Catalysis, hydrolysis

Keywords Carbohydrates, Dehydration, Heterogeneous catalysis. Hydrolysis, Oxidation... 

Base catalysis—hydrolysis. Pohl studied the hydrolysis in aqueous solutions of a series of trialkoxysilanes of R Si(OCH,CH,OCH,), structures in which R was an alkyl or a substituted alkyl group [42]. The reactions were followed using an extraction/quenching technique. Silanes were studied at concentrations ranging from 0.001 to 0.03 M and pHs adjusted from 7 to 9. The hydrolysis was found to be first order in silane. The order in water was not determined because the reactions were carried out in a large excess of water (water was the solvent). The rate constants for the hydroxide anion catalyzed hydrolysis reactions and reaction half-lives are reported in Table 1. 

Acid catalysis—hydrolysis. Several series of alkylsilane esters were studied to determine the effect of silane structure on the hydronium ion catalyzed hydrolysis reaction. The hydronium ion catalyzed hydrolysis rate constants for a series of alkyl tris-(2-methoxyethoxy)silanes in aqueous solution were used to define the modified Taft equation log(A /Ah ) = 0.39a + 1.06ES, where Ho is the rate of hydrolysis for methyl tris-2-(methoxyethoxy)silane [42], The hydronium ion catalyzed hydrolysis rate constants and the reaction half-lives are reported in Table 2. In a similar manner, the hydronium ion catalyzed hydrolysis rate constants for a series of trialkylalkoxysilanes in 55% aqueous acetone were used to obtain the modified Taft equation log(/cH//cHo) = -0.37 a + 2.48 E where kHo is the rate of hydrolysis for trimethylalkoxy-silane. 

The best way to immobilize an enzyme while affecting its structure as little as possible is to encapsulate it Out of all the different encapsulation methods the most prominent and widely used is the sol gel technique [5, 7, 62). Sol gels are highly porous silica materials that are readily prepared and modified (Scheme 2.5). The sol gel obtained is a chemically inert glass that be formed into any desired shape and can be designed to be thermally and mechanically very stable. Most importantly the synthesis proceeds under condihons that are relatively benign for many enzymes. In the first step a tetra-aUcoxysilane such as tetramethoxylsilane (TMOS) is hydrolysed by acid catalysis. Hydrolysis is followed by condensation and the sol is formed, which is a mixture of partially hydrolyzed and partially... 

The latest results on imprinted chiral footprints [154] have shown that enantioselective catalysis (hydrolysis) does occur, and based on kinetic measurement the authors believe that this is due to an enantioselective mechanism. Kaiser and Andersson also chose aluminium doped silica as a polymeric material to obtain phenanthrene imprints and their work has been discussed earlier [52]. No selectivity towards the template was observed when imprinted silica was used as stationary phase. Only relative retention and capacity factors increased. Furthermore, even after careful extraction in a Soxhlet, the polymer still leaked phenanthrene. They also found that diazomethane yields a side reaction forming long alkyl chains. Finally they attempted to rej at the work of Morihara et al. [150-155]. but were not able to detect any selectivity using dibenzamide as the template and instead found that the template decomposes into at least five different products when adsorbed on the silica. Clearly further work is required on these systems. 

Figure 9.5. Covalent Catalysis. Hydrolysis by chymotrypsin takes place in two stages (A) acylation to form the acyl-enzyme intermediate followed by (B) deacylation to regenerate the free enzyme.

BOX 9.2 Catalysis - Hydrolysis of Ethyl Acetate by Means of a Cation... 

So far, only very little attention has been focussed on the use of zeolites in biocatalysis, i.e., as supports for the immobilization of enzymes. Lie and Molin [116] studied the influence of hydrophobicity (dealuminated mordenite) and hydrophilicity (zeolite NaY) of the support on the adsorption of lipase from Candida cylindracea. The adsorption was achieved by precipitation of the enzyme with acetone. Hydrolysis of triacylglycerols and esterification of fatty acids with glycerol were the reactions studied. It was observed that the nature of the zeolite support has a significant influence on enzyme catalysis. Hydrolysis was blocked on the hydrophobic mordenite, but the esterification reaction was mediated. This reaction was, on the other hand, almost completely suppressed on the hydrophilic faujasite. The adsorption of enzymes on supports was also intensively examined with alkaline phosphatase on bentolite-L clay. The pH of the solution turned out to be very important both for the immobilization and for the activity of the enzyme [117]. Acid phosphatase from potato was immobilized onto zeolite NaX [118]. Also in this study, adsorption conditions were important in causing even multilayer formation of the enzyme on the zeolite. The influence of the cations in the zeolite support was scrutinized as well, and zeolite NaX turned out to be a better adsorbent than LiX orKX. 

Abstract This chapter explores the role of abiotic reactions such as acid catalysis (hydrolysis) as well as the adsorption of methyl tert-butyl ether (MTBE) and other fuel oxygenates in environmental issues as the remediation of these substances is notoriously difficult. First of all, these methods are briefly classified with other abiotic technologies. The suitability of hydrolysis and adsorption for the remediation of water contaminated by fuel oxygenates is then discussed in detail, with information being provided about the principle of the reactions, potential catalysts and sorbents, limitations of the reactions, and practical implications. To conclude, the possible application of hydrolysis and adsorption in combination with other remediation techniques is also examined. 

Keywords Adsorption Acid catalysis Hydrolysis Abiotic remediation... 

Scheme 73. Intramolecular general base catalysis and intramolecular nucleophilic catalysis Hydrolysis of aspirin and3,5-dinitroaspirin.

Esters. Organic esters of all kinds, including the esters of carbohydrates, are quite subject to hydrolysis with acids, bases, and, in many cases, enzymes. In this connection, the term saponification is commonly used instead of hydrolysis. The most important example is that of the saponification of fats and oils to make glycerol and either soap or fatty acids. The decomposition of ethyl and methyl acetates by water plus a catalyst has probably received more attention than any other case of hydrolysis by reason of its bearing on the general subject of catalysis. Hydrolysis of esters is reversible, unlike that of the carbohydrates, so that the equilibrium point may be approached from both sides. The following equations represent typical cases of ester hydrolysis ... 

The feasibility of designing polymer-chelator conjugates from which release of the chelator is triggered reversibly by the appearance of the target (toxic) metal in the circulatory system was tested by synthesis and measurement of the rates of metal-catalysis hydrolysis of the ester of poly(vinyl alcohol) and quinaldic acid at pH 7.5. Hydrolysis in 50% aqueous ethanol solution in the presence of the four m.etals, Cu(II), Ni(II), CodI) and Zn(II) at pH 7.5 was first order, with half lives of 67, 71, 172 and 476 min., respectively. The rate of hydrolysis in the absence of metals was not measurable. A double reciprocal plot of kobs vs. [M] for Ni(II) exhibited the expected linearity. For films of the ester, the deviation from first order kinetics was consistent with a contributing diffusion process. 

Cheng, S., Zeng, X., Meng, X., Yu, X. MetaUomiceUar catalysis hydrolysis of p-nitrophenyl pieolinate catalyzed by copper(II), nickel(II), and zinc(II) complexes of long alkylpyridine ligands in micellar solution. 7. Colloid Interface Sci. 2000, 224(2), 333-337. 

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