17217-66-2Relevant articles and documents
Autooxidation of tetrachlorohydroquinone in aqueous media
Pisarenko
, p. 881 - 886 (1999)
The oxidation of tetrachlorohydroquinone in an aqueous solution at pH 7.40 is an autocatalytic reaction (sigmoid kinetic curves). The interaction of the tetrachloro-1,4-semiquinone radical anion with dioxygen occurs with the rate constant k2 equal to 9±3 L mol-1 s-1 (22-37 °C). Superoxide dismutase does not affect the maximum rate of tetrachlorohydroquinone oxidation.
A radical intermediate in the conversion of pentachlorophenol to tetrachlorohydroquinone by sphingobium chlorophenolicum
Rudolph, Johannes,Erbse, Annette H.,Behlen, Linda S.,Copley, Shelley D.
, p. 6539 - 6549 (2015/02/19)
Pentachlorophenol (PCP) hydroxylase, the first enzyme in the pathway for degradation of PCP in Sphingobium chlorophenolicum, is an unusually slow flavin-dependent monooxygenase (kcat = 0.02 s-1) that converts PCP to a highly reactive product, tetrachlorobenzoquinone (TCBQ). Using stopped-flow spectroscopy, we have shown that the steps up to and including formation of TCBQ are rapid (5-30 s-1). Before products can be released from the active site, the strongly oxidizing TCBQ abstracts an electron from a donor at the active site, possibly a cysteine residue, resulting in an off-pathway diradical state that only slowly reverts to an intermediate capable of completing the catalytic cycle. TCBQ reductase, the second enzyme in the PCP degradation pathway, rescues this nonproductive complex via two fast sequential one-electron transfers. These studies demonstrate how adoption of an ancestral catalytic strategy for conversion of a substrate with different steric and electronic properties can lead to subtle yet (literally) radical changes in enzymatic reaction mechanisms.
Substituent effects in oxime radical cations. 1. Photosensitized reactions of acetophenone oximes
De Lijser, H.J. Peter,Kim, Jason S.,McGrorty, Suzanne M.,Ulloa, Erin M.
, p. 575 - 585 (2007/10/03)
A variety of ortho-, meta-, and para-substituted (-H, -F, -Cl, -CF 3, -CN (meta and para only), -CH3, -OCH3, and -NO2) acetophenone oximes were synthesized and studied using laser flash photolysis (LFP) and steady-state photolysis experiments in acetonitrile with chloranil as the photosensitizer. In addition, semi-empirical (AM1) calculations were performed on the neutral species, the radical cations, and the corresponding iminoxyl radicals. The data was analyzed in terms of the electrochemical peak potentials of the oximes, the quenching rates of triplet chloranil (LFP), the calculated ionization potentials, and the measured conversions of the oximes in the steady-state photolysis experiments. Photolysis of the oximes in the presence of chloranil results in the formation of the chloranil radical anion, which reacts rapidly with the oxime radical cation to form the semiquinone radical and an iminoxyl radical. Evidence for the formation of the chloranil radical anion and the semiquinone radical was obtained from LFP studies. The measured quenching rates from the LFP studies represent the rates of electron transfer from the oximes to triplet chloranil. This data was correlated to various radical and polar substituent constants. The Hammett studies suggest that steric, polar, and radical effects are important for ortho-substituted acetophenone oximes, polar effects are important for parasubstituted oximes, and radical stabilization is more important than polar effects for the meta-substituted substrates. The calculated ionization potentials of the oximes show an excellent correlation with the measured quenching rates supporting the electron transfer pathway. On the basis of calculated charge densities, we conclude that the measured substituent effects are transition state effects rather than ground state effects. At this point all of the available data suggests that the conversion of the oximes is controlled by two energetically opposing reactions, namely oxidation of the neutral oxime, which is favorable for oximes with electron-donating substituents, and deprotonation of the oxime radical cation, which is favorable for oximes with electron-withdrawing substituents. The overall result is a reaction with little selectivity as far as substituent effects are concerned.
