Two-path mechanism

Fig. 6.2.7 Two-path mechanism for the reaction of a square planar complex, MA3X, with Y to yield MA3 Y. The upper path is the solvent path (ks) and the lower one is the direct path (ky) (from Ref. N72).

The reaction [Mn04] +[W(CN)gl is presumably outer-sphere in the ratedetermining steps. The hydrogen ion dependence is consistent with the two-path mechanism shown in equations (106)—(108). Using a literature value of the two... 

The equation for rate su ests that a substitution reaction on ptA3X] to give ptA3Y] follows a two-path mechanism, as shown in Figure 13. 

Figure 13 A two-path mechanism for substitution reaction on [PtAsX] to give [PtAsY].

The reaction of 1-methyl-1,2,3,4-tetrahydropyridine 62 and azides 50a-c in dry ether at 25°C afforded the l-methylpiperidylidene-2-sulfon(cyan)amides 65a-c in good yield (82JHC1259). The reaction proceeds via a triazoline intermediate 63, which loses nitrogen to afford 65. The elimination of nitrogen from triazoline intermediate 63 occurs by two possible mechanisms [68JCS(C)277]. In path A, the 63 could eliminate nitrogen to give first an unstable 2,7-diazabicyclo[4.1.0]heptane... 

Pasinszki and Westwood investigated the dimerization of chloronitrile oxide CICNO to 3,4-dichloro-l,2,5-oxadiazole-2-oxide 78 (Scheme 48) [98JPC(A) 4939]. From B3-LYP/6-31G calculations, they conclude that the reaction path can be characterized as a typical Firestone-type cycloaddition, a two-step mechanism with a C—C bond forming characterizing the first reaction step. The activation... 

Kemkes256 assumes that the overall order relative to the esterification of terephthalic acid by 1,2-ethanediol in oligo(l,2-ethanediyl terephthalate) is two no mechanism has however been suggested. Mares257 considers that during the esterification of terephthalic acid with 1,2-ethanediol, two parallel kinetic paths take place, one corresponding to a reaction catalyzed by non-dissociated add and the other to a non-catalyzed process. In fact, Mares257 is reserved about the existence of protonic catalysis. Some other orders were found for the system terephthalic atid/l,2-ethanediol 0 (overall)318 2 (add) andO (alcohol)203 1 (add) and 1 (alcohol)181 1 (add)194 . These contradictory results could be partly due to the low solubility of terephthalic acid in 1,2-ethanediol. 

Recombination of the ion radicals within the cage is thought of as forming the path to rearrangement whilst escape of the radicals and subsequent reaction with the hydrazo compound leads to the formation of disproportionation products often observed. The theory is mainly directed at the two-proton mechanism and does not accommodate well the one-proton mechanism, since this requires the formation of a cation and a neutral radical, viz. 

Within the general mechanism for the oxidation of Ci molecules, proposed by Bagotzsky, formic acid is one of the simplest cases, since it requires only the transfer of two electrons for the complete oxidation to CO2 [Bagotzky et al., 1977]. In fact, it has the same oxidation valency as CO both require two electrons for complete oxidation to CO2. When compared with CO, the reaction mechanism of formic acid is more complex although the catalysis of the oxidation reaction is much easier. In fact, formic acid can be readily oxidized at potentials as low as 0.2 V (vs. RHE). Its reaction mechanism takes place according to the well-established dual path mechanism [Capon and Parsons, 1973a, b] ... 

The energies for the stepwise intermediates for the two paths are within 11 kcal, suggesting a concerted mechanism is possible for S-0 cleavage, but that the reaction would be very slow. In fact, of course, C-0 cleavage predominates for alkyl esters. 

FIGURE 2.27 Two possible mechanisms to explain deuterium distributions resulting from deuteriumation and exchange of methyl vinyl ketone. Deuterium is assumed to move subsurface and to attack adsorbed carbons from threefold hollows. Path A multiple exchange by repeated addition-rotation-abstraction at the [3-carbon (classic mechanism) path B I -A hydrogen shift followed by desorption as enol. 

Fig. 12. a) Mechanisms for the Grob fragmentation, (i) is a two-step mechanism, (2) is a concerted single-step mechanism, b) Level diagram for the second step of path (i) fragmentation vs. reclosure. 

One of the possible mechanisms of cis-trans isomerization of olefins is excitation to the triplet or diradical state.98-96 The two paths, one by way of singlet and triplet states and the other solely by way of singlet states, are diagrammed in Fig. 1. The two lines with minima at 0° and... 

The fluorescence and phosphorescence of luminescent materials are modulated by the characteristics of the environment to which these materials are exposed. Consequently, luminescent materials can be used as sensors (referred also as transducers or probes) to measure and monitor parameters of importance in medicine, industry and the environment. Temperature, oxygen, carbon dioxide, pH, voltage, and ions are examples of parameters that affect the luminescence of many materials. These transducers need to be excited by light. The manner in which the excited sensor returns to the ground state establishes the transducing characteristics of the luminescent material. It is determined by the concentration or value of the external parameter. A practical and unified approach to characterize the luminescence of all sensors is presented in this chapter. This approach introduces two general mechanisms referred as the radiative and the nonradiative paths. The radiative path, in the general approach, is determined by the molecular nature of the sensor. The nonradiative path is determined by the sensor environment, e.g., value or concentration of the external parameter. The nonradiative decay rate, associated with the nonradiative path, increases... 

The reaction of CIO- with methyl chloride can only proceed via the Sn2 process. An inverse KIE of 0.85 is measured (Table 10.3). The reaction with /-butyl chloride presumably proceeds via an E2 mechanism (since Sn2 attack on the Cl substituted carbon is blocked) and the observed KIE of 2.31 (Table 10.3) is consistent with that conclusion. The isotope effects for both species are nearly the same as the effects measured in the condensed phase (compare Tables 10.3 and 10.4) and measure the relative contributions of the two paths. The results indicate that the E2 pathway becomes the dominant channel as the substrate becomes more sterically hindered. 

The Diels-Alder reaction is the best known and most widely used pericyclic reaction. Two limiting mechanisms are possible (see Fig. 10.11) and have been vigorously debated. In the first, the addition takes place in concerted fashion with two equivalent new bonds forming in the transition state (bottom center, Fig. 10.11), while for the second reaction path the addition occurs stepwise (top row, Fig. 10.11). The stepwise path involves the formation of a single bond between the diene (butadiene in our example) and the dienophile (ethylene) and (most likely) a diradical intermediate, although zwitterion structures have also been proposed. In the last step, ring closure results with the formation of a second new carbon carbon bond. Either step may be rate determining. 

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