Hammett o* parameter

The pKa values determined by Brand et aL (36) (corrected to the Paul and Long Ho scale in Table XVIII) do not correlate well with Hammett o- parameters as they should if their conjugate acids behaved in response to substitution as the isoelectronic carboxylic acids do. These authors attempted to explain the discrepancy in terms of solvation-entropy factors. However, if one employs Brown s or parameters (which were not available to the original workers cited above) one obtains a better correlation (5). Furthermore, one may note that if the pKa for p-nitrophenol (141) is included on the conjugate acids of the phenols (11) it is found to be far removed from the correlation line. One may conclude from this that p-nitrophenol protonates on the nitro, rather than on the hydroxy group. 

In subsequent studies attempting to find a correlation of physicochemical properties and antimicrobial activity, other parameters have been employed, such as Hammett O values, electronic distribution calculated by molecular orbital methods, spectral characteristics, and hydrophobicity constants. No new insight on the role of physiochemical properties of the sulfonamides has resulted. Acid dissociation appears to play a predominant role, since it affects aqueous solubiUty, partition coefficient and transport across membranes, protein binding, tubular secretion, and reabsorption in the kidneys. An exhaustive discussion of these studies has been provided (10). 

Relative activation enthalpies (Aif) in Table 2 were converted to o% kx k ) at 298 K, and were plotted against Hammett a constants. Here, we used enthalpies, because the size of the entropy and hence the free energy depend much on low frequencies, which are less reliable than higher frequencies, especially for compounds with weak interactions such as TS (8). The use of free energy (AG ) gave similar correlations with more scattered points. As for the Hammett o constant, we used dual-parameter o constants in the form of the Yukawa-Tsuno equation (LArSR equation) (9) as defined in eq 3. Here, the apparent a constant (aapp) has a variable resonance contribution parameter (r), which varies depending on the nature of the reaction examined for t-cumyl... 

Five new pyridazinones were synthesized with substitutions in the two-position of the phenyl ring as given in Table II. Using the Hansch approach, correlations were made between the experimentally determined 18 2/18 3 ratios and the values and a,a values taken out of the data collection of Hansch and Leo (15) (Table II and Equations 1-3). The hydrophobic parameter is derived from the 1-octanol/water partition coefficient and o is the Hammett electronic parameter. 

Hammett s success in treating the electronic effect of substituents on the equilibria rates of organic reactions led Taft to apply the same principles to steric, inductive, and resonance effects. The Hammett o constants appear to be made up primarily of two electronic vectors field-inductive effect and resonance effect. For substituents on saturated systems, such as aliphatic compounds, the resonance effect is rarely a factor, so the o form the benzoic acid systems is not applicable. Taft extended Hammett s idea to aliphatics by introducing a steric parameter ( .). He assumed that for the hydrolysis of esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base. Rate differences would be caused only by the field-inductive effects of R and R in esters of the general formula (XCOOR), where X is the substituent being evaluated and R is held constant. Field effects of substituents X could be determined by measuring the rates of acid and base catalysis of a series XCOOR. From these rate constants, a value a could be determined by Equation (5.9) ... 

Improved understanding and treatment of the phenyl case was developed from the recognition that the derived o-si (C6H5-) is almost identical with the "standard Hammett o- value, a parameter containing both mesomeric and inductive contributions. Since published data on the krei of ozonation of substituted phenyldimethylsilanes were available (11), and included the common point, phenyldimethylsilane, simple mathematical manipulation could bring these and the present data on to the same scale. When the Hammett o- for the substituted phenyls was... 

A number of workers have reported the NMR spectra of mono-substituted phenyl groups and correlated the shifts vdth molecular parameters. It has been shown for organic compounds that the chemical shift of the para-carbon in monosubstituted benzenes is linearly related to the total TT-electron density at the para position in these compounds. Also the shift separation of the meta- and para-carbons appears to be linearly related to the rr-electron density on the para-carbon due to resonance interaction with the substituent 49, 156). Spiesecke and Schneider have reported a good linear relationship between the para-carbon chemical shift of monosubstituted benzenes and the Hammett, o-para constant, but no such relationship appears to exist for the other carbon chemical shifts, except between the chemical shift for the substituted carbon atom (corrected for magnetic anisotropy effects of the substituent) and the electronegativity of the substituent 210). 

Scheme 33. Kinetic studies of the cation radical Diels-Alder reactions of aryl vinyl sulfides with cyclopentadiene and of aryl propenyl ethers with 2,3-dimethyl 1,3-butadiene. In both solvents, both extended reaction series have rates which correlate excellently with the Hammett a parameters and poorly with the Hammett-Brown o-+ values. The preference is statistically significant at or above the 95 % confidence level.

Ewing, D.F. (1978). Correlation of nmr Chemical Shifts with Hammett o Values and Analogous Parameters. In Correlation Analysis in Chemistry (Chapman, N.B. and Shorter, J., eds.). Plenum Press, New York (NY), pp. 357-396. 

Rates of anaerobic dechlorination of aromatic hydrocarbons (Peijnenburg et al. 1992) and of the hydrolysis of aromatic nitriles under anaerobic conditions (Peijnenburg et al. 1993) have been correlated with a number of parameters including Hammett o-constants, inductive parameters, and evaluations of the soil/ water partitioning. 

Hammett a values are adequate for reactions and equilibria involving separation of substituent and reaction centre by a phenylene grouping. Examination of aliphatic species necessitates a more sophisticated set of o values as transmission effects are now markedly different from the model. Resonance and inductive effects may be separated if we can use as models reactions with no resonance interactions so that o type values may be defined. Entirely new approaches have led to the definition of an inductive scale of o parameters (o,). Although some of these parameters are now only of historic value we shall include a brief discussion of early attempts to define the a, scale. 

The rates summarized in Tables 3.51 and 3.52 were correlated with substituent effects using Hammett a parameters [225]. Hammett plots for the reactions of Zn with aryl bromides (THE, 70°C), aryl iodides (THE, 0°C), Mg with aryl bromides (Et20, THE, 0°C), and aryl iodides (Et20, 0°C) [226] are represented in Figure 3.3. The wide range of reactivity exhibited and o values covered, as well as the good fitting of the lines for the Zn reaction, permits an estimation... 

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