Hammett parameters

H-nmr chemical shifts of N-1—H and N-3—H signals have been used as a criterion for distinguishing between N-l-substituted and N-3-substituted hydantoin derivatives (22). They can often be related to electronic properties, and thus good linear correlations have been found between the shifts of N—H and Hammett parameters of the substituents attached to the aryl group of 5-arylmethylenehydantoins (23). 

Irichloromethyl-I unctional initiators, as ATRP macroinitiators 546 trichloromethyl radicals Hammett parameters... 

From the results of Malek et al.49,561 m 205,206,211) it may be concluded that the various metal derivatives used in esterifications exhibit many common catalytic characteristics. Thus, these authors190 established relationships concerning the free energies of esterification of thirteen ortho-, meta- and para-substituted benzoic adds by 1,2-ethanediol. They obtained Hammett parameters which do not differ greatly from those found for the add-catalyzed esterifications of the same adds with various alcohols. They concluded... 

As previously mentioned above, the chiral recognition abilities of the phenyl-carbamates of polysaccharides are greatly influenced by the substituents on the phenyl groups. In order to evaluate the effect of the substituents on the interaction between CSPs and solutes, the retention times of acetone and the first-eluted isomer of l-(9-anthryl)-2,2,2-trifluoroethanol (39) on 3- and 4-substituted CSPs are plotted against the Hammett parameter a of the substituents (Figure 3.33).130 The retention times of acetone tend to increase as the electron-withdrawing power of the substituents increases, whereas those of the first-eluted isomer of 39 tend to decrease. These results indicate that... 

Figure 3.33 Plots of retention times t of (a) acetone and (b) first enantiomer of 39 to elute on cellulose tris(phenylcarbamate) derivatives against Hammett parameter a of substituents. (Reprinted with permission from Ref. 130. Copyright 1986 by Elsevier Science.)...

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

Lindberg et al.17 have shown that the carbon Is shifts of some substituted benzenes are linearly correlated with the Hammett a parameters of the substituents. However, Hammett parameters are of rather limited applicability a given set of a values can be used to correlate data only for similar chemical systems. It has recently been shown18 that a wider variety of core binding energy shifts can be correlated by the four-parameter relation... 

FIGURE 1. Hammett plots [o (a) and cr+ (b), respectively] for the formation of semimercaptals from GSH and substituted nitrosobenzenes. Log k /k values were calculated from the rate constants reported for pH 7.4, 37 °C18 22 35 and pH 7.49, 25 °C, ionic strength 1 M30. Correction for pH is unnecessary as k and k ° are affected in the same manner. Correction for the different temperatures has been revealed to be insignificant. (The Hammett parameter p appears to vary with 1 IT while a seems to be independent of temperature54. Hence, log k /k 0 has been assumed to correlate with 1 IT.) For want of a for the complex nitrosochloramphenicol substituent (20), a of 4-CHOHMe was used55... 

AE° = the difference between the redox potential of the complex containing the substituted ligand and that of the complex containing the unsubstituted ligand p — constant which measures the sensitivity of the redox potential to the electronic effects of the substituents a = Hammett parameter which assigns a numerical value to the electronic effects of the substituents... 

Oxidative addition. The oxidative addition of a phosphine to a low valent transition metal can be most easily understood by comparing the Ph2P-fiagment with a Cl- or Br- substituent at the phenyl ring electronically they are very akin, cf Hammett parameters and the like. The phosphido anion formed during this reaction will usually lead to bridged structures, which are extremely stable. 

Extensive collections of pK values are available in the literature, e.g., [98-101]. It is also possible to predict pK values for a broad range of organic acids and bases using linear free energy relationships based on a systematic treatment of electronic (inductive, electrostatic, etc.) effects of substituents which modify the charge on the acidic and basic center. Quantitative treatment of these effects involves the use of the Hammett Equation which has been a real landmark in mechanistic organic chemistry. A Hammett parameter (a), defined as follows ... 

Next, a quantitative model, referred to as the E and C equation, is presented for predicting and correlating the enthalpies of adduct formation. The use of this equation and the interpretation of the parameters which result is discussed. Exceptions to the correlation are considered and the valuable insight about intermolecular interactions that can be gained by firmly establishing these exceptions is demonstrated. The parameters we obtain and valid transformations of these parameters are considered in the light of both the HSAB model and Donor Strength model of acid-base chemistry. Both of these concepts are shown to be at best incomplete models of coordination. The relationship between our parameters and the a—q Hammett parameters is quantitatively demonstrated. 

