benzoquinones. Part 4. The substitution reactions of fluoranil

Abstract

Fluoranil reacts with excess triphenylphosphoniocyclopentadienide to yield the monosubstituted and 2,6-disubstituted quinone derivatives through two parallel, irreversible, second-order reactions which are well separated in time. The rate of formation of the monosubstitution product was resolved using the stopped-flow technique and its half-life was found to be in the millisecond range. Kinetic data for the mono- and di-substitution products indicate that both reactions proceed in two steps involving rate-limiting addition to form a polar betaine intermediate followed by elimination of hydrogen halide through an E1 mechanism. The empirical rate law was established by carrying out a multi-response non-linear least-square analysis of the absorbance of the reaction mixtures.