Bronsted relationship

Although isothiazole (pK = 1.90) is less basic than thiazole, its rale of quaternization by dinitrophenyl acetate in water at 52°C is approximately 2.5 times higher (447). This deviation from the Bronsted relationship (A log k - 0.ApK, with positive) is interpreted as a consequence of the or effect of the adjacent sulfur lone pair in isothiazole that is responsible for its higher nucleophilicity (448, 449). 

Figure 4.10 is plot of the Bronsted relationship for hydrolysis of an enol ether. The plot shows that the effectiveness of the various carboxylic acids as catalysts is related to their dissociation constants. In this particular case, the constant a is 0.79 ... 

As follows from the Bronsted relationship, the symmetry factors for the cathode and anode processes are related to each other [see Eq. (6.17a)] ... 

Concerted mechanism . For the concerted mechanism, the kinetic expression (16) is used from which it can be seen that the overall Bronsted relationship is... 

DR. THOMAS MEYER (University of North Carolina) I feel that you have glossed over something that is actually very interesting, and that is the whole question of the Bronsted relationship. You have now observed a large number of systems, chromium, molybdenum, tungsten, which exhibit tricarbonyl anions. Have you looked at rate constants for all those species with the same hydride You would then have three bases, all with identical structures. 

DR. DALE MARGERUM (Purdue University) You need to be careful because you may not obtain a true Bronsted plot from what you have just described. You don t want to make a plot where each system has a different rearrangement. You want to be certain to have a common type of acid or base with which you are reacting. The literature is full of inverse Bronsted relationships in which there has been a poor choice of reaction acid-base pairs. Much confusion has been generated as a result. 

DR. MARGERUM Right. Otherwise, you will have different reorganizational terms or different work terms, all of which is going to confuse your Bronsted relationship. 

The Bronsted relationship can be strictly accurate only over a certain range of acid and base strengths. When has diffusion-controlled values, which of course cannot be exceeded, the linear plot of log k/ y vs log must level off to a zero slope, that is a = 0. As well as being reported, although rarely, in simple metal complexes, the resultant curvature in the Bronsted plot is also shown by the zinc enzyme carbonic anhydrase (Chap. 8. Zn(II)). In... 

B19. R. P. Bell, The Proton in Chemistry, 2nd edit. Chapman and Hall, London, 1973. Full discussion of Bronsted relationships. 

A major development in the study of EGB s is the recently reported measurements of rates of protonation by acids of known pK. The correlation of such rates with pK, the Bronsted relationship, also enables bases of determined pK to be used in the measurement of kinetic acidities of weak acids. This quantitative approach will eventually lead to the optimisation of reaction conditions for preparative reactions by providing data which can be used to match the acid/base pairs more exactly. In many organic reactions involving bases the base chosen is stronger than is strictly neccessary and consequently such reactions are often complicated by side reactions such as condensation reactions and isomerisations. The advantage of an EGB of moderate strength has been seen in the vitamin A preparation described in Scheme 18, where the facile cisftrans isomerisation is avoided. 

Illustrative Example 13.6 Estimating Hydrolysis Rate Constants Using the Hammett Relationship Bronsted Relationship... 

Alkyl halides react with diazines less readily than with pyridines. All the diazines are, nevertheless, more reactive toward methyl iodide than predicted by their pKa values and the Bronsted relationship. The significant although modest rate enhancements found are considered to arise from interactions between the two lone pairs on the nitrogen atoms this interaction is largest in pyridazine. Use of oxonium ions can convert the diazines into diquatemary salts. Quinoxalines and phenazines similarly yield diquatemary salts under forcing conditions. 

The Bronsted relationships. The effectiveness of a specific base as a general base catalyst can usually be related to its basicity (pfCa) via the Bronsted equation (Eq. 9-90). Here, kB is defined by Eq. 9-88 and G is a... 

Polarimetry is extremely useful for monitoring reactions of optically active natural products such as carbohydrates which do not have a useful UV chromophore, and samples for study do not need to be enantiomerically pure. Nevertheless, compared with spectrophotometry, the technique has been applied to relatively few reactions. It was, however, the first technique used for monitoring a chemical reaction by measuring a physical property when Wilhemy investigated the mutarotation of sucrose in acidic solution and established the proportionality between the rate of reaction and the amount of remaining reactant [50]. The study of a similar process, the mutarotation of glucose, served to establish the well-known Bronsted relationship, a fundamental catalysis law in mechanistic organic chemistry. 

In the Bronsted relationship, the reference equilibria are dissociations of protic acids, used directly as ApK i, and the constant of proportionality r = fi (or a, see Chapter 11). In the Hammett relationship, acid dissociation constants are also used for the reference equilibria, but indirectly. The dissociation constants of substituted benzoic acids in water at 25°C are used to define a set of substituent parameters, cr, which are then used in the equation of correlation, and r = p in Equation 9.18. 

Plots of the logarithms of the individual catalytic rate coefficients kHAi, as defined in (12), against p/irHA, give linear Bronsted relationships (13) with a... 

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