Polar media, kinetics

With a non-polar medium, carbon tetrachloride, third-order kinetics were obtained (in the initial reagent concentration range of 4-50 x 10"4 M)749, whereas with a polar medium, methanol, the order with respect to iodine was reduced to one750. 

Interestingly, the cycloaddition of 2-azadiene 44 with N-methylmaleimide in 2.5m LT-DE gave predominantly exo-adduct in contrast to the thermal cycloaddition that is mainly enJo-selective (Scheme 6.27). A similar but not so dramatic increase in cxo-selectivity was also observed [47] for the cycloaddition of 44 with N-phenylmaleimide. The reaction is kinetically controlled, but the origin of the high cxo-selectivity observed in LT-DE is unclear the polar medium probably favors the more polar exo transition state. 

German ED, Kuznetsov AM, Dogonadze RR (1980) Theory of the kinetic isotope effect in proton transfer reactions in a polar medium. J Chem Soc, Faraday Trans 2 76 1128-1146... 

It is commonly assumed, therefore, that solvent reorganization will dominate electron transfer kinetics and that the reorganization energy in the same medium will be constant within a series of closely related redox partners. With a value of 2.4 kcal/mole for solvent reorganization (as obtained by Rehm and Weller (7) for fluorescence quenching of a series of arenes by substituted anilines in a polar medium) the curve shown in Fig. 2 is obtained. It is clear that substantial solvent-dependent barrier to electron exchange can be encountered. 

Much fundamental work, concerned mostly with the mechanism and kinetics of photoinduced electron transfer in polar medium, has been reported that helps in the design of new solar energy storage systems. General treatments of electron transfer... 

Direct metalation of the pyridine ring could be accomplished using the complex of n-BuLi and f-BuOK [7]. While an equilibrium mixture of all 3 possible potassio species (16,17, and 18) initially occurred, the polarity of the reaction medium and whether thermodynamic or kinetics conditions were employed influenced the equilibrium, thus giving rise to a preferred product. A polar medium favored formation of 4-potassio species 18, thermodynamically or kinetically, while 2-potassio species 16 was favored under weakly polar conditions. 

Zhong, X. F. Francois, B. Kinetics of 1,3-cyclohexadiene polymerization initiated by organolithium compounds in anon-polar medium, 1. Pure propagation step. Mafcrc>mc>/. Chem. 1990,797,2735-2741. 

The dynamical influence of a polar medium on the electron transfer process kinetics can be taken into account quantitatively only within the framework of specific models for describing the solvent. As shown in Ref. [4], to calculate transition probabihties for slow reactions, for which the distribution over vibrational states can be considered to be equilibrium, the inertial polarization of the medium can be represented as a set of effective oscillators... 

The kinetical model that we have developed to fit the experimental femtosecond IR and visible curves considers the existence of two distinct electronic and configurational states (figure 5). The correspondence that we observed between the time relaxation of the IR absorbing species (1250 nm) and the risetime of the visible absorbance (720 nm) suggest that the initial IR trapped electron is the direct precursor of the configurationaly relaxed final quantum state of solvated electron. The time constant for the appearance of the IR precursor and its relaxation are similar in pure water and an anionic aqueous solution. This implies that the existence of the trapped electron is neither influenced by the method of electron photoejection nor by the ionic strength of the polar medium. 

We note that the forces are very high at contact (actually much higher than the kinetic energy) and thus they dominate in the case of vacuum. However, they reduce dramatically with distance or when the salt is in a polar medium like water and this explains why NaCl dissociates easily to ions in water. 

Kinetic studies on the nitration of nitrobenzene by nitronium borofhioride in the polar solvents sulphuric acid, methane-sulphuric acid, and acetonitrile show the reaction to be first-order in both nitronium salt and aromatic110. With the first two solvents, the rate coefficients are similar for nitration by nitric acid and by the nitronium salts, indicating a common nitrating entity. With acetonitrile the rate coefficients are very much lower, consistent with a much lower concentration of free nitronium ions in this medium and thus with the nitronium salts existing as ion pairs in organic solvents (see Table 25). 

Rideout and Breslow first reported [2a] the kinetic data for the accelerating effect of water, for the Diels Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile and the cycloaddition of anthracene-9-carbinol with N-ethylmaleimide, giving impetus to research in this area (Table 6.1). The reaction in water is 28 to 740 times faster than in the apolar hydrocarbon isooctane. By adding lithium chloride (salting-out agent) the reaction rate increases 2.5 times further, while the presence of guanidinium chloride decreases it. The authors suggested that this exceptional effect of water is the result of a combination of two factors the polarity of the medium and the... 

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