Hammett-type relationship

Physical phenomena other than rates and equilibrium constants can be correlated by Hammett-type relationships. For example, as Figure 2.4 shows, in 13C nuclear magnetic resonance spectroscopy (called Cmr) the chemical shift of the cationic carbon in 17 is correlated by Brown s cr + values.21 And the C=0... 

R-type para substituents lead to large increases in the rates of thermolysis of azidobenzenes.44 hi nitrobenzene at 120 °C the rates follow the Hammett-type relationship log A = —5.44 — 2.33indicating conjugative stabilization of a nitrene-like transition state. + R-type ortAo-substituents cause even larger rate increases, suggesting a special resonance proximity effect (34). 

Step A—Association of Ozone with the Silicon Atoms. The linear Hammett-type relationship of Figure 1 and Equation 1 indicates a slope, /o, of —1.25. This negative value denotes electrophilic attack by the ozone and/or the development of a partial positive charge on silicon in the transition state. Since the silicon is relatively electropositive, an electrophilic attack by ozone on silicon seems unlikely. The hydrogen bound to silicon, however, is hydridic in character and is the likely site of attack... 

The rate constants for the solvent-recombination process of the carbocations [3C (X,Y,Z)] were determined by the use of the azide clock method (Richard etal., 1984 Richard and Jencks, 1984a,b,c McClelland et al., 1991) and the rate constant of the forward reaction was derived using (38b) as /Ch = /CwXr+ (McClelland et al., 1989,1991). While ordinary Hammett-type relationships were found to be inapplicable to the substituent effects on solvent recombination, there is a rate-equilibrium correlation for all available data on triarylmethyl cations, shown as the linear log/c , vs. p/Cr<- plot, in Fig. 34 with a slope of 0.64. Such a relationship was earlier suggested by Arnett and Hofelich (1983) and Ritchie (1986). The correlation of ky, with the cr scale was... 

The Hammett-type relationship for this conceptual idea of distribution is... 

To estimate the and constants for unsubstituted pyridyl groups, Pasternak and Tomasik attempted to use the dependence of the frequencies of symmetric (vj and asymmetric (VaJ NO2 stretching vibrations in substituted nitrobenzenes (2) on the Hammett substituent constants (75BAP923). The pyridyl group constants obtained from the data of the IR spectra of the nitrophenyl pyridines in CHClj and CHBrj have considerable scatter in their values depending on the solvent. Most of the and values estimated from Vs-NOa Jiff widely from the respective values obtained from v noj even for the same solvent. Four of the seven Hammett-type relationships used by the above authors to calculate the a values have a low correlation coefficient (r = 0.949-0.965). The method in question, just as polarography, is suitable only for a rough estimation of constants. 

Fujita, T. (1983) Substituent effects in the partition coefficient of disubstituted benzenes bidirectional Hammett-type relationships. Prog. Phys. Org. Chem., 14, 75-113. 

In 1962, Hansen [14] derived a first Hammett-type relationship between the toxicities of substituted benzoic acids and the electronic a constants of their substituents. However, later, it turned out that this was a chance correlation that only resulted from a close interrelationship between the Hammett a parameter and the lipophilicity constant % (Sec. 4 Eqs. (42) and (43)). In the same year, for the very first time, a nonlinear multiparameter equation (Eq. 6) [15] was used to describe biological activity values ... 

From a cheminformatic perspective, the most important consequence of the Hammett equation is that it separates explicitly the contribution of environment from that of chemical structure in the prediction of an outcome (in this case, a reactivity property). As such, the Hammett equation represents one of the earliest attempts to predict molecular behavior on the basis of chemical structure alone. Notably, however, later investigators experienced difficulties when trying to apply Hammett-type relationships to biological systems, indicating that additional structural determinants need to be considered [32, 37]. 

To summarise, then, it is possible to predict kinetic characteristics from Hammett-type relationships, provided that electron directing and steric characteristics are similar in a series of substituted amines. The mechanism proposed by De La Mere [29] for the oxidation of, for example, a secondary amine, see reaction (18), underlines the complexity of the kinetics that can be expected, unless one particular transition state and one reaction path is the major reaction route. 

The reaction of substituted benzoic acids with c/s-[Co(en)2Cl(OH2)] has been studied and Hammett-type relationships developed. ... 

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