Hammett-type effects

The problems encountered in any attempt to treat the transmission of the effects of one substituent in a disubstituted heterocycle through the heterocyclic nucleus to a reaction site in the other substituent (i.e. the side-chain) are enormous, and it is consequently not surprising that relatively little work has been done in this area. First, while in benzene derivatives there are three positions, i.e. three relations between substituent and reacting side-chain to be considered, the number of complexities is much greater in heterocycles. Thus, e.g., in pyridine alone, after elimination of the orientations involving a vicinal relationship between substituent R and the side-chain Y to which no Hammett-type relation is likely to be applicable, the following cases should be considered ... 

There are numerous other transmission mechanisms for substituent effects. Those connected with unsaturated frameworks, such as mesomeric effects and correlations with it charge densities and Hammett-type reactivity constants, and intramolecular hydrogen-bonding effects are beyond the scope of this article and are discussed elsewhere (1,8,25,57). 

A detailed discussion of the effects of sulfenyl, sulflnyl, and sulfonyl groups on acidities and homolytic bond dissociation energies of adjacent C—H and N—H bonds has included interpretation of the following Hammett-type correlations val-... 

According to the reaction medium the results in the third column of the Table V should be approached in a different way. A rigid polyethylene film suppresses phenol formation almost entirely by keeping the radicals in the solvent cage. Furthermore, all substituents decrease the quantum yield relative to hydrogen. This effect cannot be explained solely by a Hammett-type treatment of electronic effects. The effect of the substituent size is particularly pronounced in the case of the p-isooctyl substituted ester whose quantum yield is reduced drastically when compared to that of the p-methyl substituted... 

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). 

Bowden94 studied the transmission of substituent effects through C=C in the esterification of 3-substituted acrylic acids with diazodiphenylmethane (DDM) and in the ionization of the same acids. The ap values of the substituents were used to characterize their electronic effects for Hammett-type correlations. In esterification with DDM at 30 °C, the p values of the trans acids and the cis acids were 1.682 and 1.772, respectively, in ethanol... 

For the reaction of polyvinyl acetate radicals and substituted phenols (Fig. 1), Simonyi et al. (1967c) suggested a Hammett-type correlation for the isotope effect dependence ... 

For the quantitative treatment of substituent effects in such reactions, Brown proposed (Brown and Okamoto, 1957) a new Hammett-type structure-reactivity relationship, the Brown equation (1), in terms of substituent constant instead of a in the original Hammett equation. 

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... 

Stereoelectronic effects that cannot be viewed as dipole-dipole interactions most probably operate in many concerted epoxidations, however, they are mostly difficult to identify unambiguously because their influence is modest and easily masked by steric or polar effects they can be visualized in Hammett-type correlations by a series of compounds differing only in remote substitution125-127 or substitution with substituents in the plane of the double bond208. 

Most of the Hammett-type constants pertain to aromatic systems. In evaluating an electronic parameter for use in aliphatic systems, Taft used the relative acid and base hydrolysis rates for esters. He developed equation 1.38 as a measure of the inductive effect... 

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