Rate enhancement, electrostatic

As shown in Figure 2, adsorption of dispersants on particle surfaces can increase 2eta potential further, enhancing electrostatic repulsion. Increased repulsion between particles is evidenced by lower viscosity in concentrated slurries, or decreased settling rates in dilute suspensions. The effect of added dispersants on settling of (anhydrous) iron oxide particles is shown in Figure 3. 

The solvent dependence of the reaction rate is also consistent with this mechanistic scheme. Comparison of the rate constants for isomerizations of PCMT in chloroform and in nitrobenzene shows a small (ca. 40%) rate enhancement in the latter solvent. Simple electrostatic theory predicts that nucleophilic substitutions in which neutral reactants are converted to ionic products should be accelerated in polar solvents (23), so that a rate increase in nitrobenzene is to be expected. In fact, this effect is often very small (24). For example, Parker and co-workers (25) report that the S 2 reaction of methyl bromide and dimethyl sulfide is accelerated by only 50% on changing the solvent from 88% (w/w) methanol-water to N,N-dimethylacetamide (DMAc) at low ionic strength this is a far greater change in solvent properties than that investigated in the present work. Thus a small, positive dependence of reaction rate on solvent polarity is implicit in the sulfonium ion mechanism. 

These reactions have an analogy in the Sn2 reactions of a-haloketones such as phenacyl bromides.175 The rate-enhancing effect of x-carbonyl groups on SN2 processes at carbon is well known, and has been attributed to conjugation of the p-orbital on carbon in the SN2 transition state with the carbonyl re-bond,164 175 177 stabilisation of ionic character at the central carbon as outlined by Pross,164,178 as well as electrostatic attraction to the carbonyl carbon.176 Although there... 

A. Warshel, G. Naray-Szabo, F. Sussman, J. K. Hwang, How Do Serine Proteases Really Work , Biochemistry 1989, 28, 3629-3637 G. Naray-Szabo, Electrostatic Effects on Catalytical Rate Enhancements in Serine Proteinases , Int. J. Quantum Chem. 1982, 22, 575-582. 

Zeffren and Hall (1973) have commented that, since reactions with polar transition states in nonpolar solvents can be accelerated by several orders of magnitude by the presence of low concentrations of salts (Winstein et al., 1959), the rate enhancement of tetramethylglucose mutarotation provided by the presence of acid-base pairs such as phenol and pyridine may be due to formation of ion pairs in benzene solution. Salts which do not act as acids and bases catalyse mutarotation of tetramethylglucose in aprotic solvents (Eastham et al., 1955 Blackall and Eastham, 1955 Pocker, 1960). The efficiency of enzymatic catalysis could arise largely from electrostatic catalysis... 

R was varied. Since the mechanism for the methyl ester is certainly A-1 and since intramolecular general acid catalysis should give a different transition state structure and therefore a different p value, it was concluded that the mechanism was A-1 in both cases. The rate enhancement provided by the carboxyl group substituent was ascribed to electrostatic catalysis whereby a proton is stabilized on the acetal oxygen, thus lowering the dissociation constant of the conjugate acid. Complete protonation of methoxymethoxybenzoic acids might be required because of the unstable carbonium ion intermediate. 

The monoanionic species is most reactive, but its associated rate constant for intramolecular general acid catalysis is only 65 times greater than that for the unionized species. Most of the large rate enhancement in comparison with the dimethyl ester is due to participation by one carboxyl group, as is the case with the unionized acetal [77]. If the carboxylate anion of the monoanionic species is electrostatically stabilizing the incipient carbonium ion in the reaction [8], its effect on the rate must be small. 

In terms of enzyme catalysis, the following factors are likely to influence the magnitude of the rate enhancement in enzymatic processes (a) proximity and orientation effects (b) electrostatic complementarity of the enzyme s active site with respect to the reactant s stabilized transition state configuration (c) enzyme-bound metal ions that serve as template, that alter pK s of catalytic groups, that facilitate nucleophilic attack, and that have... 

The sample kinetic data listed in Table 5.2 shows that the size of rate enhancement critically depends on the substrate-metal ion combination, and is markedly influenced by the solvent. The largest effect is displayed by 2-AcO-21 C6, which reacts with EtOBaBr half a million times faster than with EtONMe4. The conclusion was reached [6] that the huge rate enhancements observed in the ethanolysis reactions are a consequence of the fact that not only cation-anion electrostatic binding but also coordinative binding to the polyether chain in the metal-bound transition states are much more efficient in EtOH than in MeOH. 

In the area of catalysis, the esterolysis reactions of imidazole-containing polymers have been investigated in detail as possible models for histidine-containing hydrolytic enzymes such as a-chymotrypsin (77MI11104). Accelerations are observed in the rate of hydrolysis of esters such as 4-nitrophenyl acetate catalyzed by poly(4(5)-vinylimidazole) when compared with that found in the presence of imidazole itself. These results have been explained in terms of a cooperative or bifunctional interaction between neighboring imidazole functions (Scheme 19), although hydrophobic and electrostatic interactions may also contribute to the rate enhancements. Recently these interpretations, particularly that depicted in Scheme 19, have been seriously questioned (see Section 1.11.4.2.2). 

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. 

From these results, it can be concluded that the rate enhancement of polysaccharide hydrolysis obtained with the present copolymer catalyst was attributable to the hydrogen bonding interactions between the substrate and the catalyst and to the electrostatic interactions between the catalyst polyanions and protons. A drawing of this concept is shown in Figure 10. A polymer molecule is surrounded by a proton atmosphere. The substrate molecules are pulled into the atmosphere by hydrogen bonding interactions and hydrolyzed in the presence of a high concentration of proton. 

The amazing rate enhancement observed in enzymatic catalysis results from stabilization of the transition states and/or destabilization of the substrates. These effects are achieved by interactions of the reactants with the protein residues. Depending on the particular reaction some interactions may play a dominant role, or many different types, such as electrostatics, hydrophobic interactions, geometric distortion, or hydrogen bonds, may concurrently contribute to catalysis. Understanding these interactions is the key factor in exploiting enzymatic reactions for the purpose... 

It is clear from Table 4—2 that the rate enlunc ment at low ethanol contents is derived from the increase in k m rather than the decrease in The pff-rate profile of the solvolysis in ca. 30 vol% ethanol gave a curve of increasing dope with the increasing fraction of the neutral imidazole unit, which suggests that the electrostatic attraction is not a major factor for the rate enhancement. 

A more recent reappraisal study shows diat the catalytic activity of the free pyridine residue in partly quatemized polyvinylpyridines is similar to or lower than that of ethylpyridine 2 in the PNPA solvolysis (90). Rate enhancements were observed for NASA 4 because of the electrostatic attraction. 

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