Rate constants with reactive cations

Comparison of the relative alkene reactivites reported in Section III.D.4 with known copolymerization parameters [57] indicates qualitative agreement between the two sets of data. It is thus possible to use the reactivity scales in Section III.D.4, for predicting approximate copolymerization parameters, but the calculation of accurate rate ratios in copolymerization suffers from the fact that alkene reactivities are not completely independent of the nature of the attacking carbocation. This is especially true if the propagation rates get close to the diffusion limit. It is not possible to directly measure propagation rate constants by investigating cationic telomerizations, but extrapolations to such data on the basis of Eq. (23) in Section V are conceivable. 

Examples of Grunwald-Winstein treatments of reactivity trends have become rare in inorganic chemistry. Some more information has been presented on such plots for aquation of [M(NH3)5Br] + cations with M=Cr or Co in aqueous alcohols. The variation of rate constant with solvent Y value for the isomerization of a chlorophenylacetylene complex of platinum to its chloro-phenylalkynyl form, equation (3), is so small that an intramolecular mechanism is indicated. ... 

In aquation of the cations [M(NH3)5Br] + (M = Cr or Co) in alcohol-water mixtures, solvent composition variation has much more effect on A.S than on iiH. This is attributed to solvation-shell ordering effects in the transition state. Enhancement of reactivity in the reaction of cobalt(ii) with chlorophyllic acid in methanol on addition of lithium nitrate is attributed to inhibition of transition-state solvation. The effective radius of a transition state can be guessed from the variation of rate constant with dielectric constant. This approach has been used for the bromide-bromate reaction. In contrast to this concentration of attention on the transition state, it may be noted that it is the stabilization of the reactant in relation to water structure that is thought to control the variation of the racemization rate of the ( + )-[Co(phen)3] + cation in t-butyl alcohol-water mixtures. ... 

The existence of ion pairs of hydroxy aromatic anions with polar groups of cationic micelles was proposed by Zaitsev et al. [66] to explain the effective charge of anions close to zero, observed in acid-base photoreactions of hydroxyaromatics in CTAB solutions. Such a value for the effective charge was found by simulation of the values of the diffusion rate constants of hydrogen ions to excited anions of hydroxyaromatics to make the calculated diffusion-controlled protonation reaction of the excited anions rate constants close to experimentally observed ones. In aqueous solution, the excited anions are protonated with diffusional values of the rate constant with some nonsignificant steric factor [67,68]. The three-phase model can help to interpret the reactivity of polar and charged substances in micellar solutions. 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

The rate constant k iv for solvolysis is assumed to reflect the stability and reactivity of (i.e., faster solvolysis gives a more stable cation, which, therefore, reacts more slowly with nucleophiles). The ratio measured by product distribution... 

The sum of all results is consistent with the formation of both the aryl cation and the aryl radical in the aqueous acid system without copper, and with the dominance of the aryl radical in the presence of copper. The product ratios are also qualitatively consistent with the hypothesis that the reactivity of aryl cations with nucleophiles is close to that of a diffusion-controlled process (see Sec. 8.3), and that aryl radicals have arylation rate constants that are about two orders of magnitude smaller than that for diffusion control (0.4-1.7 X 107 m-1s-1 Kryger et al., 1977 Scaiano and Stewart, 1983). Due to the relatively low yields of these dediazoniations in the pentyl nitrite/benzene systems, no conclusions should be drawn from the results. 

When one compares the brutto polymerization rate constants, a measure of the reactivity of monomers during cationic homopolymerizations is obtained. It was found for p-substituted styrenes that lg kBr increased parallel to the reactivity, which the monomers show versus a constant acceptor 93). The reactivity graduation of the cationic chain ends is apparently overcomed by the structural influence on the monomers during the entire process of the cationic polymerization. The quantitative treatment of the substituent influences with the assistance of the LFE principle leads to the following Hammett-type equations for the brutto polymerization rate constants ... 

The extent to which the ion-radical pair suffers a subsequent (irreversible) transformation (with rate constant k characteristic of highly reactive cation radicals and anion radicals) that is faster than the reverse or back electron transfer (/cBET) then represents the basis for the electron-transfer paradigm that drives the coupled EDA/CT equilibria forward onto products (P)20 (equation 8). 

Furthermore, kinetic analysis of the decay rate of anthracene cation radical, together with quantum yield measurements, establishes that the ion-radical pair in equation (76) is the critical reactive intermediate in osmylation reaction. Subsequent rapid ion-pair collapse then leads to the osmium adduct with a rate constant k 109 s 1 in competition with back electron-transfer, i.e.,... 

This is the third report on attempts to measure the propagation rate constant, kp+, for the cationic polymerisation of various monomers in nitrobenzene by reaction calorimetry. The first two were concerned with acenaphthylene (ACN) [1, 2] and styrene [2]. The present work is concerned with attempts to extend the method to more rapidly polymerising monomers. With these we were working at the limits of the calorimetric technique [3] and therefore consistent kinetic results could be obtained only for indene and for phenyl vinyl ether (PhViE), the slowest of the vinyl ethers 2-chloroethyl vinyl ether (CEViE) proved to be so reactive that only a rough estimate of kp+ could be obtained. Most of our results were obtained with 4-chlorobenzoyl hexafluoroantimonate (1), and some with tris-(4-chlorophenyl)methyl hexafluorophosphate (2). A general discussion of the significance of all the kp values obtained in this work is presented. 

AN+- (Reitstoen and Parker, 1991). In other words, the triad of reactive fragments produced in (63) in the charge-transfer excitation of the EDA complex with A-nitropyridinium ion is susceptible to mutual (pairwise) annihilations leading to the Wheland intermediate W and the nucleophilic adduct N (Scheme 12), so that the observed second-order rate constant ku for the spectral decay of ArH+- in Table 3 actually represents a composite of k2 and k2. A similar competition between the homolytic and nucleophilic reactivity of aromatic cation radicals is observed in the reaction triad (55)... 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

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