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Bimolecular catalyzed reactions

Bimolecular rate constants determined at temperatures giving conveniently measurable rates and calculated for the temperature given in parentheses, except for some of the catalyzed reactions (lines 1-4 and 14—19) which are third-order. [Pg.275]

Bimolecular Reactions. Models of surface-catalyzed reactions involving two gas-phase reactants can be derived using either the equal rates method or the method of rate-controlling steps. The latter technique is algebraically simpler and serves to illustrate general principles. [Pg.360]

Zeolites have led to a new phenomenon in heterogeneous catalysis, shape selectivity. It has two aspects (a) formation of an otherwise possible product is blocked because it cannot fit into the pores, and (b) formation of the product is blocked not by (a) but because the transition state in the bimolecular process leading to it cannot fit into the pores. For example, (a) is involved in zeolite catalyzed reactions which favor a para-disubstituted benzene over the ortho and meso. The low rate of deactivation observed in some reactions of hydrocarbons on some zeoUtes has been ascribed to (b) inhibition of bimolecular steps forming coke. [Pg.65]

Apart from these typical acid-catalyzed reactions, the method is capable of dealing with bimolecular processes, and even reactions that are not acid-catalyzed. For any process that takes place in non-ideal aqueous acid media, it should be the first technique tried. Admixture of the aqueous acid with moderate amounts of inert organic solvent should not be a problem. The broad variety of reaction mechanisms given above should display the versatility and utility of the excess acidity method in physical organic chemistry. [Pg.58]

Only Cram (36) has published a rationale for the very high (99%) enantiomeric excess achieved in the reaction of methyl vinyl ketone and the hydrindanone in the presence of the chiral crown ether. This mechanism envisions a bimolecular complex comprising the potassium cation and chiral host as one entity and the enolate anion of the hydrindanone as the counterion. Methyl vinyl ketone lies outside this complex. The quinine-catalyzed reaction appears to have a termo-lecular character, since the hydroxyl of the alkaloid probably hydrogen bonds with the methyl vinyl ketone—enhancing its acceptor properties—while the quin-uclidine nitrogen functions as the base forming the hydrindanone—alkaloid ion pair. [Pg.99]

Pathway b is the specific base catalyzed (HO -catalyzed) hydrolysis. This bimolecular 5n2 reaction leads to the alcohol and nitrate. A peculiar pathway is carbonyl elimination (Fig. 9.1,c). This bimolecular reaction is catalyzed by strong bases and produces a dismutation of the two moieties, the organic group being oxidized to a carbonyl compound and nitrate being reduced to nitrite. Note that proton-catalyzed hydrolysis does not appear in Fig. 9.1 since this mechanism either does not occur or is negligible. [Pg.559]

In Figure 6.18, the catalyzed reaction consists of a two-step mechanism. The uncatalyzed reaction consists of a one-step mechanism. To see how a catalyst works, in general, consider a simple, one-step, bimolecular reaction ... [Pg.302]

Furthermore, antibodies should be capable of efficiently catalyze reactions with unfavorable entropies of activation by acting as entropy traps the binding energy of the antibody being used to freeze out the rotational and translational degrees of freedom necessary to form the activated complex. This principle has been applied to the design of antibodies that catalyze both unimolecular and bimolecular reactions (see below). [Pg.309]

For a solid-catalyzed reaction between two different molecules all the possibilities for the rate-determining step of the one-molecule reaction still exist. Thus, adsorption of either molecule can be rate determining a surface reaction involving only one of the molecules can be the rate-determining step and so forth. The two-molecule case introduces some new possibilities for the rate-determining step. In both the one-molecule and the two-molecule case the rate-determining step can be a surface bimolecular reaction. With the surface reaction in the two-molecule case, however, the reaction depends upon the relative abilities of the molecules to adsorb on the active sites. There are several possible cases. Both can adsorb weakly one can adsorb moderately well as the other adsorbs weakly they can both adsorb moderately well, competing effectively with each other for sites and one can adsorb very well as the other adsorbs weakly. Another problem arises when the two... [Pg.99]

It is not yet understood how life began on Earth nearly four billion years ago, but it is certain that at some point very early in evolutionary history life became cellular. All cell membranes today are composed of complex amphiphilic molecules called phospholipids. It was discovered in 1965 that if phospholipids are isolated from cell membranes by extraction with an organic solvent, then exposed to water, they self-assemble into microscopic cell-sized vesicles called liposomes. It is now known that the membranes of the vesicles are composed of bimolecular layers of phospholipid, and the problem is that such complex molecules could not have been available at the time of life s beginning. Phospholipids are the result of a long evolutionary process, and their synthesis requires enzymatically catalyzed reactions that were not available for the first forms of cellular life. [Pg.208]

