Kinetics complex anions

Kinetics. Details of the kinetics of polymerization of THF have been reviewed (6,148). There are five main conclusions. (/) Macroions are the principal propagating species in all systems. (2) With stable complex anions, such as PF , SbF , and AsF , the polymerization is living under normal polymerization conditions. When initia tion is fast, kinetics of polymerizations in bulk can be closely approximated by equation 2, where/ is the specific rate constant of propagation /is time [I q is the initiator concentration at t = 0 and [M q, [M and [M are the monomer concentrations at t = 0, at equiHbrium, and at time /, respectively. 

Further data on this reaction are summarised in Table 20. The role of complexing anions is clear from the kinetics and also from relative rates. It appears that strongly bound ligands are associated with second-order reduction but that weakly bound ligands such as H2O result in a third-order reaction. One view of the third-order term for dilute sulphuric acid (as for aqueous HCIO4) is that the active reductant is a bridged species of the type (Fe S04 Fe ) . 

A comprehensive kinetic, spectroscopic, and analysis study into the Rh-catalyzed carbonylation of ROH (R = Me, Et, Pr) has been reported.4,5 In all cases, the reaction rate is first order in both [Rh] and added [HI] and independent of CO pressure. The only Rh species observed under catalytic conditions was (1). The rates of carbonylation decreased in the stated order of R, with relative rates of 21 1 0.47, respectively at 170 °C. All the data are consistent with rate-determining nucleophilic attack by the Rh complex anion on the corresponding alkyl iodide. 

Kinetic studies on IrC CO) compounds in methanol have confirmed initial attack by Cl . However, protonation by CF3S03H, where the anion is non-coordinating, gives [IrHCl(CO)L2(MeOH)]+. The more basic complex IrCl(COD)(PEtPh2)2 is also first attacked by Cl , followed by immediate protonation of the resulting complex anion. The 3-coordinate intermediates are fluxional, and hence the stereochemistry of the final product is determined by thermodynamic stability. 

The common ion effect does not influence the kinetics of collapse of the ion pairs to dormant covalent species since it is a unimolecular reaction. In this case, however, deactivation of the ion pair can be increased by using less stable and more nucleophilic counteranions. The nucleophi-licity of both pure halides and complex anions with halide ligands increases in the order FOther examples of polymerizations which are well behaved because the equilibrium is favorable due to nucleophilic counteranions include Hl/I2 initiated polymerizations of vinyl ethers and polymerizations of isobutene and styrene using acetate-based initiators in the presence of BC13. 

Apparently the problems with slow equilibration can be avoided by increasing the equilibration time, but it is not always the case. For example, the presence of complexing anions often enhances dissolution of metal oxides and related adsorbents. This results in the following kinetic pattern the uptake as a function of time rises, peaks and then declines. In order to account for this effect, the declining segment was extrapolated to / = 0 [21], and the intersection with the axis of ordinates was interpreted as uptake corrected for dissolution of the adsorbent. Anyway, negligible dissolution of the adsorbent is a substantial advantage of experiments with relatively short equilibration times. 

The unusual reversal of isomers may be rationalized according to Scheme 19, where an equilibrium is proposed between the kinetically controlled anionic C-2 a-adduct 54 and the thermodynamically stable anionic C-6 CT-adduct 55. Another equilibrium exists between the dianionic <7-adducts 56 and 57. Support for the existence of dianionic cr-adducts has been presented by Novikov et al. (76CHE210) and Kessar et al. (73IJC825). The function of the partial pressure of ammonia is to inhibit the formation of the dianionic a-adducts. Once the dianionic cr-adducts are formed, they become good hydride donors and aromatize to products 58 and 59. As the concentration of products slowly becomes significant, the principal amination pathway is taken over by autocatalysis by means of a six-membered transition complex, as described in Section II,A,2 and as shown for the 2,5-isomer in Scheme 20. 

The base-catalysed cis-trans isomerization of l-aryl-2-phenylcyclopropanes has been subjected to kinetic analysis and a complexed anion, e.g. (190), is thought to be produced initially. Configurational inversion of the cyclopropyl sulphoxide (173) can also be effected by base. A kinetic analysis of the hydrolysis of 2- and 2,2-di-substituted bromocyclopropanes suggests that for 2-vinyl-substituted compounds considerable progress towards an allyl cation has been been made at the transition state. Metallation studies show that, for nortricyclanes at least, the cyclopropyl hydrogen atoms are most readily attacked by pentylsodium in the presence of potas-... 

Although this analysis indicates that the transformation in equation 3 should be favored for sodium over potassium and cesium, there are several other factors to be considered. First, as was pointed out for the previous thermodynamic analyses, no allowances have been made for entropy changes (25) nor differing heats of solution for MF and MCI in a fused salt medium (30). In fact, these effects must be important as it is well-known that the solvolysis of each of the chlorides LiCl, NaCl, KCl, RbCl, and NH4CI by HF generates a solution of the respective fluoride (31,32). Bulkier cations would also be favored based on the fundamental stabilization of a complex anion as a salt by the use of a bulky cation (6,27). With this uncertainty, it was of interest to broadly determine the optimum conditions for equation 3 with respect to cation or mixtures of cations, cation concentration, and percent conversion. Other interests included studying the reaction equilibria, kinetics, mechanisms, and phase behavior. 

Obviously the structures and yields of Birch reduction products are determined at the two protonation stages. The ring positions at which both protonations occur are determined kinetically the first protonation or 7t-complex collapse is rate determining and irreversible, and the second protonation normally is irreversible under the reaction conditions. In theory, the radical-anion could protonate at any one of the six carbon atoms of the ring and each of the possible cyclohexadienyl carbanions formed subsequently could protonate at any one of three positions. Undoubtedly the steric and electronic factors discussed above determine the kinetically favored positions of protonation, but at present it is difficult to evaluate the importance of each factor in specific cases. A brief summary of some empirical and theoretical data regarding the favored positions of protonation follows. 

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