Proposed reactions

For a proposed reaction mechanism to be valid the sum of its elementary steps must equal the equation for the overall reaction and the mechanism must be consistent with all experimental observations The S l mechanism set forth m Figure 4 6 satisfies the first criterion What about the second d... 

Table A.4, taken from the CCPS Guidelines for Chemical Reactivity Evaluation and Application to Process Design, shows the questions which need to be asked regarding the safety of the proposed reaction, the data required to answer those questions and some selected methods of investigation. The experimental analysis is extremely specialized, and companies should consider outsourcing the tests if they do not have specialists in this area.

The results for the tunneling splitting calculated with the use of some of the earlier proposed reaction paths for a single PES (4.40) (with the parameters adopted here) are collected by Bosch et al. [1990]. All of them underestimate by at least an order of magnitude the numerically exact value 10.6 cm which is also given in that paper. The parameters C and Q hit the intermediate region between the sudden and adiabatic approximations, described in sections 2.5 and 4.2, and neither of these approximations is quantitatively applicable to the problem. 

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. 

After metallization of the plasma-modified polyimide, there were a number of peaks in the TOF-SIMS spectra that confirmed the existence of Cu-N bonds as required by the proposed reaction mechanism (see Fig. 28). The most significant... 

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

Proposed reaction mechanisms have suggested that in many rate processes the nature of the conducting species may change during the course of decomposition. There is evidence [364] that, even in the absence of pyrolysis, the conducting entity may vary with the physical form of the reactant. 

A reaction interface is the zone immediately adjoining the surface of contact between reactant and product and within which bond redistributions occur. Prevailing conditions are different from those characteristic of the reactant bulk as demonstrated by the enhanced reactivity, usually attributed to local strain, catalysis by products, etc. Considerable difficulties attend investigation of the mechanisms of interface reactions because this thin zone is interposed between two relatively much larger particles. Accordingly, many proposed reaction models are necessarily based on indirect evidence. Without wishing to appear unnecessarily pessimistic, we consider it appropriate to mention here some of the problems inherent in the provision of detailed mechanisms for solid phase rate processes. These difficulties are not always apparent in interpretations and proposals appearing in the literature. 

No single criterion has been recognized as constituting a satisfactory basis for the systematic classification of the kinetics of solid-phase reactions (Chapt. 1, Sect. 3). A classification based on the anion is preferred here since it is this constituent which undergoes breakdown in most reactions of interest and proposed reaction mechanisms for substances containing a common anion often include similar features. 

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

In this, as in many catalysed reactions, the protonated substrate is postulated as an intermediate, and although the proposed reaction scheme in fact accords with all the known experimental facts it perhaps would be instructive to determine the dependence of the rate coefficient on the Hammett acidity function at high acid concentration and also to investigate the solvent isotope effect kD2JkH20. Both these criteria have been used successfully (see Sections 2.2-2.4) to confirm the intermediacy of the protonated substrate in other acid-catalysed aromatic rearrangements. 

Reaction scheme. Propose reaction steps consistent with the rate law for the hydrolysis of benzhydryl chloride relate a and /3 to the rate constants. 

To verify that a proposed reaction mechanism agrees with experimental data, we construct the overall rate law implied by the mechanism and check to see whether it is consistent with the experimentally determined rate law. However, although the constructed rate law and the experimental rate law may be the same, the proposed mechanism may still he incorrect because some other mechanism may also lead to the same rate law. Kinetic information can only support a proposed mechanism it can never prove that a mechanism is correct. The acceptance of a suggested mechanism is more like the process of proof in an ideal court of law than a proof in mathematics, with evidence being assembled to give a convincing, consistent picture. 

Amidinate complexes of copper(I) and copper(II) have been found to catalyze the polymerization of carbodiimides. The copper catalysts are highly active even under air and oxygen. It was shown that the catalytic activity of an air-stable copper(II) amidinato complex is almost equal to that of reported air-sensitive titanium(IV) amidinate initiators. Scheme 225 illustrates the proposed reaction mechanism. ... 

Scheme 2. Proposed reaction between bis(tributyltin)oxide and cellulose ( 30, 532).

Fig. 11. Proposed reaction mechanism for the SbaR bond cleavage reaction...

