Reactivity intrinsic

Pertiaps the most obvious experiment is to compare the rate of a reaction in the presence of a solvent and in the absence of the solvent (i.e., in the gas phase). This has long been possible for reactions proceeding homolytically, in which little charge separation occurs in the transition state for such reactions the rates in the gas phase and in the solution phase are similar. Very recently it has become possible to examine polar reactions in the gas phase, and the outcome is greatly different, with the gas-phase reactivity being as much as 10 greater than the reactivity in polar solvents. This reduced reactivity in solvents is ascribed to inhibition by solvation in such reactions the role of the solvent clearly overwhelms the intrinsic reactivity of the reactants. Gas-phase kinetic studies are a powerful means for interpreting the reaction coordinate at a molecular level. 

Since the free energy of a molecule in the liquid phase is not markedly different from that of the same species volatilized, the variation in the intrinsic reactivity associated with the controlling step in a solid—liquid process is not expected to be very different from that of the solid—gas reaction. Interpretation of kinetic data for solid—liquid reactions must, however, always consider the possibility that mass transfer in the homogeneous phase of reactants to or products from, the reaction interface is rate-limiting [108,109], Kinetic aspects of solid—liquid reactions have been discussed by Taplin [110]. 

As glycon affinity of glycosidases is generally low for monosaccharides (see Tables III and IV), problems with unspecific labeling may arise with glycon-derived inhibitors of high intrinsic reactivity which do not have additional features to provide enhanced affinity, for example, a suitably positioned... 

The principle of active-site-directed inactivation of glycosidases by gly-con-related epoxides can be extended to compounds having an exocyclic oxirane ring, either directly attached to the six-membered ring (32) or at some distance (33,34). Studies with -o-glucosidase from sweet almonds and intestinal sucrase-isomaltase revealed that, in spite of the higher intrinsic reactivity of these epoxides, this shift of the position of the epoxide function causes a 10- to 30-fold decrease of kj(max)/Ki, an effect which probably reflects the limited flexibility of the catalytic groups involved in the epoxide reaction. 

Despite a higher intrinsic reactivity, epoxides of type 35 and 36 show a lower inactivation rate kj(max), as seen in Table XI, than the conduritol epoxides. This is probably caused by the greater flexibility of the epoxyalkyl chain in the active-site cleft, and by non-productive binding in positions where the oxirane is not within reach of the catalytic groups of the active site. For epoxypropyl oligosaccharides, this would hold even when the inhibitor occupies the correct subsites. 

As described above, the activity of a catalyst can be measured by mounting it in a plug flow reactor and measuring its intrinsic reactivity outside equilibrium, with well-defined gas mixtures and temperatures. This makes it possible to obtain data that can be compared with micro-kinetic modeling. A common problem with such experiments materializes when the rate becomes high. Operating dose to the limit of zero conversion can be achieved by diluting the catalyst with support material. 

Temperature-programmed reactions between small amounts of adsorbed species are an excellent way to study the intrinsic reactivity of catalytic surfaces. Such experiments on rhodium (100) and (111) surfaces covered by small amounts of CO and... 

O showed a profound difference in CO2 formation rate [M.J.P. Hopstaken and J.W. Niemantsverdriet, J. Chem. Phys. 113 (2000) 5457]. Hence, care should be taken to interpret apparent structure sensitivity found under normal operating conditions of high pressure and coverage in terms of the intrinsic reactivity of sites. From the theory of chemisorption and reaction discussed in Chapter 6 it is hard to imagine how the concept of structure insensitivity can be maintained on the level of individual sites on surfaces, as atoms in different geometries always possess different bonding characteristics. 

Irreversible inhibition is probably due to the alkylation of a histidine residue.43 Chymotrypsin is selectively inactivated with no or poor inhibition of human leukocyte elastase (HLE) with a major difference the inactivation of HLE is transient.42,43 The calculated intrinsic reactivity of the coumarin derivatives, using a model of a nucleophilic reaction between the ligand and the methanol-water pair, indicates that the inhibitor potency cannot be explained solely by differences in the reactivity of the lactonic carbonyl group toward the nucleophilic attack 43 Studies on pyridyl esters of 6-(chloromethyl)-2-oxo-2//-1 -benzopyran-3-carboxylic acid (5 and 6, Fig. 11.5) and related structures having various substituents at the 6-position (7, Fig. 11.5) revealed that compounds 5 and 6 are powerful inhibitors of human leukocyte elastase and a-chymotrypsin thrombin is inhibited in some cases whereas trypsin is not inhibited.21... 

