Kinetics and mechanism of oxidation of thiosulphate by the complex ion [MnIV3(μ-O)4(phen)4(H2O)2]4+ (phen = 1,10-phenanthroline) in weakly acidic aqueous media

The trinuclear complex ion [Mn^IV ₃(-O)₄(phen)₄(H₂O)₂]⁴⁺ (1) is quantitatively reduced by an excess of S₂O₃ ^2– to Mn^II, but the binuclear complex [Mn^IIIMn^IV(-O)₂(phen)₄]³⁺, (2) is the only manganese product when [S₂O₃ ^2–] = 1.5 [(1)]. With an excess of S₂O₃ ^2– biphasic kinetics, (1) k ₁ ⁰ (2) k ₂ ⁰ Mn^II is observed, while the reaction with S₂O₃ ^2– = 1.5 [(1)], follows one-step second order kinetics with the second order rate constant k = k ⁰ ₁/[S₂O₃ ^2–]. The rate constant k ⁰ ₁ is independent of c _phen ( = [phen] + [Hphen⁺]) but directly proportional to [H⁺] and [S₂O^2– ₃]. Rapid formation of an adduct between (1) and S₂O^2– ₃, followed by rate-determining one-electron, one-proton reduction of the adduct, appears logical. A comproportionation reaction in one of the subsequent rapid steps leads to (2) without any Mn^II co-product. Kinetic dependences for the second step are same to those for an authentic complex of (2), and further support the assigned reaction sequence.     

Oxidation of l‐Methionine by Bisperoxo(1,10‐phenanthroline)oxovanadate(V): A Mechanistic Study

In phosphate buffer media (pH 5.8–8.0), bisperoxo(1,10‐phenanthroline)oxovanadate(V) ( 1 ) oxidizes l ‐methionine to methionine sulfoxide. The stoichiometry of the reaction is 1:1. The reaction occurs in two subsequent first‐order steps. In the first step, one of the peroxo ligands of 1 gets substituted by l ‐methionine. The observed first‐order rate constants for both steps increase linearly with increasing [H⁺] as well as with increasing [l ‐methionine]. The EPR spectra prove that the reaction involves a cysteinyl radical‐type intermediate and that V^V gets reduced to a V^IV species.     

Kinetics and mechanism of the oxidation of oxalic acid and bioxalate ion by [ethylene bis (biguanide)] silver (III) cation in aqueous perchlorate media

In acidic aqueous media the complex [Ag^IIILL⁻)³⁺[L = ethylenebis (bi-guanide)] reacts quantitatively with oxalic acid and bioxalate ion to produce CO₂, Ag⁺ and L. Kinetic evidence indicates preequilibrium adduct formation, but the reactions appear to be outer-sphere with these adducts (I₁ and I₂) acting as kinetic dead ends.No Ag⁺ catalysis could be detected.     

In alkaline media,Fremy’s salt oxidizes alkanols by a hydrogen atom transfer mechanism

In aqueous alkali, Fremy’s salt (potassium nitrosodisulfonate dimer), homolyses nearly exclusively to the monomer radical anion, nitrosodisulfonate (NDS). In this media, NDS almost quantitatively oxidizes benzyl alcohol (PhCH₂OH) to benzaldehyde (PhCHO), itself being reduced to hydroxylamine disulfonate (HNDS). The reaction is very nearly first-order in [NDS], [alkanol] and in [OH⁻]. However, with progressive addition of HNDS, decay kinetics of NDS gradually deviates from first-order. Ultimately, with sufficient excess of HNDS, the reaction becomes second-order in [NDS]. The consumption ratio, (ΔPhCH₂OH]/Δ[NDS]), is ∼2. PhCD₂OH manifests a large primary kinetic isotope effect (k_H/k_D = 11.6). Substituted benzyl alcohols (RBzCH₂OH) with R-groups withdrawing electron density from the O–H bond accelerated the reaction; those with R-groups donating electron density to the O–H bond retarded the reaction. The conversion of 2-propanol to 2-propanone is much slower compared to that of benzyl alcohol to benzaldehyde. An alpha-H atom transfer mechanism seems logical.