Alexandra Masarwa

New Mechanistic Aspects of the Fenton Reaction

The kinetics of the Fenton reaction was studied in detail. A second reaction step in the presence of excess H2O2 is attributed to formation of the complex Fe(III)(-O2H)(aq). Therefore, the reaction of Fe(H2O)(6)(2+) with Fe(III)(-O2H)(aq) in the presence of Fe(II) to form Fe(III)(aq) (k=(7.7+/-1.5) x 10(5) M(-1) s(-1)) may contribute to the overall Fenton reaction, and could account for some of the debate in the literature concerning its detailed mechanism. If this is correct for LFe(III)(-O2H)(aq) also, then it might be of significant biological importance. The activation parameters DeltaH(not equal), DeltaS(not equal), and DeltaV(not equal) for the Fenton reaction were measured under various experimental conditions, and are used in the mechanistic interpretation.

Copper(I) as a Homogeneous Catalyst for the Ullmann Reaction in Aqueous Solutions − The Transformation of 2-Bromobenzoate into Salicylate

The CuI-catalyzed transformation of 2-bromobenzoic acid (2-Br-BA) into salicylic acid (main product), benzoic acid, and diphenoic acid was studied. The reaction was monitored by UV/Vis spectroscopy, HPLC, and ESR. Up to 25 turnovers were achieved for low CuI concentrations. An intermediate was observed that was assigned to a complex between copper and a follow-up product of the aromatic acid, probably a complex with a copper−carbon σ-bond. The process is probably initiated by the formation of a d-π* complex of CuI with the aromatic ring of 2-Br-BA followed by the reaction with a second CuI ion. The effect of the addition of CH3CN or NH3 on the catalytic process is reported. Both were found to accelerate the process when present at relatively low concentrations and slow it down at higher concentrations. A side product, 2-aminobenzoate was observed in the presence of NH3.

pH dependence of the stability constants of copper(I) complexes with fumaric and maleic acids in aqueous solutions

Abstract The stability constants of the copper(I) π-complexes with the different acidic forms of fumaric and maleic acids (H 2 L, HL − and L 2− ) were derived as (0.23, 1.2 and 2.8)×10 4 M −1 (maleic acid) and (0.73, 1.1 and 1.5)×10 4 M −1 (fumaric acid), respectively, by either electrochemical or pulse radiolytic techniques. The influence of electron-donating/withdrawing substituents on the π-ligands are discussed.

Compact accelerated precipitation softening (CAPS) as pretreatment for membrane desalination II. Lime softening with concomitant removal of silica and heavy metals

Abstract In a previous paper the CAPS softening process is suggested as a pretreatment step for RO: in addition to removal of calcium it is an effective filtration and may thus replace part of the customary pretreatment. In the present study the removal of silica and heavy metals in addition to water softening was investigated. Precipitation with both sodium hydroxide and with lime was studied. With lime precipitation in CAPS, it was found that magnesium is necessary for silica removal, if no other additives such as Al and Zn are used. With conventional methods of precipitation, the well-known reagents for silica removal, aluminium salts, lead to very slow precipitation or difficult filtration. In contrast, aluminium chloride used as an additive in CAPS can remove a major part of the silica without interfering with filtration, if the precipitation conditions are chosen correctly; optimal conditions were in keeping with the regime of softening plants. It was found that zinc chloride, which is environmentally more acceptable than the aluminium salt, could also be used for silica removal, but in larger amounts than aluminium chloride. With these two salts, a substantial fraction of the silica can be removed, but the precipitation of silica results in decreased calcium removal. To achieve simultaneous softening and silica removal, it is necessary to add carbonate. Heavy metals are coprecipitated with the calcium carbonate.

PROPERTIES OF TRANSITION METAL COMPLEXES WITH METAL–CARBON BONDS IN AQUEOUS SOLUTIONS AS STUDIED BY PULSE RADIOLYSIS

Publisher Summary This chapter discusses properties of transition metal complexes with metal-carbon bonds in aqueous solutions as studied by pulse radiolysis. Transition metal complexes with metal-carbon σ-bonds are key intermediates in many important industrial processes, in biochemical reactions, organic synthesis, and processes involving aliphatic radicals. However, because of the high reactivity of the transition metal complexes, the study of their properties is difficult as their steady-state concentration is in most cases, far below the detection limit. The most common detection technique is the spectrophotometric one, but changes in specific conductivity, EPR, resonance Raman, etc., can be applied and are often helpful in elucidating the nature of the short-lived intermediate observed. The reactions of the radicals thus formed, or their reaction products, can be followed by a variety of physical techniques including spectrophotomety (this technique is used most commonly), electron paramagnetic resonance (EPR), electrical conductivity, polarography, resonance Raman, nuclear magnetic resonance (NMR) etc.