The values of ks/kp for partitioning of carbocations are most conveniently determined as the ratio of the yields of products from the competing nucleophile addition and proton transfer reactions (equation 1 derived for Scheme 2). The determination of these product yields has been simplified in recent years by the application of high-pressure liquid chromatography (HPLC). Typically, the product peaks from an HPLC analysis are detected and quantified by UV-vis spectroscopy. In cases where the absorbance of reactants and products is small, substrates may be prepared with a chromophore placed at a sufficient distance so that its effects on the intrinsic reactivity of the carbocationic center are negligible. For example, the aliphatic substrates [1]-Y have proved to be very useful in studies of the reactions of the model tertiary carbocation [1+].21,23... 

Develop novel transformations that perform with the selectivities typical of enzymatic reactions, so that geometric factors are more important than the intrinsic reactivity of a molecule. 

The specific synthesis reactions for aikoxides depend on both the nature of the metal and the intrinsic reactivity, as well as on the parent alcohol. A typical reaction for a highly reactive metal would be represented by... 

Assuming that the allyl Cl and C3 centers have an intrinsic reactivity, which is independent of the direct coupling partner (Cl or C3 of the second allyl radical), the results of a variety of different experimental investigations can be examined comparatively. If this assumption holds true, the relative product distribution for an allyl radical with intrinsic reactivity of A at Cl and of B at C3 should be given by equation 21. 

Studies on the extracted FeMo cofactor has allowed researchers to answer questions about the intrinsic reactivity associated with free clusters.31 The reference 31... 

In Figure 9.1(c), the opposite extreme case of a very porous solid B is shown. In this case, there is no internal diffusional resistance, all parts of the interior of B are equally accessible to A, and reaction occurs uniformly (but not instantaneously) throughout the particle. The concentration profiles are flat with respect to radial position, but cB decreases with respect to time, as indicated by the arrow. This model may be called a uniform-reaction model (URM). Its use is equivalent to that of a homogeneous model, in which the rate is a function of the intrinsic reactivity of B (Section 9.3), and we do not pursue it fiirther here. 

V-Acyloxy-/V-alkoxyamides are intrinsically reactive at the amide nitrogen. Three... 

Quantum mechanical approaches have been successfully used to predict hydrogen abstraction potentials and likely sites of metabolism of drug molecules [78-81]. AMI, Fukui functions, and density functional theory calculations could identify potential sites of metabolism. Activation energies for hydrogen abstraction were calculated by Olsen et al. [81] to be below 80 kj/mol, suggesting most CH groups can be metabolized which particular one depends on steric accessibility and intrinsic reactivities. 

Traditional wool dyeing methods have often involved a rapid unlevel initial strike at low temperature, followed by a prolonged migration treatment at the boil to attain optimum levelness. To fit in with these requirements, ranges of reactive dyes developed for wool needed to react slowly with the fibre and this implied reactive systems with low intrinsic reactivity. One such group that was found to react too slowly for exploitation on cellulosic... 

Here are some intrinsic reactivity hazards and their definitions. Incompatible materials will be considered in the next section of this document, as reactive interactions. 

As remarked already, kc/ku measures the maximal acceleration at levels of the CD sufficient to saturate complexation of the substrate. By looking carefully at the variations of this ratio with structure one may obtain insights into the mode of transition state binding (VanEtten et al., 1967a,b Bender and Komiyama, 1978). More useful is the ratio k2/ku (=kc/K ku) because it takes into account the effect of substrate binding and it scales the reactivity of S towards the CD to its intrinsic reactivity in the absence of CD. 

The calculation for dibenzylphosphoenolpyruvate assumes that the enol dibenzyl phosphate will have the same reactivity towards bimolecular attack by RCOOH as the dialkyl phosphate group of A.5.5. The comparison between A.5.3 and A.5.4 shows that the substitution of an alkoxy group by phenyl increases the reactivity by an order of magnitude towards COOH, and this factor allows us to put the phosphonate A.5.1 on the scale. The intrinsic reactivity of A.5.1 and A.5.2 are assumed the same b Gordon el al., 1964 Blackburn and Brown, 1969 c van Holst el al., 1974... 

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