Hammett relationships, oxidation

However, the effects on the rates in nonpolar solvent systems is dramatically larger and often of apparent contradiction. For example, the Hammett rho value for the oxidation of substituted methyl cinnamates and cinnamic acids by tetrabutylammonium permanganate in methylene chloride solutions is positive (33,49). See Figure lb. However, a rho value of converse sign (-0.6) is obtained from a Taft plot (Figure Ic) for the oxidation of vinyl ethers in aqueous tetrahydrofuran (33,50). For many other compounds the Hammett relationships are no longer linear, but concave upward ... 

The kinetics of oxidation of allyl alcohol with quinolinium fluorochromate have been studied.11 This reagent has also been used to oxidize benzyl alcohols the activation enthalpies and entropies obtained for the substituted derivatives are linearly related, and a Hammett relationship (p = —1.04 0.08) is obeyed.12... 

The oxidation of glutamic acid to cyanopropionic acid with CAB in acid solution showed an inverse fractional dependence on acidity. Similarly in alkaline medium, the order in alkali is fractional inverse.143 Kinetics of ruthenium(III)-catalysed oxidation of diols with CAB have been obtained. The products arise due to a fission of the glycol bond.144 The oxidation of isatins with CAB, in alkaline solutions, showed a first-order dependence on CAB and isatin and fractional order in alkali. The rates correlate with the Hammett relationship, the reaction constant p being —0.31. The observed results have been explained by a plausible mechanism and the related rate law has been deduced.145 The oxidation of cysteine with CAB in sulfuric acid medium is first order in CAB and cysteine and the rate is decreased with an increase in the hydrogen ion concentration.146... 

Figure 4.2 Hammett relationship (log 11/2 versus a) in the benzoylation reaction of benzene and substituted aromatic compounds with benzoyl chloride over H-BEA zeolite modified by indium oxides...

Reference to Chapter 3 (Scheme 3) shows that the alkaline hydrolysis of aryl carbamate esters has a large Hammett p coefficient (2.54) whereas that for the alkaline hydrolysis of aryl acetate esters is only small (0.67) (Scheme 2). The small p value for alkaline hydrolysis of the acetate case is consistent with little bond fission in the transition structure and the large p for the carbamate case (see Chapter 3 for mechanism) indicates substantial bond fission and a gross difference in mechanism between the two reactions. The simple Hammett relationship only correlates the alkaline hydrolysis rates of the carbamate when meta substituents are employed. Para substituents, which withdraw negative charge by resonance (4-NO2, 4-CN, etcf require values to obtain a good correlation. This is interpreted to mean that the transition structure has sufficient aryl oxide ion character for resonance to occur. In... 

The predictive importance of the Hammett relationship is impressive. It applies not only to hydrolysis reactions but also to substitution and oxidation reactions of aromatic compounds and even to enzyme-catalyzed reactions like the oxidation of phenols and aromatic amines by peroxidase (Job and Dunford, 1976). According to Exner (1972), data available in 1953 allowed prediction of 42,000 rate or equilibrium constants, of which only 3180 had been measured at the time. 

For monoalkyl benzenes, there is evidence of a second mechanism for hydrocarbon decomposition. Since substantial amounts of benzalde-hyde (representing a 5-25% yield) and traces (< 1% yield) of peroxy-benzoyl nitrate have been identified (10) in smog chamber experiments with aromatic hydrocarbons having at least two benzylic hydrogens, hydrocarbon oxidation apparently may occur at the a-carbon atom. Application of the usual Hammett treatment and electrophilic substituent constants (23) to any of our rate measurements for toluene, fert-butyl-benzene, and benzene indicates no linear Hammett relationship toluene... 

The evidence that electron-directing groups also affect the rate and nature of autoxidation of nitrogen-containing compounds has been well reviewed by Hoft and Schultze [51], particularly for the oxidation of amyl phenylhydrazines where it has been possible to relate rates of oxidation with predictions based on the Taft—Hammett relationship [76]. Similar relationships have been established with phenylhydrazones, which have been found to oxidise to produce a hydroperoxide [77]... 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

A comparison of the rate constants for the [Cun(FLA)(IDPA)]+-cata-lyzed autoxidation of 4/-substituted derivatives of flavonol revealed a linear free energy relationship (Hammett) between the rate constants and the electronic effects of the para-substituents of the substrate (128). The logarithm of the rate constants linearly decreased with increasing Hammett o values, i.e. a higher electron density on the copper center yields a faster oxidation rate. 

