Water Activation Catalytic Hydrolysis

Other reactions that may be influenced by water activity are hydrolysis of protopectin, splitting and demethylation of pectin, auto-catalytic hydrolysis of fats, and the transformation of chlorophyll into pheophytin (Loncin et al. 1968). 

Menger et al. synthesized a Ci4H29-attached copper(II) complex 3 that possessed a remarkable catalytic activity in the hydrolysis of diphenyl 4-nitrophenyl phosphate (DNP) and the nerve gas Soman (see Scheme 2) [21], When 3 was used in great excess (ca. 1.5 mM, which is more than the critical micelle concentration of 0.18 mM), the hydrolysis of DNP (0.04 mM) was more than 200 times faster than with an equivalent concentration of the nonmicellar homo-logue, the Cu2+-tetramethylethylenediamine complex 9, at 25°C and pH 6 (Scheme 4). The DNP half-life is calculated to be 17 sec with excess 1.5 mM 3 at 25°C and pH 6. The possible reasons for the rate acceleration with 3 were the enhanced electrophilicity of the micellized copper(II) ion or the acidity of the Cu2+-bound water and an intramolecular type of reaction due to the micellar formation. On the basis of the pH(6-8.3)-insensitive rates, Cu2+-OH species 3b (generated with pK3 < 6) was postulated to be an active catalytic species. In this study, the stability constants for 3 and 9 and the thermodynamic pvalue of the Cu2+-bound water for 3a —> 3b + H+ were not measured, probably because of complexity and/or instability of the metal compounds. Therefore, the question remains as to whether or not 3b is the only active species in the reaction solution. Despite the lack of a detailed reaction mechanism, 3 seems to be the best detoxifying reagent documented in the literature. 

A feature of Cs2.5 is its mesopores, in which bulky molecules can access the active sites. Thus, potential of Cs2.5 as a water-tolerant catalyst is especially evident in reactions involving large size reactants and products. Table 11 shows catalytic activity for hydrolysis of 2-methylphenyl acetate. Cs2.5 exhibits significantly superior activity against other... 

The objective of the present work is to discover whether the same sort of acidity/activity enhancements seen for polymer-supported sulfonic acids can be obtained on more rigid inorganic supports. Acid strengths of the two types of catalyst have been measured in both the absence of solvent as molar enthalpies of ammonia adsorption, and in the presence of water through molar enthalpies of neutralisation with aqueous NaOH solution. Catalytic activities have been measured in water for the hydrolysis of ethylethanoate. 

The acetylene process was developed in Germany in the early 1940s to supply the synthetic rubber industry [19]. Acetylene is reacted with hydrogen cyanide in an aqueous medium in the presence of catalytic amounts of cuprous chloride. The reaction is maintained at 80 90°C at a pressure of 1-2 atm. The reaction is highly exothermic forming a gaseous reactor effluent. This crude product is water-scrubbed and the pure acrylonitrile product is recovered from the resultant 1-3% aqueous solution by fractional distillation. The major drawbacks of this process are the large number of by-products formed by hydration, the loss of catalyst activity from hydrolysis reactions, and the buildup of ammonium chloride and tars. 

Dehydroascorbic acid hydrolysis yields biologically inactive 2,3-dioxogulonic (L-threo-hexo-2,3-diulosonic) acid. In aqueous solutions, this acid is present as a dihydrate (Figure 5.31). The reaction is generally acid-base catalysed. The activity of important ions and undissociated molecules decreases in the order hydroxyl ions (HO ) > hydronium ions (H3O+) > anions of carboxylic acids R-COO ) > undissociated carboxylic acids (R-COOH) > water. The catalytic effect of hydroxyl ions is about 15x10 times higher than that of hydronium ions. This means that the reaction rate in the solution of pH 4 is 15x 10 times lower than in solution of pH 10. Dehydroascorbic acid is most stable in solutions of pH 2.S-5.5, where the reaction is only catalysed by undissociated water molecules and is rapidly hydrolysed in neutral and alkaline solutions. 

The activity of cerium for DNA hydrolysis can be enhanced further by one order of magnitude by addition of a praseodymium(III) salt ( 0" + Pr + ratio is 2) (Tikeda et al., 1996). The two metal ions form a mixed hydroxo cluster, which is the active catalytic species. The function of praseodymium(III) is to provide metal-bound water to act as the acid catalyst in the cleavage of the intermediate (Komiyama et al., 1999). Cooperative effects were also observed for the ternary system cerium(IV)-lanthanide(III)-dextran (Sumaoka et al., 1994, 1997, 1998b). 

The Ce + ion is one of the most active catalysts for peptide hydrolysis. Its activity is much higher than that of the trivalent lanthanide ions and other transition metal ions. In particular, Ce + is far superior to other tetravalent ions like Zr" or Hf +. Yashiro et al. (1994) reported that dipeptides and tripeptides were efficiently hydrolyzed under neutral conditions by the y-cyclodextrin complex of cerium(IV). Komiyama and coworkers (Takarada et al., 2000) studied the catalytic hydrolysis of oligopeptides by cerium(IV) salts. The hydrolysis is fast, especially when the oligopeptides contain no metal-coordinating side-chains. The hydrolysis rates of the dipeptides, tripeptides and tetrapeptides is similar. The hydrolysis reaction was performed at pH 7 and 50 °C and under these conditions, the half-life of the amide bond was only a few hours. The authors found that ammonium hexanitratocerate(IV) is more active than other cerium(IV) compounds like ammonium cerium(IV) sulfate, cerium(IV) sulfate and cerium(IV) hydroxide. The lower reactivity of ammonium cerium(IV) sulfate is ascribed to the competitive inhibition by sulfate ions, while the low reactivity of cerium(IV) sulfate and cerium(IV) hydroxide can be explained by their poor solubility in water. However, in the reaction mixtures at the given reaction conditions, most of the cerium(IV) consists in a gel of cerium(IV) hydroxides. No oxidative cleavage has been observed. 

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