Slow step of the reaction

Formation of a relatively stable carbocation is important in an SnI reaction => low free energy of activation (AGJ) for the slow step of the reaction. 

Three mechanisms have been proposed for this reaction (Scheme 21). The reaction is first order in each of the reactants. In another study, Reutov and coworkers159 found a large primary hydrogen-deuterium kinetic isotope effect of 3.8 for the reaction of tri-(para-methylphenyl)methyl carbocation with tetrabutyltin. This isotope effect clearly demonstrates that the hydride ion is transferred in the slow step of the reaction. This means that the first step must be rate-determining if the reaction proceeds by either of the stepwise mechanisms in Scheme 21. The primary hydrogen-deuterium kinetic isotope effect is, of course, consistent with the concerted mechanism shown in Scheme 21. 

The pyridine-catalysed lead tetraacetate oxidation of benzyl alcohols shows a first-order dependence in Pb(OAc)4, pyridine and benzyl alcohol concentration. An even larger primary hydrogen kinetic isotope effect of 5.26 and a Hammett p value of —1.7 led Baneijee and Shanker187 to propose that benzaldehyde is formed by the two concurrent pathways shown in Schemes 40 and 41. Scheme 40 describes the hydride transfer mechanism consistent with the negative p value. In the slow step of the reaction, labilization of the Pb—O bond resulting from the coordination of pyridine occurs as the Ca—H bond is broken. The loss of Pb(OAc)2 completes the reaction with transfer of +OAc to an anion. 

The absence of both carbon-13 and carbon-14 kinetic isotope effects at the 1-, the 1-and the T-, the 4- and the 4- and 4 -carbons in the formation of diphenyline (12) confirms beyond any doubt that this compound is formed in a two-step rearrangement. Thus, the nitrogen-nitrogen bond ruptures in the slow step of the reaction and then the product is... 

Polymerization is assumed to occur only through the imide linkages—crucial to the formation of a star-branched nylon 6 species. If an imide-containing species (such as N-acetylcaprolactam) is added to the reaction, the slow step of the reaction is by-passed, allowing polymerization to take place very rapidly. This should also be the case for a tri-imide for generating star nylon species. 

In most cases the slow step of the reaction is not simply the activation or chemisorption of hydrogen, but involves other chemisorbed species. Thus, the exchange of deuterium with methane and with other saturated hydrocarbons is much slower than with hydrogen and probably proceeds through dissociative adsorption of the hydrocarbon. 

At this stage, it is still difficult to determine whether the conclusion is appropriate for the fundamental part of the multicomponent bismuth molybdate catalyst. Unfortunately, we have no available information on the number of active reaction sites on the catalyst system. In the heterogeneous catalysis, apparent activation energy does not necessarily correspond to the real energy barrier of the elementary slow step of the reaction. Multicomponent bismuth molybdate catalyst has been established industrially, whereas only parts of the fundamental structure and working mechanism have been elucidated. In addition, important roles of alkali metals and other additives such as lanthanides remain unknown. Apparently, further investigations should be done to clarify the complete working mechanism of the multicomponent bismuth molybdate catalyst. 

Finally, different mechanisms must obtain in those cases where the breakdown of the tetrahedral intermediate is the slow step of the reaction. Two kinetically equivalent mechanisms are possible in this case also. Here, too, the catalyst may be involved as a general base, in which circumstances it would catalyze the elimination of the leaving group by the E-2 mechanism, viz-... 

This simple expression is consistent with the slow step of the reaction being the breaking of one of the six acetate bridges by acetate or acetic acid. The subsequent decomposition to dimer is fast. 

The vanadium(V) oxidation of the sulfide PhCH=CHSPh has been studied in aqueous acetic acid containing perchloric acid. The reaction is first order in vanadiiun(V) and fractional order in sulfide. An intermediate complex of vanadium and the sulfide forms and its decomposition is the slow step of the reaction.181 Two Indian groups have reported on the use of ruthenium(VI) and ruthenium(III).182 183 The kinetics and mechanism of the oxidation of diethylene glycol by aqueous alkaline potassium bromate in the presence of Ru(VI)182 and the Ru(III)-catalysed oxidation of aliphatic alcohols by trichloroisocyanuric acid183 have been examined. 

The non enzymes, the lipoxygenases, catalyse the oxidation of 1,4-diene fatty acids to alkyl hydroperoxides and the slow step of the reaction involves H atom abstraction from the carbon adjacent to the two double bonds of the fatty acid by a Fe(OH)3 species. This mechanism has now been shown to be correct by use of the lipoxygenase model (244).219 Two papers discussed earlier are relevant to this section.138,175... 

