Catalysts Hydrolysis Rate

The acetylation reaction is stopped by the addition of water to destroy the excess anhydride, causing rapid hydrolysis of the combined sulfate acid ester (Eig. 7). This is followed by a much slower rate of hydrolysis of the acetyl ester groups. The rate of hydrolysis is controlled by temperature, catalyst concentration, and, to a lesser extent, by the amount of water. Higher temperatures and catalyst concentrations increase the rate of hydrolysis. Higher water content slightly iacreases the hydrolysis rate and helps minimize degradation (85). The amount of water also influences the ratio of primary to secondary... 

PETP flakes produced from used soft drinks bottles were subjected to alkaline hydrolysis in aqueous sodium hydroxide. A phase transfer catalyst (trioctylmethylammonium bromide) was used to enable the depolymerisation reaction to take place at room temperature and under mild conditions. The effects of temperature, alkali concentration, PETP particle size, PETP concentration and catalyst to PETP ratio on the reaction kinetics were studied. The disodium terephthalate produced was treated with sulphuric to give terephthalic acid of high purity. A simple theoretical model was developed to describe the hydrolysis rate. 17 refs. 

Peptide hydrolysis by platinum(II) (436) and palladium(II) complexes (437). In the latter case there is selective hydrolysis of the unactivated peptide bond in iV-acetylated L-histidylglycine the hydrolysis rate depends on the steric bulk of the catalyst. 

Ethylene oxide aqueous solutions, physical properties of, 20 635t Ethylene oxide catalysts, 20 648-649 Ethylene oxide hydrolysis, rate constants for, 20 638t... 

Fig. 21. Hydrolysis of acetals at 20°C on a Dowex 50W X10 resin catalyst [513]. Rate coefficients of the resin-catalysed reaction (feres) versus rate coefficients of the reaction catalysed by dissolved inorganic acid (fehom)- 1 Formaldehyde dimethylacetal 2, formaldehyde diethylacetal 3, formaldehyde di-2-propylacetal 4, acetaldehyde ethyleneacetal 5, acetone ethyleneacetal 6, acetaldehyde dimethylacetal 7, acetaldehyde diethylacetal. The slope for acetals 1—3 is 1, for the acetals 3—7 0.5.

Fiqure 2.4 Determination of the rate constants for the general-base catalysis of the hydrolysis of ethyl dichloroacetate. The first-order rate constants for the hydrolysis are plotted against various concentrations of the base. The slope of the linear plot is the second-order rate constant (k2). The intercept at zero buffer concentration is the "spontaneous hydrolysis rate constant for the particular pH. A plot of the spontaneous rate constants against pH gives the rate constants for the H+ and OH" catalysis. It is seen that pyridine is a more effective catalyst than the weaker base acetate ion. [From W. P. Jencks and J. Carriuolo, J. Am. Chem. Soc. 83,1743 (1961).]... 

Polymer catalysts showing interactions with the substrate, similar to enzymes, were prepared and their catalytic activities on hydrolysis of polysaccharides were investigated. Kinetical analyses showed that hydrogen bonding and electrostatic interactions played important roles for enhancement of the reactions and that the hydrolysis rates of polysaccharides followed the Michaelis-Menten type kinetics, whereas the hydrolysis of low-molecular-weight analogs proceeded according to second-order kinetics. From thermodynamic analyses, the process of the complex formation in the reaction was characterized by remarkable decreases in enthalpy and entropy. The maximum rate enhancement obtained in the present experiment was fivefold on the basis of the reaction in the presence of sulfuric acid. 

The effect of salt addition on the hydrolysis rate of dextrin in the presence of the random copolymer catalyst were investigated. The results are summarized in Table I. The catalytic activity of the copolymer is... 

Figure 4. Dependence of amyhse hydrolysis rates (v) on the substrate concentration in the presence of the random copolymer (A,A,+,0) and sulfuric acid ( J) at various temperatures. Temp (A) 85°, (A,U) 80°, (%) 75°, and (O) 70°C. [Catalyst] = 2.00 X 10 3N.

The substrate concentration dependence of the reaction rates was investigated kinetically to analyze the substrate binding effect. Figure 4 shows the relationships between the hydrolysis rate of amylose in the presence of the random copolymer catalyst and the concentration of the substrate at some reaction temperatures. The reaction rate clearly showed the saturation phenomenon at each reaction temperature. If the reaction proceeds via complex formation between catalyst and substrate, the elementary reaction could be described in the most simplified form as... 

Figure 7. Lineweaver-Burk plots of amylose hydrolysis rates catalyzed by the random copolymer in the presence of poly(vinyl alcohol). [Catalyst] = 2.00 X 10 3N. [Poly(vinyl alcohol)] = (O) 0 ( ) 4.0 X 10 2M (A) 8.0 X 10 2M. Km and Kr calculated according to Equation 4 are 0.083 and 0.22 M, respectively.

Alkaline hydrolysis rates of a series of thiophenyl 4-X-benzoates (47 X = H, Me, N02) was significantly enhanced in the presence of cyclodextrins (CDs), and this was attributed to strong binding of the benzoyl moiety within the CD cavity and covalent catalysis by secondary hydroxy groups of the CDs (48).63 The effect of MeCN and MeOH on the alkaline hydrolysis of acetylsalicylic acid in aqueous micellar solutions was reported.64 Butylaminolysis of p-nitrophenyl acetate in chlorobenzene in the presence of different kinds of phase-transfer catalysts (crown ethers and gly-mes) supported the existence of a novel reaction pathway exhibiting a first-order dependence on the concentration of the phase-transfer catalyst and a second-order... 

