Rate constants for reaction of azide

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. 

Figure 3.14 Pseudo-first order rate constants for reactions of azides and nitrile oxides with alkynes to give the cycloadducts illustratecP ...

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). 

Choride ion is considerably less reactive than the azide ion. Thus, although values of kc 1/ kn2o have been quite widely available from mass law effects of chloride ion on the solvolysis of aralkyl halides, normally the reaction of the chloride ion cannot be assumed to be diffusion controlled and the value of kn2o cannot be inferred, except for relatively unstable carbocations (p. 72). Mayr and coworkers251 have measured rate constants for reaction of chloride ion with benzhydryl cations in 80% aqueous acetonitrile and their values of logk are plotted together with a value for the trityl cation19 in Fig. 7. There is some scatter in the points, possibly because of some steric hindrance to reaction of the trityl cations. However, it can be seen that chloride ion is more... 

A mechanism that explains some of the more important observations in the acid-catalyzed hydrolysis of epoxides 49a-d is outlined in Scheme 15. The cis/trans diol product ratios from the acid-catalyzed hydrolysis of 49a-c, which have either hydrogen- or electron-donating groups in the para position of the phenyl ring, are 74 26, 83 17 and 65 35, respectively. An intermediate carbocation 52a is trapped by azide ion in the acid-catalyzed hydrolysis of 49a and the rate constant for reaction of 52a with water in 10 90 dioxane-water solvent is estimated, by the azide clock technique, to be 1.7 x 108 s 1. Azide ion also traps an intermediate 52b in the acid-catalyzed hydrolysis of 49b, but somewhat less efficiently. The rate constant ks for reaction of 52b with solvent is estimated to be 2 x 109 s-1. The somewhat greater reactivity of 52b compared to that of 52a is consistent with the observation that... 

For benzo[a]pyrene benzylic carbocations (82 and 83)88,89 and 4-methoxyphenyl-stabilized carbocations (52a, 54b, 66b, 68b and 127),59,63,70,108 values of kaz/ks are in the range 25-600 M-1. It is reasonable to assume that these carbocations also react with azide ion at the diffusional limit of 1010M-1s-1. The rate constants for reaction of these carbocations with water (ks) are estimated to be (8 x 106) to (2 x 108)s, which correspond to energies of activation of 6.5-8.0kcalmol-1. Benzo[a]pyrene 7,8-diol 9,10-epoxides (80 and 81) and tetrahydronaphthalene epoxide each has two optimized conformations similar to structures 132 and 134, which are shown in Scheme 42. The carbocation formed from reaction of each of these... 

Figure 2. Estimated rate constants for reactions of nucleophiles with substituted 1-phenylethyl carbocations, plotted against the effective Hammett constant of the ring substituent with r+ = 2.1 (+) trifluoroethanol (A) methanol acetate anion ( ) trifluoroethoxide anion propanethiol (o) azide. (Reproduced from reference 17.

The data in Tables I and II, together with extensive additional evidence, allow several generalizations to be made about micellar effects upon bi-molecular reactions (5). First, overall rate constants follow the distribution of both reactants between water and micelles. Second, second-order rate constants for reactions of nonionic nucleophiles are lower in micelles than in water. Third, second-order rate constants for reactions of anionic nucleophiles are similar in water and micelles except for some reactions of azide ion (37). 

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

Just as nucleophiles can interact differently with protic and dipolar aprotic solvents, so can leaving groups, and solvation energies can affect the energies of transition states. The data in Table 8.16 show considerable variation in the experimental values for the relative rate constants for reaction of methyl derivatives with a series of nucleophiles, Y . For example, methyl tosylate reacts with azide ion nearly ten times faster than methyl iodide in... 

