Enhancement of rate

In Scheme 14 the effect of pressure on Diels-Alder reactions with acyclic heterodienophiles or heterodienes is presented. The application of high pressure leads also in these reactions to an enhancement of rates and improvement of yields. The hetero-Diels-Alder reaction (entry 3) is a good example of the interplay between pressure and temperature. At high pressure the rate of reaction as well as the diastereoselectivity are increased. The pressure-induced acceleration allows the temperature of reaction to be lowered, which leads to a further increase of diastereoselectivity. 

The rate of biodegradation of the second addition may be the same as the final rate evident during the active phase of breakdown of the first addition [106, 107, 118-120]. However, it is far more common to have a greater rate of biodegradation, which is usually measured as the loss of parent compound or the formation of 14C02 from labeled compound, following the second rather than the first application. The rate is further enhanced with still more additions. This enhancement of rate upon repeated additions of chemical substrate has been reported frequently for several pesticides and surfactants [14,15, 99,100,113, 123-126] as follows ... 

MICELLAR CATALYSIS. Chemical reactions can be accelerated by concentrating reactants on a micelle surface or by creating a favorable interfacial electrostatic environment that increases reactivity. This phenomenon is generally referred to as micellar catalysis. As pointed out by Bunton, the term micellar catalysis is used loosely because enhancement of reactivity may actually result from a change in the equilibrium constant for a reversible reaction. Because catalysis is strictly viewed as an enhancement of rate without change in a reaction s thermodynamic parameters, one must exercise special care to distinguish between kinetic and equilibrium effects. This is particularly warranted when there is evidence of differential interactions of substrate and product with the micelle. Micelles composed of optically active detergent molecules can also display stereochemical action on substrates. ... 

To circumvent the problems of ring cleavage in cyclobutabenzene (38) and thereby extend the series, the related reaction of protodesilylation has been used. The rate constants and partial rate factors for protodesilylation of benzocycloalke-nes in perchloric acid-methanol at 50 °C were measured (Table 3). In this system the enhancement of rate between o-xylene (5) and tetralin (2) is much less dramatic, but the loss of a-reactivity as a function of decreasing ring size stands out clearly a factor of 15 separates the rates of a exchange for tetralin versus cyclobutaben-... 

In PhCl, reaction of [IrMe(CO)2l2]2 with CO to give [IrMe(CO)3l2] and onward reaction to give [Ir(C(0)Me)(C0)3l2] was followed by IR at temperatures up to 85 °C. The reaction is 700 times faster than the migratory carbonylation of the corresponding ionic spedes, [IrMe(CO)2l3] . A further enhancement of rate for the neutral species is observed in MeOH/PhCl. 

Provides substantial enhancement of rate and extent of dechlorination. Produces maximization of desired and selective metabolic performances. 

The enhancement of rate qualitatively follows Michaelis-Menton kinetics, with both the initial slope and the final plateau increasing with increasing length of the alkyl tails of the surfactant. 

Use these data and the Debye-Hiickel theory of electrolyte nonideality to criticize or defend the following proposition Indifferent electrolytes always inhibit the rates of ion combination reactions because the activity coefficients are fractions. The data for CTABr show an enhancement of rate so this cannot be due to an activity effect. In these data, the k s for pure water and aqueous NaCl are essentially identical, so no activity effects operate in the absence of micelles either. 

Evidence comes from comparative rate studies.216 Thus 71 was hydrolyzed about 105 times faster than benzamide (PhCONH2) at about the same concentration of hydrogen ions. That this enhancement of rate was not caused by the resonance or field effects of COOH (an electron-withdrawing group) was shown by the fact both o-nitrobenzamide and terephthal-amic acid (the para isomer of 71) were hydrolyzed more slowly than benzamide. Many other examples of neighboring-group participation at a carbonyl carbon have been reported.2 7 It is likely that nucleophilic catalysis is involved in enzyme catalysis of ester hydrolysis. 

H-exchange reaction between CH3 and CH4 have been compared in the gas-phase and in rare gas solution (compressed Ar and Xe as solvents) over a wide range of internal pressures. Depending on the solvent and its internal pressure, relatively large rate enhancements have been calculated for this methyl/methane H-abstraction reaction. The major reason for this static solvent enhancement of rate in rare gas solutions seems to be the compact (tight) character of the activated complex, [CH . .. H ... 

On the other hand, the enhancements of rate and endo selection caused by adsorption of die reactants on dry silica gel (entry 5) seem to arise from an association of the nonsolvated reacting partners at the Si02 surface, which acts as a matrix. 

The enhancement of rate for both ZnX-I (X 4) and ZnX-IV ( X 12) on the introduction of the proton donor D2O strongly suggests that protons generally play an important role in the isomerization process over ZnX zeolites. This is supported by the fact that with ZnX-I the initial... 

This seemingly simple result may have far reaching consequences. For example, it may help to explain the effect of added lithium salts in nucleophilic additions to cyclohexanones as discussed earlier in this chapter. Thus, model (63) shown in Figure 472.135-137 explain the enhancement of rate and may also be relevant to the origins of stereoselectivity in this reaction. Of course, the exact location of the lithium atom and the aggregation state of the adding nucleophile are subject to speculation, since for lithium these parameters seem to be highly variable. 

Yadav, G. D., Bisht, P. M. Novelties of microwave assisted liquid-liquid phase transfer catalysis in enhancement of rates and selectivities in alkylation of phenols under mild conditions. Catat. Common. 2004, 5, 259-263. 

The effects of a- and 8-phenyl substitution can be calculated from the data of Table 5. The reaction rates for ethyl acetate a-phenyl ethyl-acetate a, /3-diphenylethylacetate are 1 45 130. Thus a phenyl group in the a-position, has a relatively large effect in the /3-position, a much smaller one. This behaviour is also observed with the halides, although in that case the enhancement of rate on a-phenylation is very much larger. 

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