H, Cl, Br, NO2, Me, MeO) by bromamine-B, catalysed in the presence of HCl in 30% aqueous methanol by RuCls have been smdied and a biphasic Hammett a-relationship derived. A kinetic study of the ruthenium(in)-catalysed oxidation of aliphatic primary amines by sodium A-bromo-j -toluenesulfonamide (bromamine-T, BAT) in hydrochloric acid medium has been undertaken and the mechanism of the reaction discussed. A concerted hydrogen-atom transfer one-electron transfer mechanism is proposed for the ruthenium(in)-catalysed oxidation of 2-methylpentane-2,4-diol by alkaline hexacyanoferrate(III). The kinetics of the oxidation of propane-... 

The reactivity of phenols in the Cu-complex-catalyzed oxidation was studied by measuring the oxidation rate, the rate constant ke, and the redox potentials of the Cu complex and of the phenol (Fig. 29)158). The logarithm of the oxidation rate is proportional to log ke, which supports the assumption that the electron-transfer step is rate-determining. A linear relationship is observed between log ke and Hammett s o value of the phenol, which is proportional to the oxidation potential of the phenol. 

Fig. 29. Relationship between rate constant of electron-transfer step (ke), oxidation rate, and Hammett s o value of phenol for the pyridine-Cu-catalyzed oxidation of phenols1 ...

Various oxidation and reduction reactions of substituted metallocenes have already been discussed. A large number of substituted metallocenes have been oxidized chronopotentiometrically at a platinum foil in acetonitrile solution (39, 46). Electron-withdrawing substituents decrease the ease of oxidation, while electron-donating substituents increase the ease of oxidation with respect to the parent metallocenes. A plot of chronopotentiometric quarter-wave potentials, El, vs. Hammett para-sigma constants shows a definite linear relationship. The Ei s for ruthenocene and osmocene indicate these metallocenes are more difficultly oxidized than ferrocene, in agreement with earlier qualitative observations (18). 

Advantage has been taken of the ready accessibility of eleven para-substituted trityl and 9-phenylxanthyl cations, radicals, and carbanions in a study of the quantitative relationship between their stabilities under similar conditions.2 Hammett-type correlations have also been demonstrated for each series. Heats and free energies of deprotonation and the first and second oxidation potentials of the resulting carbanions were compared. The first and second reduction potentials and the p/CR values of the cations in aqueous sulfuric acid were compared, as were calorimetric heats of hydride transfer from cyanoborohydride ion. For radicals, consistent results were obtained for bond dissociation energies derived, alternatively, from the carbocation and its reduction potential or from the carbanion and its oxidation potential. 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

Usually the most convenient way to use Hammett s equation is to plot log k/k0 or just log k of the reaction of interest on the vertical axis and o values for the substituents on the horizontal axis. The slope of the plot of log k vs. o may change as a result of changes in the mechanism and in response to the variable electron demand of each substituent. It is anticipated that the various substituents will exert the same kinds of effects on the rate constants as they do on the benzoic acid rate constants, but the greater separation between substitution site and reaction site in the same chemical family makes the reaction less sensitive to the substituent effects. Many different oxidation reactions have been treated using Hammett s equation. The consistent trend of linear relationship with all of the Hammett s constants holds for all chemical classes except halogenated phenols. 

Quantitative structure-activity relationship models can be used to predict kinetic rate constants for compounds similar in chemical structure under UV /TiOz photocatalytic oxidation. In formulating the Hammett correlations, the descriptors, such as on s and o, proved to be successful in deriving equations for aromatic and aliphatic compounds respectively. For the case of aromatic compounds, the ores descriptor formulated better Hammett correlations than the om descriptor. 

Because of the bulk of comparable material available, it has been possible to use half-wave potentials for some types of linear free energy relationships that have not been used in connection with rate and equilibrium constants. For example, it has been shown (7, 777) that the effects of substituents on quinone rings on their reactivity towards oxidation-reduction reactions, can be approximately expressed by Hammett substituent constants a. The susceptibility of the reactivity of a cyclic system to substitution in various positions can be expressed quantitatively (7). The numbers on formulae XIII—XV give the reaction constants Qn, r for the given position (values in brackets only very approximate) ... 

Thiazolyl metcaptans, from haloallcylthia-zoles, synthesis of, 341 2-Thiazolylphenacyi-bromide, condensation, with thiobenzamide, 194 Thiazolyl phenylketone, by oxidation, 341 Thiazolylphenylsulfoxides, Hammett s relationships, 572 oxidation of, 570... 

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