Oxygen exchange mechanisms

In the first step, a water molecule dissociates at a carbon free active site to form H2 and an oxidized surface complex. In the second step, the carbon-oxygen complex produces carbon monoxide and a new free active site. Based on this oxygen exchange mechanism, the reaction rate is obtained ... 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

Kinetic studies on racemization, oxygen exchange reaction using H2lsO, and theoretical studies clarified the mechanism for racemization of optically active chalcogenic acids.34,37 The mechanism of racemization of tellurinic acids is different from that of the corresponding seleninic acids. In fact, the mechanism for racemization of optically active tellurinic acids was found to involve an... 

Scheme 22 summarizes the mechanism proposed. As in the case of oxygen exchange between MeRe03 and H20, the proposed intermediate was not detected. Examples of analogous intermediates that carry out the hydrolysis of imines and the aminolysis of ketones are well documented (64-66). 

The oxygen exchange processes on the aqua oxo and hydroxo oxo species of these complexes proceed by different intimate mechanisms as described below. 

The behavior of chiral phenyl /-butyl sulfoxide 219 and a-phenyl-ethyl phenyl sulfoxide 220 is completely different in strongly acidic media and in the presence of halide ions. Two reactions were found (266) to occur in parallel. One results in the loss of optical activity, and the second leads to the decomposition of the sulfoxide. It was observed that the racemization process is not accompanied by [ 0] oxygen exchange. In the case of sulfoxide 220 the complete loss of optical activity at chiral sulfur is accompanied by partial racemization at the chiral carbon center. These results are consistent with a sulfenic acid-ion-pair mechanism formulated by Modena and co-workers (266) as follows (it is obvious that the formation of achiral sulfenic acid is responsible for racemization). 

The intermediate has a finite lifetime, but it is not free the less stable secondary carbonium ion is stabilized by specific interaction with two molecules of water. The same kinetic study on primary alcohols made by Dostrovsky and Klein (79) shows that oxygen exchange in dilute acid solution does not proceed by way of an ion, but by a concerted mechanism. For the same reason the elimination reaction has to be of a concerted nature and cannot proceed via an unsolvated carbonium ion. 

VioOfg- t,/2 = 7-15 h (25 °C), pH-dependent Near equivalency of all oxygens mandates reversible, extensive breakage as exchange mechanism 48-50(a)... 

Detailed studies of the kinetics and mechanisms of oxygen isotope exchange between minerals and fluids show that there are three possible exchange mechanisms (Matthews et al. 1983b, c Giletti 1985). 

N-labelling has been used to aid the investigation of the mechanisms of isomerization no oxygen exchange between the NO and N02 ligands was observed. " ... 

Carbonyl oxygen exchange was found during the cupric ion-catalyzed hydrolysis of DL-phenylalanine ethyl ester-carbonyl-O18 at pH 7.3 (11). This indicates that an additional intermediate is formed in this reaction. A mechanism (II) consistent with both the kinetic evidence and the oxygen-exchange evidence is given below. 

A determination of the pH dependence of the lanthanum hydroxide gel-promoted hydrolysis of /3-glyceryl phosphate revealed that the two maxima exist in the pH-rate profile, one at pH 8.6 which presumably involves the species La (OH)+2 and another (smaller) maximum at pH 10.4 which involves the species La (OH) 2+ (4). Presumably the same kind of catalytic mechanism is operative in both cases. These reactions may serve as models for the metal ion-promoted alkaline phosphatases which have been shown to proceed with P—O cleavage (and with no oxygen exchange). 

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