The chemical reaction step is normally composed of various steps, and a broad diversity of rate laws and reaction mechanisms are relevant for surface-catalyzed reactions. However, if the simple assumption that the chemical reaction consists of a sole unimolecular or bimolecular elementary reaction or a rate determining simple reaction followed by one or more fast steps, is made, then the reaction kinetics can be mathematically treated [92],... [Pg.432]

In the Langmuir-Hinshelwood (L-H) mechanism for surface-catalyzed reactions, the reaction takes place between two surface-adsorbed species [4,5], As a substitute for concentration, we use surface coverage, and the rate is expressed in this term. We consider that the elementary reaction in the L-H mechanism is the bimolecular surface reaction expressed by the following equations ... [Pg.441]

Because the general principles of chemical kinetics apply to enzyme-catalyzed reactions, a brief discussion of basic chemical kinetics is useful at this point. Chemical reactions may be classified on the basis of the number of molecules that react to form the products. Monomolecular, bimolecular, and termolecular reactions are reactions involving one, two, or three molecules, respectively. [Pg.89]

We apply the low-dimensional convection model [Eq. (288)] to the simple case of a isothermal bimolecular wall-catalyzed reaction occurring in a tubular reactor. For the reaction... [Pg.279]

For bimolecular, heterogeneously catalyzed reactions, two mechanisms can be distinguished ... [Pg.16]

Figure 2.5 Elementary steps involved in bimolecular, heterogeneously catalyzed reactions A) according to Eley-Rideal B) according to Langmuir-Hinshelwood. Figure 2.5 Elementary steps involved in bimolecular, heterogeneously catalyzed reactions A) according to Eley-Rideal B) according to Langmuir-Hinshelwood.
Of the mechanisms of carboxylic ester hydrolysis, that for the base-catalyzed reaction is the best understood. It generally proceeds by bimolecular attack of hydroxide ion on the carbonyl group, forming a tetrahedral intermediate, followed by elimination with acyl-oxygen fission ... [Pg.298]

Cracking of small saturated hydrocarbons, catalyzed by zeolites, can proceed via two mechanisms, both involving carbocations the bimolecular chain reaction, which involves carbenium ions that are further transformed by / -scission, and the unimolecular protolytic mechanism, involving alkanium ions that are formed by the direct protonation of the alkane by the Br0nsted acid OH groups of the catalyst. This latter mechanism, originally proposed by Haag and Dassau, is the predominant one at about 800 K in medium-pore zeolites, like HZSM-5, which favor monomolecular reactions. While rela-... [Pg.95]

In eqn. (8), the acid catalyzed reactions of HS and S2 are formulated as first-order decompositions of H2S and HS- (uncatalyzed). Consequently, a distinction between bimolecular proton transfer to a substrate and unimolecular decomposition of the conjugate acid of the substrate is not possible solely on the basis of the experimental rate equation. For both mechanisms, that represented by eqn. (7) as well as that represented by eqn. (8), the same equation is obeyed for the dependence of k on the hydrogen ion concentration, viz. [Pg.5]

Notice that the denominator is squared for a bimolecular surface reaction. In general, the exponent on the denominator is equal to the number of sites participating in a rate-determining surface-catalyzed reaction. Since trimolecular surface events are uncommon, the exponent of the denominator rarely exceeds 2. [Pg.152]

We also mentioned stereospecificity of metal-catalyzed reactions inside zeolite cavities. In acid catalysis by zeolites it is well known that shape selectivity can be imposed by (1) selective admission of reactants fitting into zeolite pores, (2) selective release of products able to diffuse through zeolite channels, while larger molecules are retained, and (3) transition state selectivity, favoring, e.g., a monomolecular transition state over a bimolecular state in a narrow cavity. New tools that have conceptually been added to this arsenal include the collimation of molecules diffusing through well-defined pores, which then hit an active site preferentially via one particular atom or group. [Pg.209]

Previous studies on amide bond formation via conversion of the bimolecular coupling reaction (see Scheme 1) into an intramolecular reaction by grafting the carboxy and amino component on a template has clearly demonstrated the strong entropic effect, i.e. the high effective local concentration on the subsequent base-catalyzed intramolecular acyl transfer reaction. [Pg.30]


See other pages where Bimolecular catalyzed reactions is mentioned: [Pg.532]    [Pg.532]    [Pg.268]    [Pg.516]    [Pg.174]    [Pg.232]    [Pg.248]    [Pg.686]    [Pg.45]    [Pg.264]    [Pg.268]    [Pg.379]    [Pg.166]    [Pg.148]    [Pg.47]    [Pg.282]    [Pg.130]    [Pg.1723]    [Pg.24]    [Pg.40]    [Pg.731]    [Pg.118]    [Pg.405]    [Pg.214]    [Pg.328]   
See also in sourсe #XX -- [ Pg.92 ]




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