Dissociative mechanism competition studies

Cleavage in aqueous solution in the presence of other anions, Y-, has been studied for Y- = N03-, SCN-, Cl-, and CH3S03 (355-358). In all these reactions variable amount of Co(NH3)5Y2+ are produced together with the aqua complex, and extensive competition studies have been reported for the uncatalyzed as well as for the acid-catalyzed cleavage reaction. The results of these competition experiments are in keeping with an essentially dissociative mechanism for bridge cleavage. 

Some years ago it was claimed that the [Fe(fz)3] anion reacted with oxalate by a bimolecular mechanism. Nucleophilic attack of oxalate at metal or ligand here seemed equally unlikely, and a reinvestigation of this reaction, with a parallel study of reactions of this complex with edta and related ligands, indicates a dissociative mechanism, with ligand competition causing the observed kinetic pattern. ... 

The relative reactivities obtained by the competitive studies are the relative values of /Cj of the alkynes. The fact that these alkynes exhibit different values, and yet have almost identical fcj values, further supports the dissociation mechanism. 

HO-initiated oxidation of the alkanes become complex with increase in carbon number. Namely, a large variety of alkyl radicals can be produced by the H-atom abstraction from the primary, secondary and tertiary C—H bonds in the parent alkane [88]. The resulting ROO ( C4) radicals have been shown by Atkinson et al. to yield R0N02 as well as RO + N02 upon reaction with NO [100-102]. A major complication in the alkane oxidation mechanism arises from the variety of competitive reaction channels that RO radicals can undergo, e.g., 02-reaction, unimolecular dissociation and internal isomerization. There have been a number of experimental and theoretical studies of these reactions [31,88]. 

For addition of Cl atoms, the dissociation energy of the radicals AX has been estimated at about 20-22 kcal./mole. At room temperature k ifki(A) should be well below 10-3 and mechanism (C) should be obeyed and has indeed been frequently observed. At higher temperatures (about 225°C.) k 2/k2(A) 10-3 and a change to mechanism (B) should occur. This has been confirmed experimentally by Adam et al. (1) in a study of the photochlorination of tetrachlorethylene. They observed a maximum in the rate and a change in mechanism at about 180°C., as a result of the increased importance of the radical decomposition reaction (—2). From their data they were able to deduce the Arrhenius parameters for this reaction. In extensions of this work Goldfinger and his collaborators have carried out competitive experiments with a number of hydrocarbons and chlorinated hydrocarbons. 

It is interesting also to compare the results of the present experiment, which shows directly that a competitive mechanism occurs in the co-adsorption of NO and CO, with previous studies on several surfaces of the platinum group metals. On Pt(lll) and Pt(110), Lambert and Comrie (65) have inferred from thermal desorption data that gaseous CO displaces molecular NO from the surface and causes also a conversion between two thermal desorption states of molecular NO. Similarly, Campbell and White (55) report that adsorbed CO inhibits the oxidation of CO by NO at low temperature on polycrystalline Rh. They attribute this to the occupation of sites by CO which are required for NO adsorption and dissociation. Conrad et al. (66) have used UV-photoelectron spectroscopy to observe directly the displacement of molecular NO by gaseous CO from Pd(110) and polycrystalline Pd surfaces. Thus, it appears that adsorption of molecular CO and NO is competitive on these... 

The best studied cases where interchange pathways may occur are those in which there is competition between I (actually an Id, dissociative interchange) and D mechanisms (Scheme 7.1)... 

---