Reactions in homogeneous aqueous solutions

Rate data for hydrolysis reactions in homogeneous aqueous solutions have been reviewed (79), but application of these data to environmental conditions involving mineral surfaces remains difficult due to the unknown effects sorption may have. Several studies have demonstrated that acid-catalyzed reactions are promoted if the substrate is sorbed at clay surfaces (70-74 and other works reviewed by Theng, 8), but inhibition may also occur if substrate hydrolysis is base-promoted (74). 

This cycle, often referred to as the Shilov-cycle converts methane into methanol and chloromethane in homogeneous aqueous solution at mild temperatures of 100-120 °C (11). However, while Pt(II) (added to the reaction as PtCl ) serves as the catalyst, the system also requires Pt(IV) (in the form of PtCle-) as a stoichiometric oxidant. Clearly, this system impressively demonstrates functionalization of methane under mild homogeneous conditions, but is impractical due to the high cost of the stoichiometric oxidant used. A recent development by Catalytica Advanced Technology Inc., often referred to as the Catalytica system used platinum(II) complexes as catalysts to convert methane into methyl-bisulfate (12). The stoichiometric oxidant in this case is S03, dissolved in concentrated H2S04 solvent. This cycle is depicted in Scheme 3. 

For a decade or so [CoH(CN)5] was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic add (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rii(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4]. No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. 

In general, the mechanism of alkene hydroformylation with an [RhH(CO)P3] catalyst in water or in aqueous/organic biphasic systems (P = TPPTS) is considered to be analogous [61] to that of the same reaction in homogeneous organic solutions (P = PPh3) [84], a basic version of which is shown on Scheme 4.8. 

Glod, G., W. Angst, C. Holliger, and R. P. Schwarzenbach, Corrinoid-mediated reduction of tetrachloroethene, trichloroethene, and trichlorofluoroethene in homogeneous aqueous solution Reaction kinetics and reaction mechanisms , Environ. Sci. Technol., 31, 253-260 (1997a). 

Murakami et al. utilized catalytic bilayer membranes to catalyze the (1-replacement reaction of serine with indoles [44], The bilayer vesicle formed with 32 and 36 drastically accelerated the (1-replacement reaction by 51-fold (krel) relative to pyridoxal in homogeneous aqueous solution. They attributed this to the hydrophobic microenvironmental effect provided by the bilayer vesicle, which affords effective incorporation of indole molecules and elimination of water molecules in the reaction site. The imida-zolyl group of 33 enhanced the reaction further, krd being 130, possibly due to general acid-base catalysis by the imidazolyl group. Copper(n) ions also improved the reaction. 

The back reaction of Ag° + (RuIII) is rapid in water, but is strongly retarded in silica where Ag° is ejected from the vicinity of (RuIII) which is strongly bound to the silica particle. Such a long lived separation of products is not observed in homogeneous aqueous solution. 

The catalytic activity of poiy(S-lysine) — copper(II) conqrlex in homogeneous aqueous solution for the hydrolysis of the optKal antipodes of jdienylalanine methyl ester was examined by Nozawa, Akimoto and Hatano (28). As shown in Table 7, the R antipode was hydrolyzed faster than the S antipode by 2—4 times, at pH = 7. At this pH poly(S-lysine) — copper(n) complex is in random conformation. At higher pH, where the complex assumes a-helical conformatun, ntaneous hydrolyas would proceed with higher rate than the catalytic reaction. 

While there are abundant rate data available on ester hydrolysis in homogeneous aqueous solution (e.g., Mabey and Mill, 1978), quantitative data on the effect of surfaces on reaction rates are rather scarce. Hoffmann (Chapter 3, this volume) and Stone (1989) have investigated the catalytic effect of oxide surfaces on the hydrolysis of a few carboxylic acid esters, and have found a rate enhancement for compounds for which base catalysis is important at neutral pH. 

Based on the depicted equilibrium and the observed lifetime a rate constant for the forward reaction of 10 NT s" was estimated. The slow protonation rate of the one-electron reduced fullerene n-radical anions can be understood in terms of the charge delocalization and also the hybridization of the generated carbanion. Furthermore, the heterogeneous and hydrophobic environments of the host s interior can be assumed to be beneficial for the slow-down of the protonation dynamics. In homogeneous aqueous solutions the protonation rate should be faster, a hypothesis that was substantiated by recent radiolytic experiments with bisfunctionalized fullerene derivatives. The latter compounds are soluble in aqueous solutions without employing a solubiiizer (host) and give rise to protonation rate constants of 3 x 10 M s" (38). 

When water-soluble initiators are used, most of the authors concluded that acrylamide polymerization proceeds within the monomer droplets, irrespective of the nature of the organic phase (aromatic or aliphatic) [28,30-34], Both monomer and initiator reside in the dispersed droplets and each particle acts as a small batch reactor. The process is essentially a suspension polymerization and therefore the kinetics resemble those for solution polymerization. Note that a prefix micro has been added in some cases to this type of polymerization (microsuspension) to emphasize the smallness of the reactor (d 1 pm) and the possibility of interfacial reactions [33]. A square root dependence of the polymerization rate, / p, on initiator concentration, [I] was often observed, in good accord with solution polymerization [28,32-34]. Higher orders were also found which were attributed to chain transfer to the emulsifier [30]. The reaction order with respect to monomer was found to vary from 1 [2832] to 1.7 [3031]> Orders higher than 1 are common for acrylamide polymerization in homogeneous aqueous solution and are explained by the occurrence of a cage effect [35]. 

Silica is highly insoluble in an acidic solution [23]. Thus, a convenient reaction path for silica synthesis in homogeneous aqueous solution involves the acidification of soluble silicate solutions, i.e., path BP3 of Fig. 1 ... 

The charge was introduced through oxidation of the excited polypyridyl complexes by an irreversible oxidant, 4-methoxybenzenediazonium tetrafluoroborate in acetonitrile solvent. Remarkably, the Ru excited states in the mixed-valent polymer were found not to be quenched by Ru or Os. This was attributed to the fact that these electron transfer reactions lie in the inverted region. This behavior differs from that found in homogeneous aqueous solution where excited state quenching is near diffusion controlled. Possibly the relative immobilization of the reactants on the polymer, along with the smaller value for Aout in acetonitrile, prevents their reaction at the separations and orientations at which electron transfer occurs in homogeneous solution. 

However, almost none of appropriate enzyme models have ever been reported for the reaction path control or the allosteric control in homogeneous aqueous solution(2). In biological systems, both control mechanisms are very frequently and significantly operating as in the case of the reaction path control of the amino acid metabolism by individual enzyme(3) or the allosteric control of levels of many important compounds by glutamine synthetase. 

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