Transesterification rate constants

It is known that transesterification does not proceed without the presence of a catalyst [42], Kokkolas et al. [30] showed that in the solid state the transesterification rate constant increases linearly with antimony trioxide (Sb203) concentration up to levels of 1000 ppm. In the same study, they also showed that the esterification proceeds independently of Sb2()3 concentration. 

An additional example of a cycloamylose-induced rate acceleration which may be reasonably attributed to a conformational effect is the facilitation of the transfer of the trimethylacetyl group from the phenolic oxygen of 9 to the aliphatic oxygen of the adjacent hydroxymethyl group to form 10. This intramolecular transesterification is remarkably enhanced relative to a comparable intermolecular reaction,6 and occurs, at pH 7.0 and 25.5°, with a rate constant of 0.0352 sec-1 (Griffiths and Bender, 1972). An even larger rate enhancement is achieved upon inclusion of this material within the cyclohexaamylose cavity—fc2 = 0.16 sec-1. This fivefold acceleration cannot be satisfactorily explained either by a microsolvent effect which would be expected to depress the rate of the reaction or, at this pH, by covalent... 

Compound 10 has also been used to quantify double Lewis acid activation by two cobalt (HI) ions [37]. In 12, the RNA analogue 2-hydroxypropyl-phenyl phosphate (HPPP) is coordinated to the dinu-clear cobalt site. It is well known that in this substrate the hydroxypropyl group is an efficient intramolecular nucleophile. Release of phenol by intramolecular cyclization is much faster than the reaction by nucleophilic attack of bridging oxide, as observed in 11. At pH >8, transesterification rate is linearly dependent on hydroxide concentration since OH" acts as an intermolecular base for the deprotonation of the hydroxypropyl group. The second order rate constant for the hydroxide-dependent cleavage is 4 x 105 times larger than the second-order rate constant for the hydroxide-dependent spontaneous transesterification of hy-droxypropyl-phenyl phosphate. 

The catalysis afforded by the La3 + system for the transesterifications of paraoxon in ethanol and methanol is quite spectacular relative to the background reactions that are assumed to be promoted by the lyoxide. The reaction rate constant of ethoxide with paraoxon in ethanol at 5.1 x 10-3 dm3 mol-1 s-133 is roughly a factor of two lower than the rate constant of methoxide with paraoxon in methanol (1.1 x 10 2dm3mol 1 s-1).17a However a solution 2mmoldm-3 in total [La3 + ], which contains 1 mmol dm-3 of Lal+, has a maximum rate constant of 7 x 10-4s-1 for decomposition of 1 in ethanol at pH of 7.3, and accelerates the rate of ethanolysis of paraoxon by a factor of 4.4 x 10n-fold relative to the ethoxide reaction at the same pH.34 By way of comparison, the acceleration afforded by a 1 mmol dm-3 solution of the La + dimer catalyzing the methanolysis of 1 at the maximal pH of 8.3 (kobs = 0.0175 s 1) is 109-fold greater than its background methoxide reaction. On this simple basis La2+ in ethanol appears to be catalytically superior to La2+ in methanol, but this stems almost exclusively from the pH values... 

Activation energies for the transesterification reactions involving methanol have been reported in the range of 6-20 kcal/mol. Reaction rate constants... 

As to solvents in group (c) (nitroalkanes), which hardly solvate cations, higher reaction rates were observed than with other solvents [465] the rates increased with increasing dielectric constant and the sorption capacity of the resin for the solvent. This result indicates that a solvation of the anions — SOJ by the nitroalkane molecules might occur by the formation of a hydrogen bond — S03 -"-H6+ CH2N02 consequently, the activity of the protons of the sulphonic groups increases and the transesterification rate is enhanced. 

The inline measurement of the water vapour pressure makes it possible to monitor its progress in the course of the reaction. The water vapour pressure does not stay constant but slowly decreases until a very low value is reached. In the absence of the enzyme however no such decrease was detected (Figure 6). We believe that the enzymatically catalyzed hydrolysis of isopropenyl acetate is responsible for this decrease in water content. The hydrolysis occurs as a parallel reaction to the transesterification of isopropenyl acetate to menthyl acetate. It is not yet clear how significant the contribution of this parallel reaction to the overall decrease in (transesterification)reactivity is. If we assume that the rate constants for the hydrolytic reaction are of the same order of magnitude as those for the transesterification such a parallel reaction would lead to a significant decrease in the rate of transesterification as the water activity rises. At low water activities the hydrolytic reaction might be responsible for the observed decrease in reactivity. Very likely however the accumulation of water around the enzyme... 

In condensation polymerization, the use of NIR to follow the reactant concentrations at elevated temperatures (200-300 °C) has been applied to the synthesis of aromatic and aliphatic polyesters (Dallin, 1997). This provides an accurate alternative to the routine measurement of acid values, hydroxyl number and viscosity, with the added advantage of providing continuous data throughout the polyesterification, which allows optimum conversion. Related examples of the use of NIR include studies of esterification of low-molar-mass analogues (Blanco and Serrano, 2000) or transesterification of an existing polymer or copolymer (Sasic et al, 2000). The NIR method allowed quantitative determination of rate constants, end points and yield and equilibrium constants as well as mechanistic information, which allowed the esterification to be optimized through the use of higher temperatures and an excess of acetic acid (Blanco and Serrano, 2000). 

Fig. 40 (top) Cationic portion of bifunctional zinc complexes and relative second-order rate constant values and (bottom) intramolecular transesterification reaction of 2-hydroxypropyl 4-nitrophenyl phosphate (HPNP). 

Binuclear zinc hydroxide complexes of the Htdmbpo and Hbdmbbppo ligands (Fig. 68) promote the transesterification of HPNP (Fig. 40, bottom).252 Binding constants and catalytic rate constants were determined for the respective complexes. The Htdmpo-ligated zinc complex exhibits a slighter higher rate constant (1.10 x 10-3s-1) than the Hbdmbbppo-ligated complex (8.33 x 10-4s-1). However,... 

Binuclear zinc complexes of triazacyclononane-containing ligands having different bridge structures (Fig. 71) exhibit second-order rate constants for HPNP transesterification that are only 3-5 times larger than that exhibited by the mononuclear [(L360H)Zn]2+ complex.256 This behavior is contrasted by that of... 

Many metal complexes have been designed and synthesized as catalysts for transesterification of phosphate diesters to model mechanisms of RNases. Typical examples are monomeric Zn complexes (53) and (54) and dimetallic Zn complexes such as (55) (Scheme 35). For (53), the pseudo-first-order rate constant, k, for hydrolysis of BPP and for transesterification of... 

FIGURE 5 The dependence of the transesterification reaction rate constant on metal oxides nanoparticles adaptability A in logarithmic coordinates. 

FIGURE 13 The dependence of the first order rate constant fcj on reaction product fractal dimension for transesterification reaction in logarithmic coordinates. Type of kinetic curves Q ty. 1 - linear, 2 - autoaccelerated, 3 - autodecelerated. Vertical shaded fine indicates the value D... 

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