Bronsted coefficient

General base catalysis of the cyclization of ethyl 2-hydroxymethyl 4 nitrobenzoate [35], where the acyl group is further activated by electron withdrawal by the nitro-substituent, is characterized by a Bronsted coefficient of 0 97, i.e. unity within the limits of error, suggesting that proton transfer is a diffusion-controlled process (Fife... 

Belke et al. (1971) reported general base and general acid catalysis in cyclization of 2-hydroxymethylbenzamide [equation (18)]. However, with 2-hydroxymethyl-6-aminobenzamide strict general base catalysis by buffer bases is observed with a Bronsted coefficient of O 39 (Fife and Benjamin, unpublished data). In contrast with the unsubstituted amide, the Bronsted plot is nicely linear. An amino-group in the 6-position might assist decomposition of a tetrahedral intermediate as in [37a, b] or a kinetic equivalent. The pH-rate constant profile for spontaneous cyclization at zero buffer concentra-... 

Dunn and Bruice (1970, 1971) presented evidence that the mechanism of hydrolysis of methoxymethoxybenzoic acid is actually A-1. Evidence included a Bronsted coefficient for intramolecular catalysis of DO, based upon points for the unsubstituted compound and the 5-NO2 derivative, and p -values that were the same (—3-0) for both the methyl ester and the carboxyl derivatives of [72] when... 

BR0NSTED RELATION Bronsted coefficient or exponent, BR0NSTED RELATION BR0NSTED PLOT BR0NSTED RELATION BR0NSTED THEORY Brownian motion,... 

The final step of the convolution analysis is the determination of the transfer coefficient a. This coefficient, sometimes called the symmetry factor, describes how variations in the reaction free energy affect the activation free energy (equation 26). The value of a does not depend on whether the reaction is a heterogeneous or a homogeneous ET (or even a different type of reaction such as a proton transfer, where a is better known as the Bronsted coefficient). Since the ET rate constant may be described by equation (4), the experimental determination of a is carried out by derivatization of the ln/Chet-AG° and thus of the experimental Inkhei- plots (AG° = F E — E°)) (equation 27). 

Since AAv manifests itself in the thermodynamics of the reaction, and AE is a kinetic term, the relationship between the two represents a Bronsted coefficient, and the ratio, Af /AAv, a measure of the Bronsted parameter, a. The fact that the Bransted parameter for (72) equals 1 is somewhat surprising... 

It should be remembered that no single value of the Bronsted coefficient /3 describes the relationship between the basicity of oxygen and other anionic nucleophiles and their nucleophilicity towards esters over a large variation in the structure of the nucleophile. However, the general conclusion can be drawn that oxygen nucleophiles on metal centres will have a nucleophilicity which broadly equates with their basicity. 

Streitwieser and co-workers have extended their measurements of equilibrium acidities in cyclohexylamine to determination of exchange rates.69 They have made quantitative correlations between exchange rate and the pKa s determined by equilibrium methods for various aromatic compounds and have thus been able to verify that the Bronsted relation holds for these substances and to find Bronsted coefficients a for various types of compounds. A third method for evaluating p/Ca of weak acids, which has been used by Applequist70 and by Dessy,71 involves the study of exchange reactions of organometallic compounds (Equations 3.54 and 3.55). 

Table 8.2 Dependence of Bronsted Coefficient a on Basicity of the Nucleophile in Additions to C=0...

Here, k° is the standard heterogeneous electron transfer rate constant and a is the electrochemical transfer coefficient [33], which corresponds in electrochemistry to the Bronsted coefficient in organic chemistry. It is seen from Equations 6.10 and 6.11 that kTsei and k°x are both equal to k° at E = E°. 

Bronsted coefficient (Plg) is around 0.7. Thus, Lewis acid activation, metal-hydroxide activation and leaving-group activation could add up to over 1017-fold rate acceleration for DNA hydrolysis. The pseudo-first order rate constant for hydrolysis of a phosphate diester bond of DNA has been estimated to be about 1(T19 s"1 at neutral pH and 25 °C [13]. A 1017-fold rate acceleration for hydrolyzing DNA would reduce the half-life of the phosphate diester bond from billions of years to about a minute or two. 

It is of interest to compare these data with the selectivity coefficients for two nucleophiles of the same group, e. g. kR.s-/kn.o-, and also with the Bronsted coefficient a, which we take as a measure of the extent of bond formation in the transition state (5). 

Challis and Miller suggested that Bronsted coefficients may not prop-... 

Bronsted coefficient activity coefficient surface excess (molm )... 

Similar results have been obtained by Baciocchi for the deprotonation of a-substituted 4-methoxytoluenes by 2,6-lutidine and NOs in acetonitrile [145]. In this study, the same values of the Bronsted coefficient (a = 0.24), and of the deuterium kinetic isotope effect (kn/kD = 2.0 for 4-methoxytoluene radical cation) have been obtained with the two bases these results point again towards a highly asymmetric transition state with a very small amount of C-H bond cleavage. Moreover, values of 0.53 and 0.66 eV have been calculated for the intrinsic barrier of the reactions of the radical cations with NO3" and 2,6-lutidine, respectively, again comparable with those observed for acid-base reactions involving carbon acids [140, 141]. 

---