Jencks clock

The Jencks clock, as well as pulse radiolysis experiments, indicate that oxocarbenium cations often can be quite stabilized, with a half-life on the order of milliseconds. For example, at room temperature, the structures shown here (left to right) have respective lifetimes of... 

BIFURCATION THEORY JENCKS CLOCK JENKINS MECHANISM "JMP,"... 

The Jencks clock was developed into a technique whereby the lifetimes of a whole range of oxocarbenium and carbenium ions of moderate stability could measured. Azide ion replaced thiosulfate, as the products were non-ionic and more stable, although the diifusional rate constant for anion-cation recombination of 5 X 10 s in water at 25 °C still applied, and was later confirmed... 

The use of the Jencks clock has enabled a number of oxocarbenium ion and related systems to be analysed in detail. It is clear that the intuitive expectation that the more stable the oxocarbenium ion, the slower it reacts with water and the faster it is formed, are complicated by two further phenomena, geminal interactions in the ground state, and imbalance between a bond cleavage and development of conjugation. 

Addition of bromine in methanol gives exclusive trans addition (to 2-deoxy-2-bromomethyl glycosides) in approximately equal amounts, except for tribenzylgalactal, which, as expected from the steric restrictions imposed by 04, gives tribenzyl-2-bromo-2-deoxymethyl p-galactoside and tribenzyl-2-bromo-2-deoxymethyl a-L-talopyranoside in a 2.3 1 ratio. Only in the case of the bromomethoxylation of triacetylglucal was it possible to intercept the oxocarbenium ion with azide ion. The Jencks clock, using a value of 7 x 10 s ... 

Why the lifetime of the acetyl-substituted species should be longer than that of the benzyl-substituted species, so that it alone can be timed by the Jencks clock, is not clear. 

In these circumstances, where routine kinetic measurements are uninformative and direct measurements of the product-forming steps difficult, comparative methods, involving competition between a calibrated and a non-calibrated reaction, come into their own. Experimentally, ratios of products from reaction cascades involving a key competition between a first-order and a second-order processes are measured as a function of trapping agent concentration. Relative rates are converted to absolute rates from the rate of the known reaction. The principle is much the same as the Jencks clock for carbenium ion lifetimes (see Section 3.2.1). However, in radical chemistry Newcomb prefers to restrict the term clock to a calibrated unimolecular reaction of a radical, but such restriction obscures the parallel with the Jencks clock, where the calibrated reaction is a bimolecular diffusional combination with and the unknown reaction a pseudounimolecular reaction of carbenium ion with solvent. Whatever the terminology, the practical usefulness of the method stems from the possibility of applying the same absolute rate data to all reactions of the same chemical type, as discussed in Section 7.1. 

Recognizing this, Richard and Jencks, proposed using azide ion as a clock for obtaining absolute reactivities of less stable cations. The basic assumption is that azide ion is reacting at the diffusion limit with the cation. Taking 5 x 10 M s as the second-order rate constant for this reaction, measurement of the selectivity fcaz Nu for the competition between azide ion and a second nucleophile then provides the absolute rate constant since feaz is known. The clock approach has now been applied to a number of cations, with measurements of selectivities by both competition kinetics and common ion inhibition. Other nucleophiles have been employed as the clock. The laser flash photolysis (LFP) experiments to be discussed later have verified the azide clock assumption. Cations with lifetimes in water less than about 100 ps do react with azide ion with a rate constant in the range 5-10x10 M- s-, " which means that rate constants obtained by a clock method can be viewed with reasonable confidence. 

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... 

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... 

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