Hammett’s substituent constants

In Scheme 7-1 kx and kn refer to the rate constants for a benzene derivative, in our case the benzenediazonium ion, bearing a substituent X in the 3- or 4-position, and the corresponding unsubstituted benzene derivative respectively. The term p is Hammett s reaction constant for the reaction, and o is Hammett s substituent constant which is, at least in principle, independent of the nature of the reaction but different for the 3- and 4-positions. A plot of log kx (or log kx - log kH) versus o should give a straight line. Its slope (positive or negative) corresponds to the reaction constant p. Equilibrium relationships are treated analogously. 

The F CFs chemical shifts of each of the 5- and 5e-(trifluoromethyl)di-benzothiophenium and -selenophenium triflates were linearly correlated with Hammett s substituent constants Op or Om, as evident from Fig. 1. This indicated that the electron density of the trifluoromethyl group is linearly correlated with the electronic nature of the substituents attached to the dibenzoheterocyclic salts. These F CF3 chemical shifts are in good agreement with the order of trifluoromethylating power, as discussed below. 

QSAR Models with Hammett s Substituent Constant... 

The rate constants for the reactions of eaq with aromatic compounds depend very strongly on the substituent. Benzene itself has a relatively low reactivity, k = T4 x 107 M -1 s 1, phenol and aniline are even less reactive, but substituents such as CN, Br, and N02 increase the rate constant almost to the diffusion controlled limit. The rate constants for several series of mono- and disubstituted benzenes were correlated with Hammett s substituent constants a and showed good linear relationships. 

The effect of substituents on the rate constant for reaction of H with benzene was also examined. Correlation with Hammett s substituent constants was possible and showed that the hydrogen atom behaves as a slightly electrophilic reactant (Neta, 1972a, and references therein see also Brett and Gold, 1971, 1973 and photochemical studies by Pryor et al., 1973). 

Table 4. Taft s and Hammett s substituent constants of vinyloxy and vinyl compounds...

Dialkyl, alkyl aryl, and diaryl trithiocarbonates are reduced in DMF to the radical anion, which loses a thiolate ion [146-148]. The rate of the cleavage was measured with CV for substituted diphenyltrithiocarbonates, the rate constant of the cleavage was found to vary linearly with Hammett s substituent constant a. The anion radicals of mono- and... 

Abstract The control elements that did not find mention in the earlier chapters are dealt with here. The prominent among these elements are spiroconjugation, peris-electivity in pericyclic reactions, torquoselectivity in conrotatory-ring openings, ambident nucleophiles and electrophiles, a-effect in nucleophilicity, carbene addition to 1,3-dienes, Hammett s substituent constants, Hammond postulate, Curtin-Hammett principle, and diastereotopic, homotopic, and enantiotopic substituents. 

Keywords Spiroconjugation Periselectivity Carbenes Ketenes Torquoselectivity Ambident nucleophiles and electrophiles a-effect Hammett s substituent constants Hammond postulate Curtin-Hammett principle Diastereotopic Homotopic Enantiotopic substituents... 

The more negative the a value of a group, the stronger its electron releasing properties, reflected by an increase in the pKg value of the substituted acid. The nearer the value of p is to zero, the less sensitive to substitution is the degree of dissociation. Hammett s substituent constants are quoted for only m- and p-sub-stituents o-substituents behaved irregularly because of steric factors. Substituted aliphatic acids also failed to correlate with the benzoic acid series. Deviations of o-substituted aromatics and of aliphatics are shown in Figure 5.1. 

In an early paper on this subject, the action of nuclear substituted phenoxy-acetic acid auxins on plants was investigated [79]. There is evidence that one of the points of attachment of these molecules to the active site is the position ortho to the carboxyl group. The attachment was therefore considered to be influenced by the electron density of this position, as predicted by Hammett s substituent constants (p. 210). kj was therefore expressed as op, the choice of o depending on whether the substituent was meta or para to the position of attachment. Substitution for f(a) and k in Equation (59), and taking logarithms gives Equation (60). 

The resulting empirical relationship between electrofugality and group nucleophihcity is linear and reflects the inductive substituent effect, as given by Hammett s substituent constant a... 

Abstract - Empirical parameters of solvent polarity seem now-days as useful for the prediction of solvent effects as Hammett s substituent constants for the estimation of substituent effects on chemical reactions. The extreme sensitivity of the position of the long-wavelength UV/VIS absorption band of pyridinium-N-phenoxide betaine dyes not only changes in temperature (thermo-solvatochromism), external pressure (piezo-solvatochromism), and the introduction of substituents, but also to small changes in solvent polarity (negative solvatochromism) has been used to establish a comprehensive empirical scale of solvent polarity, called the E.p-scale. In addition, the influence of solvents and substituents on the chemical reactivity of the pyridinium-N-phenoxides has now also been studied. 

It seems that empirical parameters of solvent polarity such as the E-p-scale are nowadays as useful for the prediction of solvent effects as Hammett s substituent constants for the prediction of substituent effects with one and the same standard molecule. 

Further evidence in support of this proposed two-step mechanism was obtained by studying the reactions of a series of /lara-substituted benzaldehydes with semicarbazide to determine the electronic effects upon the individual steps of the reaction . The equilibrium con- stants for the formation of semicarbazide addition compounds show a Unear logarithmic correlation with Hammett s substituent constants, with a p value of +1-81. The aldehydes are therefore destabilized, relative to the addition intermediate, by electron-withdrawing substituents. The rates of acid-catalyzed dehydration show an almost equal and opposite p value of — 1-74, indicating that dehydration is euded by substituents capable of electron donation (Figure 3)... 

Later studies by Koser and Liu also demonstrated that the heavy atom effect promoted the formation of 1 derived from the Tj state of ACN in the presence of p-X-bromobenzenes (X = OCH, CH, H, CHO, CFj) in methanol however, the cisoid/transoid ratio does not vary linearly with Hammett s substituent constant,... 

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