Sulphuric acid redox mechanism

Independently of the work on the chemical composition of sulphuric acid catalysts, attempts to develop kinetic equations to describe the rate of reaction both for mechanistic analysis and reactor design purposes have been numerous. There is much evidence to suggest that in common with its catalytically oxidative behaviour in other environments, valency states of vanadium of V " and V " are involved. Mars and Maessen in 1964 developed a rate expression based on a simple two-step redox mechanism ... 

Another noticeable improvement of the electropolymerization of conjugated heterocycles comes from the use of super-acidic conditions. Especially most of the mixtures used contain borontrifluoride etherate (BTFE) [ 145—147]. This research was tri ered by the original report of Shi and Li, who showed that PPy with unusual mechanical strength could be obtained from BTFE solutions [148-149]. This field was developed a little later by Reynolds [150] who used mixtures of solvents (acetonitrile) and BTFE and Seeber [151] who used BTFE with concentrated sulphuric acid to polymerize high redox potentials monomers like 3-chlorothiophene. However, despite the apparent efficiency of the polymerization process in that case, no mechanistic studies have been performed so far. 

Cerium(iv).— The acid-promoted redox decomposition and cerium(iv) oxidation of the tris(oxalato)cobaltate(in) ion have been studied in aqueous acid media. In IM sulphuric acid, in the absence of oxidant, there occurs an induction period prior to the internal redox decomposition of the anion. On addition of the cerium(iv), however, there results an increased rate of reduction of the cobalt(ni) centre in contrast to the behaviour of this oxidant to M(C20 ) complexes where M = Cr, Rh, or Ir. The induction in the add-catalysed decomposition is consistent with the formation of a unidentate oxalato-complex-ion which may be the main route towards the stepwise reduction to yield Co and COg. From spectral studies on the total expected absorbance values on mixing, it would appear that the cerium(iv) ion is involved in the pre-equilibrium formation of a dinuclear species which might undergo internal electron transfer with reduction to cerium(in). A possible mechanism in this system may then be written as shown in Scheme 5 (ox = C2O4 -). The variations in rate of the one-electron redox reactions of this type are dependent on the nature of the activated complex, which may differ from one metal centre to another in respect of the number of protons and sulphate anions incorporated. 

The proposed catalytic mechanism of the ferredoxin oxidoreductase [32] is shown in Fig. 4, a similar mechanism existing for the analogous citric acid cycle enzyme, 2-oxoglutarate oxidoreductase. In outline, the 2-oxoacid is decarboxylated in a TPP-dependent reaction to give an hydroxyalkyl-TPP. From this, one electron is abstracted and transferred to the enzyme-bound iron-sulphur cluster, generating a free-radical-TPP species. This intermediate can then interact direct with coenzyme-A to form acyl-CoA, the iron-cluster receiving the second electron. In each case, ferredoxin serves to re-oxidise the enzyme s redox centre. 

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