Acidity-dependence studies

The success of carbocation chemistry lies in the stabilization of carbocations in a medium of low nucleophilicity. Superelectrophiles, in turn, are reactive intermediates generated by further protonation (protosolvation). This second protonation increases electron deficiency, induces destabilization, and, consequently, results in a profound increase in reactivity. In particular, charge-charge repulsive interactions6 play a crucial role in the enhanced reactivity of dicationic and tricationic superelectrophilic intermediates. As various examples of acidity dependence studies show, without an appropriate acidity level, transformations may occur at much lower rate or even do not take place at all. In addition to numerous examples of superacid catalyzed reactions, various organic transformations, in which the involvement of superelectrophilic intermediates is invoked or superelectrophiles are de facto observed in the condensed state, are also included in this chapter. 

Acidity-dependence studies were also made using triflic acid modified with trifluoroacetic acid (TFA) and water in the range of acidity between //(l 10.1 and... 

The modem process for manufacturing nitric acid depends on the catalytic oxidation of NH3 over heated Pt to give NO in preference to other thermodynamically more favour products (p. 423). The reaction was first systematically studied in 1901 by W. Ostwald (Nobel Prize 1909) and by 1908 a commercial plant near Bochum. Germany, was producing 3 tonnes/day. However, significant expansion in production depended on the economical availability of synthetic ammonia by the Haber-Bosch process (p. 421). The reactions occurring, and the enthalpy changes per mole of N atoms at 25 C are ... 

A kinetic study of the electrophilic substitution of pyridine-N-oxides has also been carried out50b,c. Rate-acidity dependencies were unfortunately given in graphical form only and the rate parameters (determined mostly over a 30 °C range) are given in Table 4b. There is considerable confusion in Tables 3 and 5 of the original paper, where the rate coefficients are labelled as referring to the free base. In fact the rate coefficients for the first three substituted compounds in... 

Horne has studied the kinetics of exchange in aqueous perchlorate media at temperatures down to —78 °C by the isotopic method ( Fe) and dipyridyl separation. The same rate law in these ice media as in aqueous solution was observed, although the acid dependence was small. Horne concluded that the same exchange mechanism occurs in solid and liquid solvent. Evidence for a Grotthus-type mechanism has been summarised. ... 

The rates of reduction of cis- and tra/u-Co(NH3)4(N3)2 and Co(NH3)5N3 by Fe in aqueous perchloric acid have been studied by Haim . The cis isomer of the former complex reacts at an acid-independent rate whereas the rate for the trans isomer is acid-dependent. The respective rate laws at 25 °C are... 

The reaction between isothiocyanatopentaminecobalt(III) and hydrogen peroxide has been studied kinetically . Although Co " " (aq) is formed as one of numerous products, the overall reaction is essentially an oxidation of ligand by the peroxide. The reaction is first-order in both oxidant and reductant although there is a complex acidity dependence. 

These oxidations have attracted wide interest and both specialised and comparative studies have been published. The rate laws which are summarised in Table 14 are not distinctive although the acidity dependence of the V(V) oxidations of some substrates suggests by analogy that a pinacol-type of oxidation may occur cf. V(V)-pinacol complexes, p. 388), viz. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

The pioneering studies of Bender s group were followed by many attempts to increase the efficiency of esterolysis by cyclodextrins and several approaches have been tried, most notably in Breslow s laboratory. One may optimize the structure of the substrate (Trainor and Breslow, 1981 Breslow et al., 1983), modify the cyclodextrin (Emert and Breslow, 1975 Breslow et al., 1980 Fujita et al., 1980), or alter the solvent (Siegel and Breslow, 1975). The last of these is the easiest to achieve but detailed studies are made tedious by the necessity to redetermine all of the relevant equilibrium and rate constants, and the acidity dependence of the catalysed and uncatalysed processes, in the new medium. 

The work of Coombes and coworkers20 on the formation of the 4-methyl-4-nitro intermediate has already been discussed above. Here the solvent was aqueous sulphuric acid with acid concentration ranging from 55% to 90%. The final product, 4-methyl-2-nitrophenol, was formed by the expected two routes about 40% via the ipso-intermediate and 60% directly. Their kinetic studies enabled the acidity dependence of the ipso-rearrangement to be examined they argued that this dependence demonstrated that the rate-limiting stage of the conversion involved the protonated /pso-intermediate (43). They... 

The oxidation of cysteine, as well as other amino acids, was studied by Mudd et a/. Individual amino acids in aqueous solution were exposed to ozone the reported order of susceptibility was cysteine, methionine, tryptophan, tyrosine, histidine, cystine, and phenylalanine. Other amino acids were not affected. This order is similar to that for the relative susceptibility of amino acrids to radiation and to lipid peroxides. Evaluation of the ozonization products revealed that cysteine was converted to cysteic acid, as well as cystine methionine to methionine sulfoxide tryptophan to a variety of pioducrts, including kynurenine and N-formylkynurenine tyrosine also to a variety of products, includiitg dihydroxyphenylalanine histidine to ammonia, proline, and other compounds and cystine in part to cysteic acid. In some cases, the rate and end products depended on the pH of the solution. 

The pyridinium chlorochromate (PCC) oxidations of pentaamine cobalt(III)-bound and unbound mandelic and lactic acids have been studied and found to proceed at similar rates.Free-energy relationships in the oxidation of aromatic anils by PCC have been studied. Solvent effects in the oxidation of methionine by PCC and pyridinium bromochromate (PBC) have been investigated the reaction leads to the formation of the corresponding sulfoxide and mechanisms have been proposed. The major product of the acid-catalysed oxidation of a range of diols by PBC is the hydroxyaldehyde. The reaction is first order with respect to the diol and exhibits a substantial primary kinetic isotope effect. Proposed acid-dependent and acid-independent mechanisms involve the rapid formation of a chromate ester in a pre-equilibrium step, followed by rate-determining hydride ion transfer via a cyclic intermediate. PBC oxidation of thio acids has been studied. ... 

In a previous investigation, Klont and Ten Cate (1991) showed that dentin must be demineralized before its matrix can be degraded by proteases. In the present study, it was demonstrated that proteolytic degradation of the demineralized matrix enhanced the susceptibility of dentin lesions to acid-dependent demineralization. 

Removal of calcium from HRP C has a significant effect not only on enzyme activity and thermal stability, but also on the environment of the heme group. The calcium-depleted enzyme has optical, EPR, and H NMR spectra that are different from those of the native enzyme (211). Temperature dependence studies indicate that the heme iron exists as a thermal admixture of high- and low-spin states. Kinetic measurements at pH 7 show that ki, the rate constant for compound I formation, is only reduced marginally from 1.6 0.1 x 10 to 1.4 x lO M s , whereas k, the rate constant for compound II reduction, is reduced from 8.1 1.6 x 10 to 3.6 x lO M s (reducing substrate p-aminobenzoic acid), 44% of its initial value (211). There can be little doubt that this is the main reason for the loss of enzyme activity on calcium removal. 

Referring to chemical species possessing the ability to exhibit either acidic or basic properties (e.g., amino acids). This property actually depends on the medium. Eor example, sulfuric acid is an acid, when studied in water however, it becomes amphoteric when studied in superacids. 

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