Proton determining relative acidity

The kinetic method of determining relative acidity suffers from one serious complication, however. This complication has to do with the fate of the ion pair that is formed immediately on removal of the proton. If the ion pair separates and difiuses into the solution rapidly, so that each deprotonation results in exchange, the exchange rate is an accurate measure of the rate of deprotonation. Under many conditions of solvent and base, however, an ion pair may return to reactants at a rate exceeding protonation of the carbanion by the solvent. This phenomenon is called internal return ... 

Unlike the Beckmann rearrangement, the outcome of the Neber rearrangement does not depend on the configuration of the starting oxime derivative E- as well as Z-oxime yield the same product. If the starting oxime derivative contains two different a-methylene groups, the reaction pathway is not determined by the configuration of the oxime, but rather by the relative acidity of the a-methylene protons the more acidic proton is abstracted preferentially. ... 

A hydrogen-bond basicity scale has been developed that can be used to determine the relative basicity of molecules. Table 8.2 gives the p hb values for several common heteroatom containing molecules. This is obtained from the protonated form (conjugated acid) of the base in question. The larger the number, the more basic is that compound. 

Gaines (15) has previously described the use of merocyanine dyes as a nonaqueous means of determining Bronstcd acid concentration. Merocyanine dyes are protonated by strong acids to produce protonated dye which has a distinct visible absorption (Figure 1). The unprotonated dye form (3) has a solvent dependent visible absorption maxima. The present studies were performed in acetonitrile or dichloromcthane solvent where absorption maxima were at 576 nm and 610 nm respectively. The absorbance of the protonated form (4) is relatively unaffected by choice of solvent and is clearly separable from the absorbance of the free dye. The extinction coefficient of the free dye is quite large (71,000 in dichloromethane) which allows determination of small amounts of acid such as 10 6 mmol with an average error of less than 10%. 

Figure 5. Relative acidity of a-protons as determined by the conformational effect of deuterium exchange on a conformationally biased system...

In contrast, reaction of electron acceptor-substituted phenols exhibits p = +1.72, indicating the development of negative charge at the phenolic oxygen in the rate-determining step for reaction of relatively acidic, weakly nucleophilic phenols with 110, and the addition of 112f exhibits a large primary deuterium kinetic isotope effect of k /kv = 5.3. This is consistent with the electrophilic addition mechanism of equation 85, in which full or partial protonation at silicon precedes nucleophilic attack. 

Flash photolysis of misonidazole, metronidazole, and nitrobenzothiazoles has been carried out in [1369-1371], Laser flash-photolysis (355 nm) allows to determine relatively stable anion-radicals of misonidazole and metronidazole in aqueous solutions [1370], Solvated electrons have been formed at harder irradiation, the result of which interaction with nitroimidazole molecules is generation of their radical anions [1372], The authors [1372] have also found that fluorescence intensity of metronidazole is about 20 times more than that of misonidazole in same conditions. Photochromic properties of benzothiazole derivatives containing nitro and methyl groups in the ortho positions with respect to each other were studied by flash photolysis [1371], The application of the thermodynamic approach to predict the kinetic stability of formed nitronic acids is limited owing to specific intramolecular interactions. The lifetime of photoinduced nitronic acid anions tends to increase with rise in the chemical shift of the methyl protons. The rate constants photoinduced nitronic acids and their anions increase as the CH3C-CN02 bond becomes longer [1371],... 

This enzyme, which is relatively stable under reaction conditions, will retain 70% of its activity after 10 days at pH 5 and 25°C. Although it is not yet commercially available, it has been overexpressed in E. coli, making large quantities easily accessible.68 The detailed mechanism of DERA has been determined based on the atomic structure (ca. 1.0 A) combined with site-directed mutagenesis, kinetic, and NMR studies136 (Scheme 5Alb). A proton relay system composed of Lys and Asp appears to activate a conserved active-site water that functions as the critical mediator for proton transfer in acid-base catalysis. 

The ability to recognize the most acidic site in a molecule will be important throughout the study of organic chemistry. All the factors that determine acidity are therefore summarized in Figure 2.5. The following two-step procedure shows how these four factors can be used to determine the relative acidity of protons. 

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