Substituent constants, Hammet

On the basis of these considerations, the authors suggest that a roughly linear relationship might exist between the transition energies (Ft) for the charge-transfer band and Hammet-type substituent constants, am. 

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

The Hammet substituent constant o Steric effects, described by the Taft values... 

How are the Hammet Oymeta and d)para substituent constants defined Are there cases in which the Oypara values are not applicable If yes, give some examples. 

The effects of solvent (polarity), temperature, and substituents (the Hammet cr constants) have been studied in the azide-tetrazole equilibrium for thiazolo [2,3 -e]tetrazoles (75BSB1189 77CJC1728, 77JHC1299) oxazolo[2,3-e]- and isoxazolo[2,3-d]-tetrazoles (77CJC1728) and tetra-zolo[4,5- >]-1, 3,4-thiadiazoles (81CHE721). A rough parallelism has been proposed between the azido-tetrazole isomerism and prototropic tau-tomerism (75BSB1189). 

In principle, extrathermodynamic relationships that deviate from the simple Hammett equation (equation 8) can be treated by equation 14. The major problem is the determination of the different sets of o s, (e.g., set and 0 set) in a way that will indeed reflect their relation to independent properties. An example of such a procedure is the separation of polar and steric effects (10). The need for such a separation arose when a nearly complete lack of correlation was observed between substituent effects represented by the Hammet a constants and the rates for alkaline hydrolysis of aliphatic systems (12). Inspection of the structures indicated that the proximity of the substituents to the reaction site was a common feature. The steric interaction between R and X had to be considered separately from the electronic effects. Polar substituent constants were thus defined as the difference between the rate constants of base and acid catalyzed hydrolysis of esters. From the structural similarity of the transition states for these reactions (equation 15) it was assumed that the difference in their charge reflects only the polar effect of the substituent... 

FIGURE 15 The peak oxidation potentials, p, of several 3-substituted thiophene monomers are plotted against their respective Hammet substituent constants, [Pg.798]

The underlying physical basis for the failure of Hammet (r and o-p values to correlate certain reactions is that all substituent interactions are some mixture of resonance and field effects. When direct resonance interaction is possible, the extent of resonance effects increases and the substituent constants appropriate to a more normal mixture of resonance and field effects then fail. There have been many attempts to develop sets of cr values which take into account extra resonance interactions. 

The second mechanism Involves concerted cleavage of both the 0-(3 and C-C bonds to give, through a single transition state, alkyl -butoxy radical pairs. Activation parameters ( ), isotope effects (3 ) and substituent effects ( ) all indicate a change from stepwise (A-1) to concerted (A-2) mode of reaction when the decarboxylation residue has the stability of secondary alkyl or more. Hammet substituent constants... 

Halterman et al. agreed with this proposal to show the selectivity of 5-aryl-5-phenylcyclopentadiene favoring the reactions on the anti side of a more electron rich aromatic system with significant correlation between the Hammet constants for the aromatic substituents and the facial selectivity [19] (Scheme 29). 

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 substituent hydrophobicity constant, based on partition coefficients analogs to Hammet constants ... 

Regression analysis is currently the most widely used correlative method in drug design. This is because it simplifies problems within a set of compounds by using a limited number of descriptors, notably the Hansch hydrophobic constant n, Hammet constants, or other electronic characteristics of substituents, and the Taft steric constant Eg. 

Figure 13.15 Effects of substituents on the base-catalyzed hydrolysis of benzoic acid ethyl esters in ethanoliwater (85 15) at 25°C. Relative reaction rates are correlated with Hammet Oj constants (data from Tinsley, 1979).

Reaction constants p in the Hammet equation describing the influence of the substituents in the phenyl ring of 2-arylbenzoxazoles, 2-arylbenzoth-iazoles (68MI2), and 2-arylbenzotellurazoles are approximately the same value, indicating a similar mechanism of expressing the electronic effects of substituents at the nitrogen atom in different benzazoles. 

It has been pointed out [52] that there is a correlation between the Hammet S (Tpara constants of the apical substituents and the redox potentials of the corresponding iron clathrochelates. Meanwhile, such a correlation is observed only inside the two groups of complexes with (a) OH, OCH3, F, Cl, and Br and (b) CH3, CsHs, and i-C4H9 substituents. No reasons are given for the existence of two groups of complexes. In spite of the fact that the cyclic voltammetry evidence indicates that the Fe +/ + couple is quasi-reversible, the electrolytic oxidation has failed to isolate stable iron(III) complexes [52]. 

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. 

Taft s (o ) and Hammet s (of,ara) constants for substituents in a-dioximate fragments n... 

A good agreement between the Oi values and Hammet s constants is observed in some cases (Fig. 56). For sepulchrate and sarcophaginate cobalt(III)/cobalt(II) pairs with apical amino, methyl, and chlorine substituents, the Log/en values correlate with inductive constants of these substituents (Fig. 56). 

Fig. 56. Plot of Log ku (1) and ai values (2) versus the Hammet s apara constants of apical substituents.

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"...

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