Proposed mechanism between

Process 2, the adsorption of the reactant(s), is often quite rapid for nonporous adsorbents, but not necessarily so it appears to be the rate-limiting step for the water-gas reaction, CO + HjO = CO2 + H2, on Cu(lll) [200]. On the other hand, process 4, the desorption of products, must always be activated at least by Q, the heat of adsorption, and is much more apt to be slow. In fact, because of this expectation, certain seemingly paradoxical situations have arisen. For example, the catalyzed exchange between hydrogen and deuterium on metal surfaces may be quite rapid at temperatures well below room temperature and under circumstances such that the rate of desorption of the product HD appeared to be so slow that the observed reaction should not have been able to occur To be more specific, the originally proposed mechanism, due to Bonhoeffer and Farkas [201], was that of Eq. XVIII-32. That is. 

FIGURE 27 19 Proposed mechanism of hydrolysis of a peptide catalyzed by carboxypeptidase A The peptide is bound at the active site by an ionic bond between its C terminal ammo acid and the positively charged side chain of arginine 145 Coordination of Zn to oxygen makes the carbon of the carbonyl group more positive and increases the rate of nucleophilic attack by water... 

At this point in the investigation, the relationship between the pits and the arrowhead-shaped regions of corrosion was uncertain. Several possible causes for the pitting were considered, such as siphonic gas exsolution, biological and/or microbiological activity, and debris (concrete chips, etc.) lodged in the tubes, but each was tentatively dismissed as improbable since none of the proposed mechanisms adequately accounted for all observations. 

After deposition of 0.5 nm of copper onto plasma modified polyimide, the peaks due to carbon atoms C8 and C9 and the oxygen atoms 03 and 04 were reduced in intensity, indicating that new states formed by the plasma treatment were involved in formation of copper-polyimide bonds instead of the remaining intact carbonyl groups. Fig. 28 shows the proposed reaction mechanism between copper and polyimide after mild plasma treatment. 

In Chapter 1 we distinguished between elementary (one-step) and complex (multistep reactions). The set of elementary reactions constituting a proposed mechanism is called a kinetic scheme. Chapter 2 treated differential rate equations of the form V = IccaCb -., which we called simple rate equations. Chapter 3 deals with many examples of complicated rate equations, namely, those that are not simple. Note that this distinction is being made on the basis of the form of the differential rate equation. 

Despite the synthetic utility of this transformation, nearly eighty years elapsed between the discovery of the Bischler-Napieralski reaction and the first detailed studies of its mechanism. " Early mechanistic proposals regarding the Bischler-Napieralski reaction involved protonation of the amide oxygen by traces of acid present in P2O5 or POCI3 followed by electrophilic aromatic substitution to provide intermediate 5, which upon dehydration would afford the observed product 2. However, this proposed mechanism fails to account for the formation of several side products that are observed under these conditions vide infra), and is no longer favored. 

The proposed mechanism is based on the basis of the fact that ylides (Scheme 23 and Scheme 24) undergo bond fission between the phosphorus atom and the phenyl group in TPPY as reported by Nagao et al. [51] and between the sulfur atom and the phenyl group in POSY as observed in triphenylsulfonium salts [52-55] when they are irradiated by a high-pressure mercury lamp. The phenyl radicals thus produced participate in the initiation of polymerization. 

This equation is proposed for the range 0-40°C and for NaOH concentrations from 0.005 to 0.05 molar. These authors also reviewed and used the reaction between C02 and aqueous ammonia. They propose mechanisms and report kinetic data. 

The reaction of Si02 with SiC [1229] approximately obeyed the zero-order rate equation with E = 548—405 kJ mole 1 between 1543 and 1703 K. The proposed mechanism involved volatilized SiO and CO and the rate-limiting step was identified as product desorption from the SiC surface. The interaction of U02 + SiC above 1650 K [1230] obeyed the contracting area rate equation [eqn. (7), n = 2] with E = 525 and 350 kJ mole 1 for the evolution of CO and SiO, respectively. Kinetic control is identified as gas phase diffusion from the reaction site but E values were largely determined by equilibrium thermodynamics rather than by diffusion coefficients. 