Optically active Schiff-base oxovana-dium(IV) complexes catalyze the asymmetric oxidation of sulfides to sulfoxides by peroxides [86]. The catalytically active species is VO(V) rather than VO(IV) and is formed in situ under the reaction conditions. A series of related complexes based on the optically active ligand shown in Eig. 15 shows linear dependence of their oxidation Ef values on the Hammett parameters of functional group X. These values ranged from 0.18 V versus Cp2Ee/DMSO for X = NO2 to —0.18 V for X = OCH3 [87]. A few complexes of planar tetradentate non-Schiff base ligands have also been investigated [88]. 

Table 10 Natural bite angles, NMR chennical shifts, and Hammett parameter of Y in HRh(ligand)(CO)2 ...

The most-studied reaction at pyridine nitrogen is protonation, and correlation of pK values with Hammett parameters has received extensive investigation.7,47 102-114 Quite clearly, electronic interactions of a-substituents are tempered by general steric effects,47,102 and there is also doubt as to the mode by which electronic effects are conveyed from the /l- and y-positions. Groups in the 4-position of potential (-I-R) character... 

Other properties of pyridine 1-oxides have been investigated with regard to applicability of Hammett parameters. The type of parameter required and the degree of correlation achieved varies, but again the need for a sliding o scale is generally indicated. Such measurements... 

The first adequate examination of the applicability of the Hammett equation to aromatic substitution was made by Roberts and his associates (1954). New data on the nitration of the halobenzenes together with the results for other substituted benzenes (Ingold, 1953) revealed that the rates for nitration in the meta position were, indeed, correlated satisfactorily by the Hammett rr-constants. Para substituents, in particular groups directing to the ortho and para positions, exhibit important deviations from the line defined by the meta reactivities (Fig. 3). It was suggested that these deviations were the consequence of significant resonance interactions in the transition state. At the same time, de la Mare (1954) examined the application of the Hammett parameters for the correlation of the relative rate data for non-catalytic chlorination. The poor agreement achieved (Fig. 4) prompted his conclusion that variable resonance interactions in the... 

The elimination of one unknown is accomplished by the assumption that om Me = o-m-Me (— 0.069). Ample evidence is available to substantiate this assignment. Experimentally, the meta reactivities even in electron-deficient reactions are adequately correlated by the Hammett parameters, as illustrated for aromatic nitration (Fig. 3) and solvolysis (Fig. 10). Presumably this is the consequence of the unimportance of resonance interactions between the meta substituents and the reaction site. Solution of the equation (25) with Me as —0.069 yields cr+Me as —0.280. The assignment of a value to oJ.Me allows the extension of the treatment to all other substituents through relations (23) and (24). 

The first term of the expression (48) is the contribution of the Hammett parameters. The second term is formed of a substituent constant (48) measuring the resonance contribution and a modified reaction constant, pr. The success of this treatment is directly attributable to the introduction of a second reaction constant. This additional parameter permits the effect of the substituent, Areaction constant, p), to depend on the susceptibility of the side-chain to resonance interactions. 

Monroe (1985) lists Hammett parameters for nine singlet oxygen reactions, all of which were conducted in organic solvents. Values of p vary from -0.82 to -1.71 with no obvious relation between the value of p and the reaction type or class. In most cases, better correla-... 

Gustowski, D. A., Gatto, V. J., Mallen, J., Echegoyen, L., Gokel, G. W., Direct correlation of cation binding strengths to Hammett parameters in substituted TV-benzylaza-15-crown-5 lariat ether and A 7V-dibenzyl-4,13-diaza-18-crown-6 BiBLE derivatives. J. Am. Chem. Soc. 1987, 52, 5172-5176. 

Substituent effects on rate constants of base-promoted ionisation of ketones have led to the conclusion that an electron-withdrawing substituent increases the rate of ionisation, in agreement with the anionic character of the transition state. This is based chiefly on studies on halogenation and isotope exchange of aromatic ketones. Data on p-values observed by plotting ionisation rate constants versus Hammett -parameters (Table 3) for substituted... 

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