The effect of pyrophosphate, tripolyphosphate and ATP on the rate of the Fenton reaction

It has been recently reported that pyrophosphate, tri-polyphosphate, ATP and analogous ligands considerably decrease the yield of hydroxyl radicals by the Fenton reaction under conditions where [H(2)O(2)]>>[Fe(II)L(n)]. It was suggested that this effect is due to the slowing down of the Fenton reaction by these ligands. This suggestion seemed surprising as polyphosphate ligands stabilize Fe(III). Indeed, a kinetic study points out that these ligands accelerate the rate of the Fenton reaction by several orders of magnitude. Thus it is suggested that the effect of the ligands on the yield of the hydroxyl radicals is due to the stabilization of the Fe(III) complexes which slows down, or inhibits, their reduction by the radicals formed in the system and thus decreases the overall yield of hydroxyl radicals.

Kinetics and Reaction Mechanisms of Complexes with Cobalt−Carbon σ Bonds of the Type {(NH3)5Co−R}n+ in Aqueous Solutions, a Pulse Radiolysis Study

The kinetics of formation of complexes with cobalt−carbon σ bonds in the reaction between {CoII(NH3)5H2O}2+ or {CoII(NH3)6}2+ with different aliphatic radicals was studied in aqueous solutions using the pulse radiolysis technique. The formation of the complexes {(NH3)5CoIII−R}n+ obey pseudo first order rate laws and the rate constants for R = CH3 and CH2CO2− are (3.0±0.3)×107 and (2.1±0.3)×107M−1s−1, respectively. The UV/Vis spectra of the complexes are reported and found to be in agreement with previous results for analogous complexes. The results point out that the reactions (NH3)5CoIII−R + R· + NH3 Co(NH3)62+ + R2 are fast thus limiting the radical route as a synthetic tool for the (NH3)5CoIII−R complexes.

Conductive and SERS-active colloidal gold films spontaneously formed at a liquid/liquid interface

We show that water-soluble gold thiocyanate salt [KAu(SCN)4] spontaneously formed electrically-conductive and surface enhanced Raman scattering (SERS)-active films at the interface between water and pentane. Microscopy and spectroscopic analyses reveal that the films comprised of condensed Au nanoparticles, assembled via crystallization of KAu(SCN)4 and self-reduction of the Au3+ ions by the thiocyanate ligands. Importantly, the pentane phase was crucial in promoting the crystallization/reduction reactions. The Au films exhibited useful applications, including excellent conductivity and SERS sensing. We also show that the new film self-assembly process could be readily harnessed for producing macro-scale Au patterns.

Reduction of ethylene by Ni(I)(cyclam)+ in aqueous solutions.

Ni(I)(1,4,8,11-tetraazacyclotetradecane)(+) reduces ethylene quantitatively to ethane plus butane (and hexane traces) in neutral aqueous solutions. A radical mechanism is proposed.

Mechanism of reaction of alkyl radicals with (NiIIL)2+ complexes in aqueous solutions

The reactions of methyl radicals, *CH3, with the macrocyclic complexes Ni(II)L(1-5) (L(1-5) = cyclam derivatives, vide infra) and Ni(II)edta in aqueous solutions were studied. Methyl radicals react with all these nickel complexes, forming intermediates with Ni(III)-C sigma-bonds. The L(m)Ni(III)-CH3 complexes are formed in equilibria processes with relatively fast forward rate constants of k(f) > 1 x 10(8) M(-1) s(-1) (except in the case of NiL2-trans I cyclam, where the reaction is slower). In all cases the decomposition of the transient complexes occurs via the homolytic cleavage of the metal-carbon sigma-bond. When the homolysis is relatively slow, an isomerisation process of the transient is also observed with the exception of NiL2, where no isomerisation was observed. The results suggest that the strength of the Ni(III)-CH3sigma-bond is mainly affected by steric hindrance.