Bunnett relationship

Having in mind the various forms of the isokinetic relationship, we can also show its physical meaning in kinetics more clearly. Let us consider a reaction series with a variable substituent, solvent, or other factor. The term reaction series was discussed by Bunnett (14), with the conclusion that the common mechanism of all reactions is a necessary condition (12). However, this condition can seldom be ascertained, and best after finishing the whole analysis. At the beginning, it may be sufficient that the reaction products are invariable and the kinetic order equal. In addition, the structural changes should not be too large of course, this condition cannot be defined precisely. 

Equation (12) is a linear free-energy relationship, since activity coefficients/can be represented as AG° values. The reason for defining the slope parameter as in equation (12) (subscript e for equilibrium) is that a little rearranging of equations (11) and (12) leads to the easy-to-use Bunnett-Olsen equation for equilibria, equation (13) 30... 

A criterion of mechanism based on the Hammett acidity function, H0 (Section 3.2, p. 130),has long been used to decide the type of question raised by the choice between Mechanisms I and II in Scheme 8. Since in strongly acidic media the concentration of the protonated substrate should be proportional to h0, the reaction rate for a unimolecular decomposition of this protonated substrate (Mechanism I) should also be proportional to h0, whereas if a water molecule is required (Mechanism II), the rate should follow H30+ concentration instead. This test, known as the Zucker-Hammett hypothesis,76 when applied to acetal and ketal hydrolysis, appears to confirm the A-l mechanism, since a linear relationship is found between rate constant and h0 at high acidity.77 Inconsistencies have nevertheless been found in application of the Zucker-Hammett hypothesis, for example failure of the plots of log k vs. — H0 to have the theoretical slope of unity in a number of cases, and failure to predict consistent mechanisms for forward and reverse reactions the method is therefore now considered to be of doubtful validity.78 Bunnett has devised a more successful treatment (Equation 8.45), in which the parameter to measures the extent of... 

TABLE 10. Analysis of rate data for hydrolysis of A-i-butylbenzaldoxime and 2-i-butyl-3-phenyloxaziridine at 24.2 C by use of Bunnett linear free-energy relationships... 

Benoit, 1974) has enabled them to use the linear free energy relationship (BO) approach of Bunnett and Olsen (1966), and the (MCP) modification by Marziano, Cimino and Passerini (1973), to evaluate pKa (see also Levi et al., 1974). Not surprisingly, it is found that those compounds whose ionization is closely governed by an acidity function give similar p/ifa values by the BO, MCP and AF procedures. Appreciable differences result, however, for other compounds such as carbon acids, and the differences have been attributed to an incorrect anchoring of the AF scale for these acids. The chosen anchor, 9-phenylfluorene, had been assigned a pATa value of 18-59 (Bowden and Stewart, 1965). However the BO and MCP methods give a value close to 20 (Table 6). 

Medium effects on acid/base equihbria in aqueous solutions of strong acids have been analyzed not only in terms of Hammett acidity functions, but also in terms of linear Gibbs energy relationships, first developed by Bunnett et al. [225] see [226] for a... 

However, there are two important points in favor of the Bunnett-Olsen relationship. First, its range of validity extends down to dilute acid concentrations where the Yates treatment fails. Second, by expressing eqn (10) in terms of activity coefficients we may obtain... 

We have approached the problem of the relationship between protonation, solvation, and acidity functions. First we have discussed the present status of the determination of p and acidity functions for individual bases in terms of the Bunnett-Olsen equation. 

Reactivities of Side-chains of Monocyclic Thiophens.—The protonation of various furan- and thiophen-carboxamides in aqueous sulphur acid solution has been investigated by u.v. spectroscopy. The values of pKbh that were calculated using the acidity function and the Bunnett-Olsen linear free-energy relationship indicate the lower basicity of furan- and thiophen-2-carboxamides with respect to that of benzamides and of the 3-derivatives. The kinetics of isomerization of cw-l-(2-thienyl)-2-phenylacrylonitrile, as well as of its 2-furyl and 2-selenienyl analogues, have been studied in a solution of decahydronaph-thalene, with methanesulphonic acid and potassium t-butoxide as catalysts, and the mechanism has been discussed. ... 

The rates of PDC oxidations of lactic acid, retarded by the added Mn(II) and Ce(III) ions, correlate with the Zucker-Hammett, Bunnett, and Bunnett-Olson relationships. It suggests that the proton is not abstracted by the water molecule in the rate-limiting step. 

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