Substituent constants, Hammet table

Influence of Physical Chemistry on Separations in Liquid Chromatography Table 6.2 Hammet s substituent constants... 

La Mar (150) and Walker (156) have found a thermodynamic cis effect in the formation of hemichrome salts [Fe(TRP)L2]Cl (->-[27]) according to equilibrium (77) which was studied by 1H-NMR and optical spectroscopy for L = l-Melm (Table 22). As the electron-donating power of the para-phenyl-substituent of the porphyrin increases, the total formation constant, /J2, increases. This is because the product of the reaction contains a positively charged center which is stabilized by electron-donating groups. As a Hammet relation exists, the mesomeric part of the electronic transmission is also operative, and hence dative porphyrin-to-metal tr-bonding seems to be involved. 

The bipyridines are dibasic, and the two acid dissociation constants Ki and K2, for all the bipyridines have been determined. Typical values are recorded in Table I. There has been considerable interest in the first dissociation constants Ki of 2,2 -bipyridine and substituted 2,2 -bipyridines because of their use as metal complexing agents. In general, the order of relative basic strengths of derivatives of 2,2 -bipyridine is as expected. Electron-attracting substituents reduce the basicity, whereas electron-donating substituents increase the basicity of the molecule. " The dissociation constants of several substituted bipyridines correlate well with the Hammet equation. 2,2 -Bipyridines with an electron-donating substituent at position 4 are monoprotonated at N-1 and not at... 

Table 1.13 Hammet s electronic constants of fluorinated substituents ...

The electrochemical behaviour of the ribbed-functionalized iron(II) [65, 68] and ruthenium(II) [78] clathrochelates with alkylamine, thioaryl, thioalkyl, phenoxyl and crown ether substituents in a-dioximate fragments was characterized by E1/2 values for Fe3+/2+ and Ru3+/2+ couples (Table 37). The Tomes criterion values of most complexes exhibited reversible or quasi-reversible anodic processes. Moreover, the quasi-reversible oxidation processes are accompanied by the formation of insoluble products followed by passivation of the working electrode. The Ev2 values depend on the electron-donating properties of the substituents in the ribbed fragments. The correlations of E1/2 values for Ru3" 2+ and Fe + 2+ couples with these substituents Hammet s Opara constants were observed in Refs. 65, 68, and 78. These correlations are rather qualitative, but they enable one to conclude that ruthenium complexes are less sensitive to the change of substituents in dioximate fragments. There was no correlation between the Em values and the inductive Taft s (cr,) constants for substituents in dioximate fragments. 

For illustrative purposes. Table 3.10 lists a few representative values of Hammet constants for substituents in benzoic acid. 

Table 3.10. Hammet Constants for Substituents in Benzoic Acids"...

The thermodynamic stability of the radical anions has been determined from log K and are displayed in Table 1.11. The tetrasubstituted derivatives showed lower values of K, thus indicating that steric factors are responsible for this behaviour. The substituent effect in positions 2 and 3 on the redox potentials of the A,AT-dicyano-1,4-anthraquinonediimine series can be correlated with the Hammet a constants. The Hammet plot of E i2 v5. (73 [133] gives a p value of 0.123 with a correlation coefficient of r = 0.971. The comparison of this value with that reported by Aiimuller and Hunig [32] for other simpler DCNQI series (p — 0.490) indicates that the substituent effect over the ease of reduction decreases as the conjugation is extended. 

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