The value of = 1 X 10 s for the first-order rate constant for collapse of an ion pair between Me-4 and pentaflourobenzoate ion is larger than the second-order rate constant rcoo = 5x10 M s reported for the bimolecular addition of alkane carboxylates to Me-4. This second-order rate constant is limited by the rate constant for formation of an ion pair between Me-4 and a carboxylate ion. The larger barrier to encounter-limited reactions of carboxylate ions compared with the diffusion-limited reactions of anions such as azide ion, = 5 X 10 represents the barrier to desolvation of nucleophile that must precede formation of an ion pair between Me-4 and a carboxylate ion (Scheme 13). ... 

The rate constant for solvolysis of the model tertiary substrate 5-Cl is independent of the concentration of added azide ion, and the reaction gives only a low yield of the azide ion adduct (e.g., 16% in the presence of 0.50 M NaNa in 50 50 (v/v) water/trifluoroethanol]." Therefore, this is a borderline reaction for which it is not possible to determine the kinetic order with respect to azide ion, because of uncertainties about specific salt effects on the reaction." ... 

X 10 M s. Direct measurements of azide trapping reactions have generally validated this assumption (see Chapter 2). Except for highly stabilized nitrenium ions, feaz. is seen to fall in a narrow range of 4—10 x 10 M s. of azide adducts relative to the competing products can be used to estimate the rate constants for formation of the latter. 

Richard has also shown that intrinsic barriers for carbocation reactions depend not only on the extent of charge delocalization but to what atoms the charge is delocalized. In a case where values of pifR for calculation of A were not available, comparisons of rate constants for attack of water kH2o with equilibrium constants for nucleophilic reaction with azide ion pKAz for 65-67 showed qualitatively that delocalization to an oxygen atom leads to a lower barrier than to an azido group which is in turn lower than to a methoxyphenyl substituent.226... 

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... 

Figure 7. The collection of cyclooctyne reagents and relative second-order rate constants for reaction with benzyl azide.

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... 

Table 2 Reaction rate constants for cycloadditions of aliphatic (entries 1-3) and aromatic (entries 4—6) azides with BCN and DIBAC in different solvent systems...

The Ir(III,IV,IV) complex [Ir3N(S04)6(0H2)3]4 undergoes ligand substitution of the three H20 ligands with azide, Cl, Br, and NCS-. The three H20 ligands, one on each metal, are all equivalent towards substitution.103 The rate constant for the reaction with azide is 17.3 x 10-4 M 1s 1. 

The determinations of absolute rate constants with values up to ks = 1010 s-1 for the reaction of carbocations with water and other nucleophilic solvents using either the direct method of laser flash photolysis1 or the indirect azide ion clock method.8 These values of ks (s ) have been combined with rate constants for carbocation formation in the microscopic reverse direction to give values of KR (m) for the equilibrium addition of water to a wide range of benzylic carbocations.9 13... 

Only low yields of the azide ion adduct are obtained from the reaction of simple tertiary derivatives in the presence of azide ion 2145 46 and it is not possible to rigorously determine the kinetic order of the reaction of azide ion, owing to uncertainties in the magnitude of specific salt effects on the rate constants for the solvolysis and elimination reactions. Therefore, these experiments do not distinguish between stepwise and concerted mechanisms for substitution reactions at tertiary carbon. 

Absolute values of the rate constants ks (s ) and kp (s ). In most cases these rate constants were determined from the values of kaz/ks (M 1) or kaz/kp (M-1) for partitioning of the carbocation between reaction with azide ion and solvent, by using the diffusion-limited reaction of azide ion, kaz = 5 x 109m 1s, as a clock for the slower reactions of solvent.7 8 13 32 82... 

Table I. Rate Constants/ k (s 1), for the Reaction of Azide with BrN3 Formed by Mixing Br2 (0.043 M) and NaN3at 25 °C.

The values, ranging from 3 to 6 M are seven orders of magnitude smaller than the selectivity observed for activation limited reactions with stable cations, and can be used as evidence that the reaction with azide ion is a diffusion controlled process. Choosing a value for the rate constant for a diffusion-limited... 

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