Competitive reactions relative rates estimation

Much is known about the lifetimes of carbocation intermediates of solvolysis, and these data have proven critical in the design of experiments to estimate absolute rate constants for reorganization of ion pairs. Consider reorganization of an ion-pair reaction intermediate that exchanges the positions of the nucleophilic atoms of the leaving group (, Scheme 9) and that occurs in competition with diffusional separation to free ions (k-d) which is much faster than addition of solvent to the ion pair. Ion-pair separation is irreversible and will result in formation of solvolysis reaction products s ). Reorganization of the ion pair will result in formation of isomerization reaction product and the yield of this reaction product will provide a measure of the relative rate constant... 

The free radical arylation of thiazole (391) has been performed either by the Gomberg-Bachmann (392) decomposition of aryldiazonium chlorides (119,393), by the thermal decomposition of benzoyl peroxide (394-397) or N-nitrosoacetanilide (398), or by the photolysis of benzoyl peroxide or iodobenzene (398). The three monophenylthiazoles are obtained in the practically constant proportions 2-phenyl, 60% 5-phenyl, 30% 4-phenyl, 10%, giving the order, 2>5>4, of decreasing reactivity of the three positions of thiazole toward phenyl radicals (398). Competition reactions with nitrobenzene (397) gave an estimation of the globed reactivity of thiazole relative to benzene of 0.75 with the partial rate factors f2 = 2.2, /s=1.9, /4 = 0.5. When the thermolysis of benzoyl peroxide is performed in acetic acid solution, the substrate in reaction is the conjugate acid of thiazole the global reactivity is enhanced to 1.25,... 

The activation energy for dissociation has been calculated to be 13.0 kcal mol" [126] thus at 277 K the rate of reaction (55) is expected to be on the order 10 s" (based on an A factor of 2 x 10 s" ). This estimate is in accord with the measured reaction rate of 1.5 x 10 s in 150torr of N2 [92]. Wallington et al. [99] showed that dissociation and reaction with O2 are competitive under atmospheric conditions by measuring a relative rate... 

To measure the relative rates of various ligands to replace CO, chemists must turn to competition experiments (see Section 8.8.3). Competition experiments using the coordi-nately unsaturated 16-electron species Ni(CO)a, Fe(CO)4, and Mo(CO)s, and alkenes, phosphines, and amines as ligands show only small differences in product ratios (between 1 and 10). Because these three 16-electron coordinately unsaturated species are not very selective in their reactions, they must all be very reactive. They react with almost any nucleophile with a very low energy of activation. In support of this view, the rate constants for the addition of the new ligands to such unsaturated systems have been estimated to be near 10 M s that is, they are approaching diffusion control. 

The existence and reactivity of the free-radical intermediate for the case R = CCI3 were examined by kinetic competition with 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy (4-htmpo). Numerical integration techniques yielded an estimate of the relative rate constants for the reaction of CCl3 with 4-htmpo (kHTMPo) vs. kco in benzene, kHXMPo/ o = 1.8 0.1 (25°C). 

In an extremely clever study, Tanner and collaborators have estimated rate constants for fragmentation of a series of a-haloacetophenones41. These rate constants were determined by the intramolecular competition outlined in Scheme 9. Radical ion 4 was generated by reaction with l,3-dimethyl-2-phenylbenzimidazoline (DMBI). The rate constant ratio (k llc2) was determined by the relative yields of the two products 5 and 6. With the assumption that the a-substituent does not affect the magnitude of k2, was assumed to be equal to 3 x 107 s 1 (which had been measured earlier by Wipf and Wightman for the dehalogenation of p-bromoacetophenone radical anion)42. 

If competition for the electrons occurs solely between Reactions 19 and 20, kS2/kSl can be deduced from the decrease in the yield of P upon adding S2. Sherman (66) has used this approach to estimate the rate constants relative to N20 for many solutes from a study of the nitrogen yield decrease. 

The olefin distribution varied slightly with reaction conditions but the results show the competitive nature of syn- relative to a/i/i-elimination in these systems. Only in the cyclohexyl system is a/iri-elimination clearly predominant. Isotope-labelling studies obviously afford a more accurate method of estimating stereochemistry of elimination reactions than do rate profile approaches. Subsequent work questioned the validity of using dimethyl-substituted compounds as models for the parent alicyclic substrates as the substituents can influence the stereochemical outcome of the reaction, viz. 

Nearly all the mass-spectrometric studies of these rearrangement reactions have yielded relative abundances of the ionic products formed in Penning ionization, associative ionization, and rearrangement ioniza-tion. " For those cases in which the Penning ionization cross sections are known or could be estimated, specific reaction rates have been calculated and these are depicted also in Table VI. Comparison of Tables V and VI shows that when rearrangement ionization and heteronuclear associative ionization are in competition, the former occurs about an order of magnitude faster. 

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