Oxidation rate determining step

Some early observations on the catalytic oxidation of SO2 to SO3 on platinized asbestos catalysts led to the following observations (1) the rate was proportional to the SO2 pressure and was inversely proportional to the SO3 pressure (2) the apparent activation energy was 30 kcal/mol (3) the heats of adsorption for SO2, SO3, and O2 were 20, 25, and 30 kcal/mol, respectively. By using appropriate Langmuir equations, show that a possible explanation of the rate data is that there are two kinds of surfaces present, 5 and S2, and that the rate-determining step is... 

The effect of conformation on reactivity is intimately associated with the details of the mechanism of a reaction. The examples of Scheme 3.2 illustrate some of the w s in which substituent orientation can affect reactivity. It has been shown that oxidation of cis-A-t-butylcyclohexanol is faster than oxidation of the trans isomer, but the rates of acetylation are in the opposite order. Let us consider the acetylation first. The rate of the reaction will depend on the fiee energy of activation for the rate-determining step. For acetylation, this step involves nucleophilic attack by the hydroxyl group on the acetic anhydride carbonyl... 

The most widely accepted mechanism of reaction is shown in the catalytic cycle (Scheme 1.4.3). The overall reaction can be broken down into three elementary steps the oxidation step (Step A), the first C-O bond forming step (Step B), and the second C-O bond forming step (Step C). Step A is the rate-determining step kinetic studies show that the reaction is first order in both catalyst and oxidant, and zero order in olefin. The rate of reaction is directly affected by choice of oxidant, catalyst loadings, and the presence of additives such as A -oxides. Under certain conditions, A -oxides have been shown to increase the rate of reaction by acting as phase transfer catalysts. ... 

At medium and high temperatures copper ultimately follows the parabolic law " . It has been shown " using radioactive tracers that the diffusion of copper ions in cuprous oxide is the rate-determining step at 8(X)-1 000°C, and there is considerable evidence in favour of the view that metal moves outwards through the film by means of vacant sites in the oxide lattice . 

Thus, if triose reductone is, in fact, the first intermediate in the periodate oxidation of malonaldehyde, the total consumption of periodate per mole of malonaldehyde should be four molar equivalents two moles of formic acid and one mole of carbon dioxide should be formed, in accordance with the sequence proposed by Fleury and his collaborators (22). As in the case of the periodate oxidation of malonic acid (32) the rate determining step should be the hydroxylation step. 

Such a complex, cw-Rh(CO)2I2, is the active species in the Monsanto process for low-pressure carbonylation of methanol to ethanoic acid. The reaction is first order in iodomethane and in the rhodium catalyst the rate-determining step is oxidative addition between these followed by... 

Several authors suggested mechanisms for esterifications catalyzed by titanium tetraalk-oxides. Bolotina et al.16,221,2221 who studied the polyesterification of 2-ethylhexyl phtha-late with 2-ethylhexanol found the same reaction order with respect to catalyst, acid and alcohol, namely 1 they suggested the following rate-determining step ... 

Nitric Add by the Oxidation of Ammonia. Here, the catalytic oxidation of ammonia under press using a Pt catalyst maintained at a temp of 900—1000° is the process used. The reaction press is the rate determining step, being directly proportional to the product nitric acid concn (Refs 6, 22, 26, 30, 34, 36, 37 41). 

The rate-determining step was, therefore considered to be reaction of bromine with peroxyacetic acid to give a species (suggested as bromine acetate)which subsequently and rapidly, brominates. Formation of bromine acetate was believed to take place according to the reaction scheme represented by equilibrium (158) (which is analogous to the mercuric oxide oxidation of bromine) followed by either equilibrium (159), (160) or (161), viz. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

Kinetic studies of the oxidation of sulphoxides to sulphones by chromium(VI) species have been carried out131-133. The reaction has been found to be first order with respect to the chromium(VI) species and the sulphoxide and second order with respect to acid. At high sulphoxide concentrations the order with respect to sulphoxide is two. The proposed mechanism involves an electron transfer from the sulphoxide to the active chromium(VI) species (HCr03+ in strong acidic media) in the rate-determining step producing a sulphoxide radical cation which further reacts to give the sulphone. 

The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

The rate-determining step in the homo-coupling reaction of aryl halides could be the oxidative step or the reduction of Ni(II) to Ni(I) step. 

MeCDOHMe, showing that the a hydrogen is removed in the rate-determining step. Note that, as in 14-6, the substrate is oxidized by three different oxidation states of chromium. 

Fig. 8. The Fe protein cycle of molybdenum nitrogenase. This cycle describes the transfer of one electron from the Fe protein (F) to one afi half of the MoFe protein (M) with the accompEmying hydrolysis of 2MgATP to 2MgADP + 2Pf. The rate-determining step is the dissociation of F (MgADP)2 from M,rf. Subscript red = reduced and ox = oxidized.

The partial oxidation of propylene occurs via a similar mechanism, although the surface structure of the bismuth-molybdenum oxide is much more complicated than in Fig. 9.17. As Fig. 9.18 shows, crystallographically different oxygen atoms play different roles. Bridging O atoms between Bi and Mo are believed to be responsible for C-H activation and H abstraction from the methyl group, after which the propylene adsorbs in the form of an allyl group (H2C=CH-CH2). This is most likely the rate-determining step of the mechanism. Terminal O atoms bound to Mo are considered to be those that insert in the hydrocarbon. Sites located on bismuth activate and dissociate the O2 which fills the vacancies left in the coordination of molybdenum after acrolein desorption. 

Consequently, the activated complex (X ) of the rate-determining step is composed of one Ce atom and one Cr atom, the average oxidation state of each atom being -t-4 [X = 2 Ce(IV)4-Cr(III)—Ce(III)]. A sequence of one-equivalent steps are in accord with the rate law, viz. 

In the case of Tl(III) the overall rate coefficient has been resolved into a product kK for the two steps The large positive AS is due almost entirely to the initial association, which was also studied spectroscopically. An alternative rate determining step in the Pd(II) oxidation is hydride ion transfer to Pd(II) . 

The rate expression suggests a rate-determining step not involving the oxidant this is very probably enolisation, viz. 

The kinetics of the initial stages of the oxidation of some a-hydroxy-carboxylic such as lactic, malic and mandelic acids by chromic acid have been studied by Bakore and Narain . The initial reaction resembles the oxidation of a secondary alcohol to ketone. The authors concluded that the rate determining step involves C-H bond rupture at the a-carbon atom. The rate of oxidation of these acids is reduced to one-half by the addition of manganous ions, when the concentration of the latter is commensurable with that of the acids. 

The rate-determining step (rds) of the reaction on platinum is the oxidation of adsorbed CO with adsorbed hydroxyl species [step (26)]. The current density of the methanol electrooxidation can be obtained from the following equatiorf ... 

Results discussed above show in several lines a distinct biomimetic-type activity of iron complexes stabilized in the ZSM-S matrix. The most important feature is their unique ability to coordinate a very reactive a-oxygen form which is similar to the active oxygen species of MMO. At room temperature a-oxygen provides various oxidation reactions including selective hydroxylation of methane to methanol. Like in biological oxidation, the rate determining step of this reaction involves the cleavage of C-H bond. 

---