No dimerization of acetic anhydride has been observed in either die liquid or solid state. Decomposition, accelerated by heat and catalysts such as mineral acids, leads slowly to acetic acid (2). Acetic anhydride is soluble in many common solvents, including cold water. As much as 10.7 wt % of anhydride will dissolve in water. The unbuffered hydrolysis rate constant k at 20°C is 0.107 min 1 and at 40°C is 0.248 min-1. The corresponding activation energy is about 31.8 kj/inol (7.6 kcal/mol) (3). Aldiougli aqueous solutions are initially neutral to litmus, they show acid properties once hydrolysis appreciably progresses. Acetic anhydride ionizes to acetylium, CH CO+, and acetate, CH - CO, ions in the presence of salts or acids (4). Acetate ions promote anhydride hydrolysis. A summary of acetic anhydride s physical properties is given in Table 1. 

Fig. 34. The effect of solvent composition of aqueous dioxane on the hydrolysis rate constant of p-nitrophenyl laurate. Catalyst ( ), imidazole (o), Cyclo-(D-Leu-His)...

Since the hydrolysis rate is small at low temperatures, it is important to raise the rate by increasing the temperature and pressure. Using a temperature higher than the melting point of PET at about 250°C leads to a better distribution of the reactants. At this temperature no catalyst is necessary. At 265°C the reaction leads to a complete conversion in 30 min, with yields for TPA of 97% and EG of 91%. The TPA produced (solubility in water at 270°C = 28.5 wt%) can be obtained by cooling down the solution and following crystallization, because of the almost insoluble character of TPA at room temperature. 

The first commercial aliphatic diisocyanate to be available is 1,6-hexa-methylene diisocyanate (HDI). It is a colourless liquid with a boiling point of 127 C at 10 mm Hg. It is less reactive than either TDI or MDI, but in the presence of a catalyst the rate of reaction is enhanced. The use of HDI leads to urethane polymers with better resistance to discolouration, hydrolysis and heat degradation than has TDI (Frisch, 1969). A number of aliphatic diisocyanates have become available commercially. These aliphatic diisocyanates have been reported to give excellent colour stability. Some important ahphatic diisocyanates are illustrated in Figure 2.16. 

Summary Hydrolytic polycondensation of octyltriethoxysilane (OTES) was studied by H and Si NMR spectroscopy, GLC, and Fischer titration. Hydrolysis rate in terms of ethoxy groups was found to exceed polycondensation rate during the reaction in ethyl alcohol with hydrochloric acid as a catalyst. The molecular mass of hydrolytic polycondensation oligomers reaches a maximum value at molar ratio water/OTES = 1.5. 

Acidic hydrolysis of CCT catalysts is described as a possible problem. Few actual measurements of the catalyst hydrolysis have been made. Addition of only 1% of acetic acid in MMA leads to a rate of cobaloxime deactivation of about 1.2% min 1 at 60 °C.296 The stability of the BF2-bridged cobaloxime is believed to be higher.119,338,342 The complex (dmgBF2)2-Co11 decays at room temperature at a rate of about 0.6% min 1 at pH = 1 but is practically stable at neutral pH.309,310 It should be noted, however, that these measurements were made in a benign solvent rather than in an active polymerization system involving free radicals. 

The synthesis conditions which lead to weakly branched systems involve the use of an acid catalyst where pH < 2.2 (iso-electric point of silica) and the use of low to moderate water content (r < 10). Hydrolysis (see reactions in Section 7.4) then takes place via a fast protonation of the alkoxide, followed by attack of water, resulting in the substitution of the alkoxy group with an hydroxyl group. Protonation becomes slower when more hydroxyls are present. The hydrolysis rate will therefore decrease with the extent of OH substitution. Acid catalysed condensation reactions proceed analogously where a protonated silanol species is attacked by water. The condensation reaction rate decreases with the number of condensed Si-O-Si groups. 

A transesterication reaction occurs that results in cleavage of the substrate and ligation of the 3 -portion of the substrate (Tsang and Joyce 1994). Just like in the case of enzyme- or catalytic antibody-catalyzed reactions, the rate depends upon substrate binding affinity and the intrinsic catalytic rate parameters. For example, in ester hydrolysis there is a hyperbolic dependence on the concentration of the ribozyme at low concentration of catalyst the rate of hydrolysis is first order, while at high concentration of catalyst the reaction rate is indepen-dent of ribozyme concentration (Piccirilli et al. 1992). This type of saturation or Michaelis-Menten kinetic behavior is typical of ribozymes and is completely analogous to the enzyme-substrate complex observed for enzymes and catalytic antibodies. 

Hydrolysis rates have been measured for a number of azolides in order to compare the effects of extra annular nitrogens and benzannelation. Rate constants for A-acyl compounds are larger than for the corresponding thioacyl analogues, and benzimidazolides are less reactive than imidazolides <83BAU1525,83BAU1934). Hydroxylated surfactant micelles are powerful catalysts for the deacylation of 1-acylimidazoles under neutral conditions <89JCS(P1)1697>. 

Table I. Hydrolysis rates of acetals (dlchloroacetlc acid as catalyst)...

The hydrolysis rates for several blcycllc orthoesters have been measured using acetic acid as catalyst, and compared to the hydrolysis rates of acyclic orthoformates (17). 

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