Carbocations azide clock

Richard and Jencks combined the above method with use of the azide clock to determine values of pA R for a-phenethyl carbocations bearing electron-donating substituents in the benzene ring and for the cumyl cation for a wider range of substituents.22,89 They inferred values for the parent... 

A further dependence of the selectivity between different nucleophiles on the stability and reactivity of carbocations was found by Richard and Amyes in a study of reactions of alcohols and carboxylate anions with -substituted a-trifluoromethyl benzyl cations (75, X = Me, OMe, SMe, N(Me)CH2CF3, and NMe2) monitored using the azide clock.305 Apart from the methyl-substituted substrate, for which the reactions approached diffusion control,... 

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... 

The rate constants for the solvent-recombination process of the carbocations [3C (X,Y,Z)] were determined by the use of the azide clock method (Richard etal., 1984 Richard and Jencks, 1984a,b,c McClelland et al., 1991) and the rate constant of the forward reaction was derived using (38b) as /Ch = /CwXr+ (McClelland et al., 1989,1991). While ordinary Hammett-type relationships were found to be inapplicable to the substituent effects on solvent recombination, there is a rate-equilibrium correlation for all available data on triarylmethyl cations, shown as the linear log/c , vs. p/Cr<- plot, in Fig. 34 with a slope of 0.64. Such a relationship was earlier suggested by Arnett and Hofelich (1983) and Ritchie (1986). The correlation of ky, with the cr scale was... 

The behaviour of a-arylethyl cations has been analysed by Richard and coworkers (Richard etal., 1984 Richard and Jencks, 1984a,b,c) using the azide clock technique. Combination of the rate constants for the reaction of the carbocations with water and the acid-catalysed cleavage of 1-phenylethanols... 

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... 

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... 

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... 

Values of (s ) for reaction of the more stable tertiary carbocations X-2+ with a solvent of 50 50 (v/v) water/trifluoroethanol, determined using the azide ion clock, were used to establish a good linear logarithmic relationship between k (s ) for reaction of X-2+ and the first-order rate constants fcobsd (s ) for their formation as intermediates in the stepwise reaction of X-2-C1 in the same solvent (Eq. 2 and Scheme 2.5)." ... 

Values of k.v, = 5 X 109 M-1 s 1 have been determined for diffusion-controlled addition of azide ion to a variety of ring-substituted benzhydryl24 and a-substituted 4-methoxybenzyl25 carbocations. This rate constant have seen extensive use as a clock to determine absolute rate constants for addition of a variety of nucleophiles to benzylic carbocations.5 Relative rate constants for addition of azide ion... 

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