Fig. 13. Proposed reaction mechanism for ACS. The reaction involves the sequential assembly of acetyl-CoA from a carbonyl, methyl, and CoA. We favor a Ni(l) nucleophile to form a catEdytically competent paramagnetic M-CO complex, but see text for discussion of Em alternative mechanism.

Film diffusion may influence the overall reaction because of the low gas flow rate. As the bulk concentrations change little with time along the length of the reactor, an assumption of constant difference between bulk and catalyst surface concentrations is used in this study and the rate constants will change with gas flow rates. Nevertheless, the activation energies will remain constant, and the proposed reaction kinetics still provides useful hint for understanding the reaction mechanism and optimizing the reactor and operation conditions. 

The proposed reaction mechanism, which is assisted by Periasamy s stoichiometric reaction [111-118], is shown in Scheme 36. Initially, the reaction of [Fe3(CO)i2]... 

In the n-paraffin reactions, let us suggest that the reaction scheme shown in Figure 6 satisfies most of the observations. The solid lines represent reactions that have been observed, while the dashed lines are proposed reactions involving products whose steady state concentrations are too small to be measured with our analytical equipment. 

Figure 6. Proposed reaction scheme during dehydro-cyclization of n-paraffins over Te-NaX.

These relationships are identical to Haldane relationships, but unlike the latter, their validity does not derive from a proposed reaction scheme, but merely from the observed hyperbolic dependence of transport rates upon substrate concentration. Krupka showed that these relationships were not obeyed by the set of data previously used by Lieb [64] to reject the simple asymmetric carrier model for glucose transport. Such data therefore cannot be used either to confirm or refute the model. 

Baretto RD, KA Gray, K Anders (1995) Photocatalytic degradation of methyl-ferf-butyl ether in TiOj slurries a proposed reaction scheme Water Res 29 1243-1248. 

As for the reaction path from pyruvic acid to citraconic anhydride, it is considered that a condensation reaction first takes place by a reaction between an oxygen atom of carbonyl group and two hydrogn atoms of methyl group in another molecule, followed by oxidative decarboxylation to form citraconic acid. The produced citraconic acid is dehydrated under the reaction conditions used. The proposed reaction path is shown in Figure 7. 

Figure 10. Proposed reaction mechanism for NO reduction by CH in the presence of O2 ...

As shown in Table 1, the reaction order with respect to NO2 on ln/H-ZSM-5 was smaller than that of NO. This is in accordance with the proposed reaction sequence that NO is firstly oxidized to NO2 and the NO2 reacts with CH4. Coincidence in the order of reaction for NO2 between ln/H-ZSM-5 and lr/ln/H-ZSM-5 catalysts means that NO2 react on a common active site which should be In species. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

A mechanistic rationale for the observed cw-selectivity has been proposed based on preorganisation of the Breslow-type intermediate and imine through hydrogen bonding 253, with an aza-benzoin oxy-Cope process proposed. Reaction via a boat transition state delivers the observed cw-stereochemistry of the product (Scheme 12.57). Related work by Nair and co-workers (using enones 42 in place of a,P-unsaturated sulfonylimines 251, see Section 12.2.2) generates P-lactones 43 with fran -ring substituents, while the P-lactam products 252 possess a cw-stereo-chemical relationship. 

Both heterogeneous and homogeneous catalysts have been found which allow the hydroamination reaction to occur. For heterogeneously catalyzed reactions, it is very difficult to determine which type of activation is involved. In contrast, for homogeneously catalyzed hydroaminations, it is often possible to determine which of the reactants has been activated (the unsaturated hydrocarbon or the amine) and to propose reaction mechanisms (catalytic cycles). 

Schemes 6-21 Proposed reaction path for Ru-catalyzed hydration of terminal alkynes...

Schemes 6-26 A proposed reaction pathway for Rh(l)-catalyzed cyclization of a,co-alkynoic acids to alkylidene lactones...

Scheme 7-1 A proposed reaction path of Pd(OAc)2-catalyzed hydrothiolation of alkyne...

The proposed reaction mechanism (Scheme 7-2) comprises (1) oxidative addition of ArSH to RhCl(PPh3)3 to give Rh(H)(Cl)(SPh)(PPli3)n, (2) coordination ofalkyne to the Rh complex, (3) ris-insertion of alkyne into the Rh-H bond with Rh positioned at terminal carbon and H at internal carbon, (4) reductive elimination of 16 from the Rh(III) complex to regenerate the Rh(I) complex. 

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