Disproportionation rates

The UV illumination of carefully selected solutions of 10 M Pu(IV) in 0.5 HClOb significantly increased the disproportionation rate and shifted the equilibrium in favor of Pu3, Pu022+, and acid. Those effects are shown in Fig. 2. 

M] = monomer concentration [l] = initiator concentration [R ] = radical concentration f = initiator efficiency k-j g = chain transfer rate "constant" ktr = termination (combination or disproportionation) rate constant... 

Condition 3 It is desirable that the toluene disproportionation reaction (kD) is not diffusion limited. Otherwise, the effective crystal size would be smaller than the actual size (r), adversely impacting on Condition 2. Experimentally we find that the observed toluene disproportionation rates are similar for equal activity HZSM 5 s having D/r2 values differing by four orders of magnitude. Thus, this condition is met. 

All the above-mentioned processes are similar in a CH3CN-pyridine (v/v = 9/1) mixture32. However, the disproportionation rate of Co1 which is electrogenerated at — 1.30 V/SCE and its oxidative addition rate to various arylhalides are divided by a factor of two compared to the rate constants determined in DMF-pyridine. It has also been shown that the use of vinyl acetate stabilizes Co1 species. Under these conditions, the disproportionation rate constant is divided by a factor of seven whereas the oxidative addition rate constants are not much affected. 

In aqueous acetic acid, the disproportionation of the platinum still occurs quite rapidly, and it can be suppressed further by adding mineral acid. Hydrochloric acid is often used, but this has a disadvantage in that the exchange rate is inversely proportional to the chloride ion concentration. Perchloric acid has been found to be more satisfactory (55). The platinum(II) catalyst most used is sodium or potassium tetrachloropla-tinate(II). An aromatic compound added to the reaction mixture also inhibits disproportionation of the platinum(II) complex—benzene, pyrene, and other aromatics have been used. A comparative study of the effect of various aromatics on the H—D exchange in alkanes has been carried out (55). Even under optimum conditions, the disproportionation [Eq. (4)] still takes place, and the catalytic platinum(II) is slowly removed from the reaction mixture. To get useful rates of exchange in alkanes, temperatures of 100° to 120°C have to be used, and the disproportionation rate increases with temperature. 

The participation of such ionic states would load one to expect disproportionation rates to increase in the series CH8, C2H5J iso-C8H7, and ferf-butyl, since the ionization potentials of these radicals form a descending series 10.0 e.v., 8.8 e.v., 7.9 e.v., and 6.9 e.v., respectively. This effect of decreasing ionization potential, which might be expected to push the ion-pair state down in energy, considerably below the radical pair state, is considerably off-set by the increasing value of rt (eqs. 5 and 6). ... 

There are, however, a number of facts which make such a proposal untenable. In the first place, the fact that disproportionation is much faster than recombination by a factor of 3.2 for fer -butyl radicals indicates that such structures cannot be general since this would imply that for recombination structure which is less favored, is arrived at through the tighter 4-centered structure (reaction H) of the disproportionation state.f It is much more reasonable to assume for er -butyl that steric repulsion of CH3 groups lowers the recombination efficiency but does not affect the disproportionation rate which goes through a different transition state. 

R. A. Sheldon and J. K. Kochi, J. Amer. Chem. Soc., 92, 4395 (1970). Note that in the t-butyl radical, with nine hydrogens, seven times as many radical pairs will disproportionate as combine even though the disproportionation rate per hydrogen is less than comb. 

The reactions shown in Eqs. (14.1) through (14.3) are known as the Alkylation reactions. They are exothermic and highly irreversible, except for Eq. (14.3). The reactions in Eqs. (14.4) through (14.6) are known as Disproportionation reactions. They are reversible and are endothermic. The alkylation reactions dictate the rate of consumption of methanol and are somewhat faster than the disproportionation rates that govern the selectivity of the three amines. 

In these cases k is a lower limit for molecular disproportionation rate constants since during the reaction a substantial amount of the donor is converted to tetralin (35b). 

It may be obvious from this explanation that the para selectivity will become virtually independent of the size of the crystallites when the disproportionation rate is severely affected by a limited diffusivity of the reactant EB. 

To obtain quantitative results the recombination and disproportionation rates of triethylmethyl radicals 19 were measured by kinetic esr spectroscopy50). The radicals 19 were generated by photolysis of 18. 

Figure 12. Plot of the hydrogen peroxide disproportionation rate vs. the concentration of Na2[Mn2(2-OH-(5-Cl-sal)pn)]2 in acetonitrile.

The U (V) disproportionation rates can best be described by a rate law consistent with a 2-1 pattern and a U(V) U(V) intermediate, although other rate laws fit nearly as well (59). 

Disproportionation equilibria have been studied for various systems. Cauquis and co-workers investigated by electrochemical means the matrix of equilibria corresponding to Scheme 2 for 3,7-dimethoxypheno-thiazine and its derivatives, and applied the measurement of the response of the equilibria to different conditions of basicity to the definition of a scale of basicity in acetonitrile. The disproportionation kinetics of the iron-thionine system were measured several years ago solvent effects on the disproportionation rate constant have been examined, and, lately, an indirect measurement of the synproportionation rate constant of thionine and leucothionine has been made. ... 

The disproportionation rate of sulfite to form sulfide and sulfate (Equation 4) has been measured. Rate measurements indicate that disproportionation is a first-order reaction which is not important at the scrubber temperature (450°C) but is rapid at reduction temperatures (850°C). Therefore the major sulfur compound undergoing reduction in the process is sulfate. 

Most likely is smaller than the propagation constant, i,e, k < 250 M sec l, However, since the concentration of the monomer is at least 10 times larger than that of the monomeric radical-anions, the addition competes efficiently with the dimerization. Nevertheless, the resulting dimeric radical-anions, M.M , play no role in the pol3nnerization because their diffusion controlled disproportionation (rate constant 10 M lsec" ) destroys them as soon as formed. Hence, radical propagation is imperceptible in such systems. 

Nitroxides derived from amines having one hydrogen atom on a-carbon atom to nitrogen oxidize with ROO to nitrones [111] and/or disproportionate easily into the respective hydroxylamine and nitrone [112]. The cis and trans nitroxides derived from 18 differ in their disproportionation rates. The disproportionation proceeds via an intermediately formed dimer ... 

In general terms, this is represented as Eq. (6.7b), where ktd is the rate constant for termination by disproportionation or simply the disproportionation rate constant. 

The dependence of the disproportionation rate constant on ionic strength confirmed that the reacting species is doubly charged [30]. 

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