1,4-addition activation parameters

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... 

A wide-ranging experimental study of carbene-alkene addition activation parameters examined a series of substituted aryUialocarbenes (16) in reactions with (CH3)2C=C(CH3)2 and 1-hexene (Fig. 7.9). ... 

Whereas Freeman and Lewis reported the first comprehensive analysis of hydroxymethylation of phenol, they were not the last to study this system. A number of reports issued since their work have confirmed the general trends that they discovered while differing in some of the relative rates observed [80,84-99], Gardziella et al. have summarized a number of these reports ([18], pp. 29-35). In addition to providing new data under a variety of conditions, the other studies have improved on the accuracy of Freeman and Lewis, provided activation parameters, and added new methodologies for measuring product development [97-99],... 

Rate constants (fifth column) usually correspond to one of the temperatures reported in the original papers and may be either experimentally determined values or those calculated from the activation parameters. In the preparation of the present review, the author has normalized a number of rate constants at arbitrary temperatures to permit direct comparisons with other data these normalized values and temperatures are tabulated (in italics) with the hope that they will offer additional useful information. The rate constants are usually expressed in liter x mole x sec when the values are followed by the symbol (A i) the units are sec. and dH are in kcal/mole JS is in eu. 

The three activation parameters, AG, AH, and AS decreased with polyelectrolyte addition. The decrease in AS suggests that the acceleration is due to the enthalpic loss. We recall that the acid hydrolyses of aliphatic esters with polymeric sulfonic acid was accompanied by decreases in AH and AS 97, 98 ... 

Recently samples of TiCl2j active in polymerization without additional activation, were prepared (156-159). The activity of TiClj in ethylene polymerization was practically the same as the activity of a conventional two-component system TiClg + AlEtjCl (see Fig. 3). The polymerization activity of TiClj depends to a large extent on the parameters tempera-... 

It is known that the penultimate unit influences the conformation of both model radicals and propagating radicals.32 3 Since addition requires a particular geometric arrangement of the reactants, there are enthalpic barriers to overcome for addition to take place and also potentially significant effects on the entropy of activation. Comparisons of the rate constants and activation parameters for homopropagation with those for addition of simple model radicals to the same monomers also provide evidence for significant penultimate unit effects (Section 4.5.4). 

It is easily possible to introduce refinements into the dilated van Laar model which would further increase its accuracy for correlating activity coefficient data. However, such refinements unavoidably introduce additional adjustable parameters. Since typical experimental results of high-pressure vapor-liquid equilibria at any one temperature seldom justify more than two adjustable parameters (in addition to Henry s constant), it is probably not useful for engineering purposes to refine Chueh s model further, at least not for nonpolar or slightly polar systems. 

In addition to the chemical inferences that can be drawn from the values of AS and AH, considered in Section 7.6, the activation parameters provide a reliable means of storing and retrieving the kinetic data. With them one can easily interpolate a rate constant at any intermediate temperature. And, with some risk, rate constants outside the experimental range can be calculated as well, although the assumption of temperature-independent activation parameters must be kept in mind. For archival purposes, values of AS and AH should be given to more places than might seem warranted so as to avoid roundoff error when the exponential functions are used to reconstruct the rate constants. 

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... 

Tables I, III, V, and VII give the kinetic mass loss rate constants. Tables II, IV, VI, and VIII present the activation parameters. In addition to the activation parameters, the rates were normalized to 300°C by the Arrhenius equation in order to eliminate any temperature effects. Table IX shows the char/residue (Mr), as measured at 550°C under N2.

No, Not, None More Less As well as Part of Reverse Other than Complete negation of design intentions Quantitative increases Quantitative decreases of any relevant physical parameters Qualitative increase A qualitative decrease Logical opposite of intention Complete substitution No part of intention is achieved and nothing else occurs Quantities and relevant physical properties such as flow rates, heat, pressure Same as above All design and operating intentions are achieved as well as some additional activity Some parts of the intention are achieved, others are not Activities such as reverse flow or chemical reaction, or poison instead of antidote No part of intention is achieved something quite different happens... 

Table 11.3 Rate constants and activation parameters for additions of alkyl radicals to propadiene (la) [27],...

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]. 

Additional insight can be obtained by comparing the electrochemical and homogeneous activation parameters. Table II... 

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