Chromium reaction rate constants

The radical rearrangement reaction, serving as a timing device, has been called a free radical clock 2 It provides a means of evaluating the rate constant for reactions of this radical with other substrates. The example shows how the radical-chromium(II) rate constant can be determined. A number of other instances have been summarized.13... 

The electron self-exchange rate constant for the [Cr(CNdipp)6] couple (CNdipp = 2,6-diisopropylphenyl isocyanide) in CD2CI2 has been measured between -89 and +22 °C using H NMR line-broadening techniques, with an extrapolated value of 1.8 x 10 M s determined for 25 The kinetics of the outer-sphere oxidations of tris(polypyridine)chromium(II) complexes by a series of tris(chelate)cobalt(III) species have been studied in aqueous solution. " The cross-reaction rate constants obey the Marcus relationship, with the exception of [Co(bpy)3] " and [Co(phen)3] ", for which mild nonadiabaticity (/[Pg.18]

Rate law flooding. The second-order rate constant for the reaction between the hydrated ions of vanadium(3+) and chromium(2+) depends on [H+ ]. From the data given, which refer to T = 25.0 °C and a constant ionic strength of 0.500 M, formulate a two-parameter equation that describes the functional dependence. Evaluate the two constants. Compare your result to the one derived in to Problem 1 -2. 

Induced reactions. Consider the scheme shown4 to account for the effect of iodide ions on the reaction of vanadium(IV) and chromium(VI), as in Eq. (2-3). Derive an expression for the ratio of [Vv]/tl2] for experiments in which [CrIV] is much less than [Vlv] and [l-]. Show what rate constants or ratios can be evaluated. 

Three reviews in the A. W. Adamson 80th birthday commemoration volume deal with mechanisms of photochemical reactions of chromium complexes (90) rate constants for photochemical and thermal solvolyses have been compared for [Cr(NCS)6]3- and for [Cr(NCS)5(H20)]2- in DMFand in MeCN (91). 

Another common approach consists of the comparison between the experimental rate constants and theoretical values calculated by the procedure developed by Marcus (1956), Marcus and Sutin (1985) as well as Hush (1958). This classical procedure is used widely. Premsingh et al. (2004) gave the relevant references and described a detailed procedure to analyze the ion-radical reaction between anilines and chromium (V) complexes of azomethyne derivatives. Lepage et al. (2003) studied transformation of para-substituted thioanisoles to corresponding methylarylsulfoxides... 

Rate Constants (10 3 k/M 1 s ) for the Reactions of Chromium(IV) and Chromium(V) Complexes with Rhodium Hydrides ... 

The rate constants for hydrogen abstraction from Rh H, O H, and C-H bonds by chromyl ions and Craq002+ are summarized in Table VI. Also listed in the table are selected relative rate constants for hydrogen abstraction by tert-butoxyl and tert-butylperoxyl radicals, expressed as .buo/AbuOO- The difference between the two sets of data is striking in that alkoxyl radicals are 105-107 times more reactive than alkylperoxyl radicals, but in the chromium series the ratio kcrolkcrOO is only about 102 for all the reactions studied. This ratio is preserved over about 103-fold change in absolute rate constants within each series. 

The recently estimated rate constants (116,161) for the identity reactions. BuO/ -BuOH (3 x 104M-1s-1) and / - (5 x 102 M-1 s-1) in non-aqueous solvents differ by a factor of 60, which will cause only a modest, 8-fold increase in the estimated /vbuo/ -BuOO-The reasonable closeness between /. buO/c-BuOH and -BuOO/c-BuOOH strengthens the assumption made for the chromium couples, i.e., that (k22/ 33)0 5 1 is probably also correct to within an order of magnitude. 

The rate constants of this bimolecular reaction for a given organo-chromium cation change in order of the driving force, with fis increasing in the series Ig- < SCNg- < Br. ... 

The kinetics of the reaction between acetic acid and epichlorohydrin in the presence of chromium acetate and chromic anhydride have been studied.29 Rate constants for the reaction of epichlorohydrin with /vcresol in the presence of basic catalysts have been measured in the temperature range 71-100 °C.30 Several simultaneous reactions occur, depending on the catalyst, and an appropriate kinetic model was developed. 

It is interesting to note the effect of chromium content on reaction rate at high pressures (,—500 p.s.i.g.). Experiments (5) were carried out with normal air-activated catalysts (Figure 4). Catalysts were used with chromium contents ranging from 0.7 to 0.0005 wt. % of the total catalyst. Results of one-hour ethylene polymerization tests at 132°C. and 450 p.s.i.g. with these catalysts, activated at 500°C., are given. As the concentration of chromium was decreased, catalyst charge was increased to compensate for poisoning of catalyst sites by trace impurities and to keep total rate of production about constant. 

An inner-sphere intramolecular electron transfer has been observed also in the reaction of chromium(II) salts with [Co(A-acacCN)(NH3)5] + °. This reaction proceeds via the binuclear intermediate [(NH3)5Co(A-acacCN)Cr]" +, which evolves to [Co(NH3)5(H20)] and [Cr(0,0 -acacCN)(H20)4] + ° . The mechanism is supported by the observation that addition of non-reducing metal ions such as Zn(II), Ni(II) or Ba(II) to the reaction mixture causes a decrease of the rate constant. ... 

The preceding analysis gives the following rate constants (M s ) for Eq. (20) at 20°C Cr (1 x 10 ), Mo (5 x 10 ), W (1 x 10 ). This translates into the relative reactivity order Mo > W > Cr (5 x 10 10 1). Because of the ethyl groups in 7, this series has a steric component, but it is likely that the same qualitative order will always obtain because studies with [(mesitylene)M(CO)3] indicate that W > Cr by ca. 10 1. In dissociative reactions of 18-electron chromium triad complexes, the normal reactivity order is Mo > Cr > W. Steric effects only serve to increase the rate for Cr compared to W. The much greater rate of W compared to Cr for associative reactions at 17-electron centers is likely due to the greater size of W (and Mo). In any event, it is apparent that the reactivity of 17-electron complexes can be very dependent on the metal within a triad. 

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