The chemoselectivity of olefin bromination is reported84 to occur after the attack of the bromine on the double bond, but the formation of the bromonium ion is the slow step of the reaction. As a consequence, the distribution of products and the selectivity of addition of nucleophiles can hardly be explained by substituent effects (both steric and electronic) bonded to the C=C double bond in a fast step of the reaction. 

O The electrophile reacts with a pair of pi electrons of the aromatic ring. This step resembles the first step of the reaction in which electrophiles react with alkenes, described in Chapter 11. This is the slow step of the reaction. 

The kinetic law is generally of the type rate = A 2 [acetylene] [H+]. In concentrated acid solutions, the plot of log koha vs H0 in the case of compounds 2 is linear with essentially unit slope (Noyce et al., 1965, 1967 Noyce and De Bruin, 1968). The study of the reaction in a series of buffers showed that it is subject to general acid catalysis (Noyce and Schiavelli, 1968a Stamhuis and Drenth, 1961) and the application of the Bunnett, Grunwald and similar treatments in the case of thio-alkoxyacetylene derivatives (Hogeveen and Drenth, 1963b) clearly indicate that the addition of a water molecule does not take place in the slow step of the reaction. 

To initiate chain growth, a "Ci" surface species may be required as in the Sachtler-Biloen mechanism, but the rate of formation of such species may be low. This scenario is comparable to conventional polymerization catalysis, in which initiation is usually the rate-limiting step. Assuming the generation of "Ci" species to be rate determining contrasts the Pichler-Schulz reaction scheme from the Sachtler-Biloen scheme, in which the slow step of the reaction is the termination. Because of the structure sensitivity of the CO dissociation reaction, and also because of the expected structure sensitivity of the chain-growth reaction, the Pichler-Schulz mechanism requires unique sites. The rate of CO insertion and consecutive steps should be fast compared with the rate of CO dissociation. Of course, the rate of termination should be low compared with that of chain growth. 

Such a scheme lends itself to several alternative descriptions of the oxidative reaction 117, 118). However, since the AT-Compound I reaction is pH-invariant 115), the pX, of distal histidine could be atypical or, more likely, its modification is not rate determining in the reaction sequence of Elq. (4). It is uncertain, however, whether fci or fcj represents the slow step of the reaction. Kinetic or analytical demonstration of a Compound I-AT complex is also lacking. Thus, under nonturnover conditions using preformed Compound I, the redox reactions are first order in Compound I and AT when [AT] <70 mM. 

Aj j V concerns the volume change in the formation of the transition state from the intermediate SH+ in the slow step of the reaction. 

However, if the slow step of the reaction involves proton transfer from the positive ion H30+ to an anionic substrate, AS is close to zero or even positive [53] (see the examples in the bottom part of Table 4). The data for these examples do not obey the Matesich relationship. Within the series of the 4-substituted salicylic acids, AS for the decarboxylation reaction increases with decreasing reactivity [53]. A positive contribution... 

This might seem like a puzzling result. If C " is involved in the reaction, why doesn t it affect the overall rate of the reaction Not only can you change the concentration of C and not affect the rate, but you also can replace it by a different anion without affecting the rate. How can this be Cr is not involved in the slow step of the reaction, so neither its concentration nor its identity affect the reaction rate. 

Although there is now agreement about the general scheme there is still discussion as to some details of the mechanism. First there is the mode of hydroxypalladation. The original suggestion was that the insertion occurred by attack of the coordinated OH on the coordinated ethylene in the slow step of the reaction. 

Isotope effects can be used to choose the most likely path. When ethylene is oxidized in deuterated water, the acetaldehyde contains no deuterium hence, all four hydrogens in the acetaldehyde must come from the ethylene. Thus, if the slow step of the reaction involves the formation of acetaldehyde, the activated complex for this slow step would involve a hydride transfer, and a primary isotope effect would be expected when deuterated ethylene is used. Actually, the isotope effect kn/ko was found to be only 1.07. In Paths 1 and 3, the slow step is, respectively, the decomposition of a 7r-complex and a a-complex to product, and they would be expected to display a primary isotope effect. However, in Path 2, the rate-determining step is the rearrangement of a 7r-complex to a (T-complex. Since no carbon-hydrogen bonds are broken, no primary isotope effect would be expected. Thus, Path 2 is consistent with all the experimental facts. Paths involving oxypalladation adducts, first suggested by the Russian workers (32), are now generally accepted (19, 28, 32). 

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