Rate constants azide radical reactions

The reaction of azide ions with carbocations is the basis of the azide clock method for estimating carbocation lifetimes in hydroxylic solvents (lifetime = 1 lkiy where lq, is the first-order rate constant for attack of water on the carbocation) this is analogous to the radical clock technique discussed in Chapter 10. In the present case, a rate-product correlation is assumed for the very rapid competing product-forming steps of SN1 reactions (Scheme 2.24). Because the slow step of an SN1 reaction is formation of a carbocation, typical kinetic data do not provide information about this step. Furthermore, the rate constant for the reaction of azide ion with a carbocation (kaz) is assumed to be diffusion controlled (ca. 5 x 109 M 1 s 1). The rate constant for attack by water can then be obtained from the mole ratio of azide product/solvolysis product, and the molar concentrations of azide (Equation 2.18, equivalent to Equation 2.14) [48]. The reliability of the estimated lifetimes was later... 

Shoute, L. C. T., Z. B. Alfassi, P. Neta, and R. E. Huie, Temperature Dependence of the Rate Constants for Reaction of Dihalide and Azide Radicals with Inorganic Reductants, J. Phys. Chem., 95, 3238-3242 (1991). 

Rate constants (k X 10 9 M-1sec-1) were determined to be 7.0, 3.5, and 1.0 for the enumerated substrates, respectively. The change in kinetics for the three cation radicals with increasing steric hindrance at the (3-carbon is in accordance with the depicted addition reaction. In contrast with that, a reaction of the azide ion with these three cation radicals in acetonitrile proceeds with rate constants that are the same in all three cases ( 3 X 109 M-1sec-1). In acetonitrile, the reaction consists of one-electron transfer from the azide ion to a cation radical. As a result, a neutral styrene and the azidyl radical are formed. The azidyl radical reacts with the excess azide ion, and the addition reaction does not take place ... 

Table X. A Selectioii of Rate Constants for Reactions of the Azide Radical...

The piT values for the semiquinones from 2-methyl- and 2,3-dimethyl-5,6-dihydroxyindole were determined to be 6.6 and 6.3, respectively (94MR343). The rate constants for reaction of azide radicals with all these 5,6-dihydroxyindoles lie in the range 4-8 x 10 M s i.e. approaching diffusion controlled rates. 

The results for reaction of azide with several 4-methoxysty-rene radical cations provide an interesting example of the effect of solvent on the competition between electron transfer and addition. In TFE the 4-methoxystyrene, P-methyl-4-methoxystyrene, and p,P-dimethyI-4-methoxystyrene radical cations react with azide with rate constants of 7.0, 3.5, and 1.0 X 10 respectively. In all three cases the transient... 

Mn(2,2 -bipyridine)2] . The oxidation of HN3 via 2 HN3 2 Mn(bipy) - 2 Mn(bipy>2 + 3 N2 2 H involves free radicals and probably two different Mn-azide complexes. Pseudo-first-order rate constants for Mn(bipy) and excess HN3 at different [H ] are listed. A kinetic model of the reaction and the calculated parameters of activation are given [57]. 

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