Activation parameters addition reactions

On addition the stereochemistry at the C=C bond of the vinyl ether remains constant. This was established by labeling studies and by the use of cis and trans ethyl propenyl ether. From the similarity of the activation parameters, the reaction characteristics, and the stereoselectivity of the cycloaddition step to those of the reactions of the same complexes with 1,3-dienes or ynamines an associative concerted nonsynchronous process was deduced.224... 

In the following, all the reactions included in the model are reported together with the values of the relevant kinetic parameters. Addition reactions, from 1 to 7, are reported in Table 2.2, whereas condensation reactions to the single dimers (DPh ) are reported in Tables 2.3, 2.4, 2.5, 2.6, and 2.7 for all condensation reactions, an activation energy of 90 kJ mol-1 has been assumed. 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

B. l,3>2>Dioxaphospholens.—The kinetics of the addition of trialkyl phosphites to benzil have been investigated spectrophotometrically. The second-order reaction of trimethyl phosphite in dioxan has activation parameters of A// = 8.4 kcal mol and AS = — 47.5 e.u. In benzene the rate constant increases linearly with low concentrations of added organic acid and decreases linearly with low concentrations of added base. The Diels-Alder mechanism is considered unlikely on the basis of these data, and the slow step is considered to be nucleophilic addition of the phosphite to the carbon of the carbonyl group (see Scheme). 

Both 1st- and 2nd-order rate expressions gave statistically good fits for the control samples, while the treated samples were statistically best analyzed by 2nd-order kinetics. The rate constants, lst-order activation parameters, and char/residue yields for the untreated samples were related to cellulose crystallinity. In addition, AS+ values for the control samples suggested that the pyrolytic reaction proceeds through an ordered transition state. The mass loss rates and activation parameters for the phosphoric acid-treated samples implied that the mass loss mechanism was different from that for the control untreated samples. The higher rates of mass loss and... 

The TGA system was a Perkin-Elmer TGS-2 thermobalance with System 4 controller. Sample mass was 2 to 4 mgs with a N2 flow of 30 cc/min. Samples were initially held at 110°C for 10 minutes to remove moisture and residual air, then heated at a rate of 150°C/min to the desired temperature set by the controller. TGA data from the initial four minutes once the target pyrolysis temperature was reached was not used to calculate rate constants in order to avoid temperature lag complications. Reaction temperature remained steady and was within 2°C of the desired temperature. The actual observed pyrolysis temperature was used to calculate activation parameters. The dimensionless "weight/mass" Me was calculated using Equation 1. Instead of calculating Mr by extrapolation of the isothermal plot to infinity, Mr was determined by heating each sample/additive to 550°C under N2. This method was used because cellulose TGA rates have been shown to follow Arrhenius plots (4,8,10-12,15,16,19,23,26,31). Thus, Mr at infinity should be the same regardless of the isothermal pyrolysis temperature. A few duplicate runs were made to insure that the results were reproducible and not affected by sample size and/or mass. The Me values were calculated at 4-minute intervals to give 14 data points per run. These values were then used to... 

Cellulose pyrolysis kinetics, as measured by isothermal TGA mass loss, were statistically best fit using 1st- or 2nd-order for the untreated (control) samples and 2nd-order for the cellulose samples treated with three additives. Activation parameters obtained from the TGA data of the untreated samples suggest that the reaction mechanism proceeded through an ordered transition state. Sample crystallinity affected the rate constants, activation parameters, and char yields of the untreated cellulose samples. Various additives had different effects on the mass loss. For example, phosphoric acid and aluminum chloride probably increased the rate of dehydration, while boric acid may have inhibited levoglucosan... 

Rappoport and Topol investigated the displacement of the halogen of bromo- and chloromethylenemalonates (287 X= Br, Cl) by several substituted anilines and that of the brosyloxy group of (4-nitrophenyl)(4-bromo-phenylsulfonyloxy)methylenemalonate (289) by morpholine and piperidine, in acetonitrile. A rate-determining nucleophilic addition of the amines was suggested as the mechanism for these reactions. Activation parameters (AH, AS ) were determined [72JCS(P2)1823]. 

Table 2 Arrhenius and Eyring activation parameters for the second, Slow reaction observed by stopped-flow spectroscopy in the oxidative addition of halogens to diorgano tellurides 17, 20, and 23-25...

The second, slow reaction was followed for 17 and 23-25 in several solvents at several different reaction temperatures. Arrhenius and Eyring activation parameters for the second, slow reaction observed in the addition of iodine to 17 and 23-25 along with those for the addition of bromine to compound 20 are compiled in Table 2. In the examples of Table 2, the rate of reaction increases as the polarity of the solvent increases from CCI4 to EtOAc to CH3CN. The slow reaction remains first-order in all three solvents. For di-4-methoxyphenyltelluride (24), values of and A// in CH3CN are 20-40 kJ moP lower than in CCI4 or EtOAc. Again, the data from the kinetics studies are consistent with the formation of an ionic intermediate via a dissociative process. 

The activation parameters for the (bimolecular) addition and the (unim-olecular) heterolysis steps have been determined [28] for the case of Ri, R2, R3 = H or CH3 and the results are shown in Fig. 1. It is obvious that the heterolysis reaction is entropy controlled which is the consequence of the highly ionic transition state which leads to freezing of water molecules with the concomitant loss of entropy. 

The reactivity of a range of alkenes in addition reactions of peroxyl radicals has been reported. Parameters that described the relationship between the activation energy and enthalpy were calculated. An activation energy of 82 kJ moP was determined for the addition of alkylperoxy radicals to isolated C=C bonds, rising by 8.5kJmor when the alkene was conjugated with an aromatic substituent. 

Addition-elimination (for the chloro compound) and elimination-addition (via an intermediate haloalkyne, for the bromo and iodo compounds) mechanisms account for the activation parameters determined for reaction of 2-(/3,/3-dihalovinyl)-5-nitrothiophenes with MeONa-MeOH. °°... 

Pseudo-first-order rate constants for carbonylation of [MeIr(CO)2l3]" were obtained from the exponential decay of its high frequency y(CO) band. In PhCl, the reaction rate was found to be independent of CO pressure above a threshold of ca. 3.5 bar. Variable temperature kinetic data (80-122 °C) gave activation parameters AH 152 (+6) kj mol and AS 82 (+17) J mol K The acceleration on addition of methanol is dramatic (e. g. by an estimated factor of 10 at 33 °C for 1% MeOH) and the activation parameters (AH 33 ( 2) kJ mol" and AS -197 (+8) J mol" K at 25% MeOH) are very different. Added iodide salts cause substantial inhibition and the results are interpreted in terms of the mechanism shown in Scheme 3.6 where the alcohol aids dissociation of iodide from [MeIr(CO)2l3] . This enables coordination of CO to give the tricarbonyl, [MeIr(CO)3l2] which undergoes more facile methyl migration (see below). The behavior of the model reaction closely resembles the kinetics of the catalytic carbonylation system. Similar promotion by methanol has also been observed by HP IR for carbonylation of [MeIr(CO)2Cl3] [99]. In the same study it was reported that [MeIr(CO)2Cl3]" reductively eliminates MeCl ca. 30 times slower than elimination of Mel from [MeIr(CO)2l3] (at 93-132 °C in PhCl). 

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