Further confirmation of the proposed mechanism is provided by the fact that the reaction rates in acetic acid and nitromethane are little affected by the addition of small amounts of water, but when larger amounts (ca. 5 %) are added (to the acetic acid medium) competition between water and the aromatic for... 

The diastereofacial selectivity of this asymmetric [3C+2S] process is explained following a model similar to that described in Sect. 2.6.4.4 for the reaction of chiral alkenylcarbene complexes and 1,3-dienes. Thus, the proposed mechanism that explains the stereochemistry observed assumes a [4+2] cycloaddition reaction between the chromadiene system and the C=N double bond of the imine. The necessary s-cis conformation of the complex makes the imine... 

The type of catalyst influences the rate and reaction mechanism. Reactions catalyzed with both monovalent and divalent metal hydroxides, KOH, NaOH, LiOH and Ba(OH)2, Ca(OH)2, and Mg(OH)2, showed that both valence and ionic radius of hydrated cations affect the formation rate and final concentrations of various reaction intermediates and products.61 For the same valence, a linear relationship was observed between the formaldehyde disappearance rate and ionic radius of hydrated cations where larger cation radii gave rise to higher rate constants. In addition, irrespective of the ionic radii, divalent cations lead to faster formaldehyde disappearance rates titan monovalent cations. For the proposed mechanism where an intermediate chelate participates in the reaction (Fig. 7.30), an increase in positive charge density in smaller cations was suggested to improve the stability of the chelate complex and, therefore, decrease the rate of the reaction. The radii and valence also affect the formation and disappearance of various hydrox-ymethylated phenolic compounds which dictate the composition of final products. 

The reaction of bisphenol-A benzoxazine under strong and weak acidic conditions was also investigated.102 The proposed mechanism for the benzoxazine ring-opening reaction in the presence of a weak acid involves an initial tau-tomerization between the benzoxazine ring and chain forms. In an electrophilic... 

FIG. 19 Proposed mechanism for the difference of adhesive force between cations of low and high hydration enthalpies. (Reprinted from Ref. 88. Copyright 2000 by Academic Press.)... 

Several ways to suppress the 2-oxonium-[3,3]-rearrangements might be envisioned. Apart from the introduction of a bulky substituent R at the aldehyde (Scheme 23) a similar steric repulsion between R and R might also be observed upon introduction of a bulky auxiliary at R. A proof-of-principle for this concept was observed upon by using of a trimethylsilyl group as substituent R in the alkyne moiety (Scheme 25, R = TMS). This improvement provided an efficient access to polysubstituted dihydropyrans via a silyl alkyne-Prins cyclization. Ab initio theoretical calculations support the proposed mechanism. Moreover, the use of enantiomerically enriched secondary homopropargylic alcohols yielded the corresponding oxa-cycles with similar enantiomeric purity [38]. 

As described previously, the two-component coupling reaction between amidox-ime 50 and DM AD generated a mixture of Z- and E-adducts 51, which was heated in xylenes to afford hydroxypyrimidinone 55 (Scheme 6.20). The previously proposed mechanism involved tautomerization of 51 to 52, followed by a Claisen [3,3]-rearrangement to yield intermediate 53. Subsequent tautomerization of the intermediate 53 to 54, followed by cyclization would afford 55 [9a,f]. 

Figure 12 Catalytic mechanism of thermolysin and stromelysin-1. (A) The mechanism of thermolysin [54], (B) The mechanism of stromleysin-1 [10]. Equivalent residues to Tyr-157 and His-231 are not observed for stromelysin-1. The proposed mechanism for collagenase-1 [S3] is similar to stromelysin-1, but also involves Asn-180 (equivalent to Asn-162 in stromelysin-1). This residue cannot participate in stromelysin-1 due to an additional residue between Ala-165 and Asn-162. (Adapted from Ref. 10.)...

The concept of simultaneous turn over between catalytic functions in multi-functional catalysis is widely accepted (for instance, in bi-functional metal/acid transformation of alkanes), and this aspect of the proposed mechanism is a normal behaviour in steady state. 

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