Catalysis cobalt-bromide

Generalizing the known data and established experimental peculiarities of the action of the cobalt bromide catalyst, we have to emphasize the following advantages of cobalt bromide catalysis ... 

Competition between Homolytic and Heterolytic Catalytic Decompositions of Hydroperoxides Reactions of Transition Metals with Free Radicals Reactions of Transition Metal Ions with Dioxygen Catalytic Oxidation of Ketones Cobalt Bromide Catalysis Oscillating Oxidation Reactions... 

In this paper we hope to clarify the features of cobalt bromide catalysis using various hydrocarbons and a neutral bromide as sodium bromide at low temperatures and at moderate concentrations of cobalt. 

Effect on the Oxidation of Polymers. In the cobalt bromide catalysis, the steric hindrance to the intramolecular hydrogen abstraction in the autoxidation of polymers is expected to be reduced remarkably. 

The oxidation of alkylaromatic hydrocarbons proceeds particularly easily in the presence of both cobalt and bromide ions (a so-called cobalt-bromide catalysis ). Carboxylic acids are the final products of the reaction. For example, terephthalic acid is selectively formed from p-xylene, the whole process being used in the industrial production of the acid [Ik, 19]. Despite the large number of works on cobalt-bromide catalysis, its mechanism has long remained speculative. 

COBALT BROMIDE CATALYSIS OF THE OXIDATION OF ORGANIC COMPOUNDS... 

Metal bromide catalysis is widely used in industry for oxidation of alkylaromatic compounds to their corresponding acids. The mechanism for this catalytic process has been investigated for over 20 years but it is still not established despite numerous papers on this subject This work presents results of our investigations on the kinetics and mechanisms of cobalt bromide catalysis. 

We have found that cobalt bromide catalysis is based on a reaction involving peroxy radical and Co(II) leading to hydroperoxide formation and Co(UI) in a rate determining step (eq. 1) which is then followed by a rapid catalytic reduction of the generated Co(III) by bromide ions in the presence of hydrocarbon (eq. 2-4). The overall scheme involves two catalytic cycles and is given by equations (l)-(4) below. 

Preparation by oxidising tris(phenanthroline)cobalt(III) tetrafluoroborate with nitric acid in sulfuric acid with potassium bromide catalysis is potentially explosive. See Nitric acid... 

Cobalt bromide is used as a catalyst in the technology of production of arylcarboxylic acids by the oxidation of methylaromatic hydrocarbons (toluene, p-xylene, o-xylene, polymethyl-benzenes). A cobalt bromide catalyst is a mixture of cobaltous and bromide salts in the presence of which hydrocarbons are oxidized with dioxygen. Acetic acid or a mixture of carboxylic acids serves as the solvent. The catalyst was discovered as early as in the 1950s, and the mechanism of catalysis was studied by many researchers [195-214],... 

When the cobalt salt of carboxylic acid and bromide ion are dissolved in acetic acid, a cobalt bromide complex is formed instantaneously. For cobalt dibromide a pronounced induction period was observed, but adding sodium acetate eliminates entirely the induction period, suggesting that cobalt monobromide is responsible for the catalysis. 

From a mechanistic point of view, the MC catalyst, that is, Co-Mn-Br, is essentially a cobalt-bromide catalyst (Co-Br) promoted by Mn ions. The reason for this definition is that the Co-Br catalyst exhibits all properties of the Co-Mn-Br catalyst, while the Mn-Br catalyst is much less active and has significant mechanistic differences from the Co-Br catalyst. For this reason, we first discuss the nature of the Co-Br catalysis mechanism and then the mechanism of the Co-Mn-Br system. 

Cobalt-Manganese-Bromide Catalysis (MC Oxidation) The Nature of Synergy between Co and Mn... 

Reactions between R02 and Go(II) or Mn(II) are discussed in terms of the methods of rate constant determination, the values of the rate constants and the effect of media on rate constants. A synergistic effect of Mn(II) addition is also discussed. The effect of the media is discussed in terms of the coordination ability of the solvent with Go(II) ion as well as the proton donor property of the solvent. It is concluded that the presence of a proton donor in the coordination sphere of Co(n) is required for hydroperoxide formation but at the same time this prevents the peroxy radical from reacting with the cobalt(II) ion. As a result, a bell-shaped dependence of the rate constant on the acetic acid and water concentration is observed. Our data explain some of the controversial literature data on solvent effects for the kinetics of metal bromide catalysis. 

A method was proposed for the preparation of p-hydroxybenzoic acid by oxidation of p-cresol with atmospheric oxygen in an acetic acid-acetic anhydride mixture under catalysis of cobalt acetate, manganese(II) acetate, and sodium bromide (Litvintsev et al. 1994). This procedure ensures 60% yield of p-acetoxybenzoic acid and 100% conversion of the initial p-cresol. 

The formation of arylzinc reagents can also be accomplished by using electrochemical methods. With a sacrificial zinc anode and in the presence of nickel 2,2-bipyridyl, polyfunctional zinc reagents of type 36 can be prepared in excellent yields (Scheme 14) . An electrochemical conversion of aryl halides to arylzinc compounds can also be achieved by a cobalt catalysis in DMF/pyridine mixture . The mechanism of this reaction has been carefully studied . This method can also be applied to heterocyclic compounds such as 2- or 3-chloropyridine and 2- or 3-bromothiophenes . Zinc can also be elec-trochemically activated and a mixture of zinc metal and small amounts of zinc formed by electroreduction of zinc halides are very reactive toward a-bromoesters and allylic or benzylic bromides . ... 

Although cobalt and bromide ion catalysis have been studied by Ravens (11) and recently by Hay and Blanchard (4), there are many important aspects yet to be elucidated. 

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