Chromium reaction rates

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... 

A wet-ashing technique used for dissolution of graphite in perchloric acid involved boiling a mixture of 70% perchloric acid and 1% of chromium trioxide as an aqueous solution. This method was later applied to 6-14 mesh charcoal, and after boiling for 30 min the reaction rate increased (foaming) and accelerated to explosion. The charcoal contained traces of extractable tar. 

The second order in catalyst points to the involvement of two chromium salen molecules in the transition state complex. Therefore several dimeric species were synthesised with suitable linkers the dimeric catalysts gave reaction rates that were one or two orders of magnitude higher than that of the monomeric catalyst. Trimeric species gave still higher reaction rates The... 

The submitters had recommended use of only slightly more [2.3 moles of chromium(II) complex per mole of halide] than the stoichiometric amount of chromium(II) complex in this reduction. However, because these concentrations of reagents lead to a very slow reaction rate in the last 5-10% of the reduction (Note 6), the checkers found it more convenient to employ excess reducing agent [3.8 moles of chromium(II) complex per mole of halide]. 

In the first example, we studied the competitive bromination of two disubsti-tuted trisacetylacetonates of chromium. The dichloro compound (X = C1) reacts with iV-bromosuccinimide much more rapidly than the dinitro compound (X =N02). The reaction is thought to involve electrophilic substitution. This substantial influence of substituents on the reaction rate is undoubtedly an electronic rather than a steric effect. 

As discussed (vide supra) disproportionation and isomerization are often competitive reactions. The reaction rates of both types depend on the temperature and the catalyst used for the reaction. Chromium(III) oxide on support, or without, favors the disproportionation of 1,1,2-trichloro-1.2,2-trifluoroethane to give l,l-dichloro-l,2.2,2-tetrafluoroethane and 1,1,1.2-tetra-chloro-2,2-difluoroethane whereas with aluminum trifluoridc the isomerization is favored.24 The higher the chlorine content of the molecules the greater is their reactivity. 

Raney nickel catalysts, unpromoted or doped with molybdenum or chromium, were prepared from the precursor alloys of the type Ni A13. The structure and phase composition of the catalysts have been deternfmetl. Hydrogenation of valeronitri le at 90°C and 1.6 MPa in cyclohexane was performed to evaluate catalyst activities and the relative amounts of amines formed. Doping catalysts by chromium improved reaction rates and yields of primary amine, whereas molybdenum addition was ineffective. 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

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. 

The reaction products of the Cr203-B203-K/NaN0s-H20 system are the chromium l rates 2Cr203 3B2O3 XH2O (x = 13 and 18) (145). A basic borate Cr(0H)(B02)2 H20 has been detected by high-frequency conductometry (50). 

Kinetics Studies. The sol-gel kinetics experiments were performed on a meth-anolic solution of 1.12 M dimer and 1.57 X 10" M chromium acetylacetonate [Cr(acac)3], a spin relaxation agent. Previous studies (12, 13) showed that Cr(acac)3 concentration does not affect the product distribution or reaction rate of TMOS-derived sol-gel solutions. The solutions were acid catalyzed (1.64 X 10" M HCl), and various amounts of water were added. To compensate for the exothermicity caused by dimer hydrolysis when water is added to the dimer-methanol solution, the alcoholic silicate solution was chilled in a thermostatically controlled bath prior to the addition of water. By adjusting the temperature to the appropriate level, the desired reaction temperature (25 1 °C) could be achieved within 60 s of mixing. At this time, the sample was removed from the thermostatically controlled bath and inserted into the spectrometer probe. 

Catalysis by metal ions is reported for molybdenum and zirconium but not for a number of other cases. With chromium(III), where complexing is well established, a marked decrease in reaction rate occurs . 

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