Hammett constants, correlation with half-wave

Most aquatic oxidation reactions are attributable to well-defined chemical oxidants. As a result, model systems can be designed where second-order rate constants can be determined precisely for families of organic congeners. The comparatively high quality of these data allows mechanistic models of electron transfer to describe aquatic oxidations of environmental interest. Kinetic studies of these processes have produced many QSARs, mostly simple empirical correlations with common convenient descriptors such as the Hammett constant (a), half-wave oxidation potential ( j/2)> energies of the highest occupied molecular orbital ( HOMO), or rate constants for other oxidation reactions as descriptors (Canonica and Tratnyek, 2003). Their predictive power has lead to engineering applications in water treatment and remediation. 

Some pyridazines were investigated by cyclic voltametry and/or preparative scale electrolysis to give mixtures of reduced compounds [77NKK990 81 ACS(B)185]. The half-wave potentials of some 3(27/)-py-ridazinones were determined at different pH and correlated with Hammett o--constants (77KGS668). Maleic hydrazide is stable, but reduced pyrida-zines undergo reductive N—N bond cleavage upon electroreduction (78-LA 1505). 

Correlation of the half-wave potentials of meta- and para-substituted phenylarsonic acids with the Hammett substituent constants, cr, has been tried . The equation 1/2 = (0.053 0,010)o- — 0.705 V vs NHE (r = 0.9946) was obtained for a series of five compounds (unsubstituted, 4-OH, 4-NHj, 4-Me, 3-NH2-4-OH) studied in 0.02 M HCl, while under the same conditions the equation jy2 = 0.173(7 — 0.703 V vs NHE (r = 0.9991) was obtained for the second reduction of a series of three N02-substituted phenylarsonic acids using the a value for —NHOH for the reduced nitro group , i.e. apparently two different correlations exist. However, the possible protonation of the amino or hydroxylamino groups under the experimental conditions was not taken into account, and the validity of assigning the a value of the reduced substituent to the combined substituent/arsonic acid reduction taking place at the potential of the second wave was not discussed. It is therefore not surprising that the data cannot be described by a simple Hammett relation. For the simple para-substituted phenylarsonic acids a linear relation (equation 15) was found between the half-wave potential and the value of the arsonic acid. 

As observed before the first polarographic half-wave potentials determined in aprotic solvents can be correlated with Hammett a values. This was done for the 4-nitro-, chloro-, methyl-, methoxy- and a series of ring chlorinated dimethylamino-benzene-diazonium salts. A linear correlation was observed when corrected constants were used... 

Based on the experimentally measured redox properties (2001T(57)1857) of the studied compounds, it was noted that the tendency to reduction is strongly dependent on the electronic nature of substituent. The half-wave potentials (A i/2) of the meta- and para-substituted derivatives are well correlated with Hammett substituent constants. Relatively high values of the reaction constant p indicate a good abihty of the —NH— bridge to transmit the substituent effects. Similar results have been observed in studies of the... 

For meta- and para-substituted aromatic carbonyl compounds, a linear relationship exists between the earbonyl absorption frequency and the Hammett reactivity constant. - - - A relationship between the carbonyl stretching vibration frequency of aromatic carbonyl compound and the pAT " of the corresponding aromatic carboxylic acid has been demonstrated. Correlations with other parameters, such as electronegativities,"" ionization potentials, Taft a values, half-wave potentials, etc., have also been made." For aromatic compounds with ortho- substituents, a combination of factors may be important, such as chelation, steric effects, and field effects (dipole interactions through space). 

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