Sigma constants

Two modified sigma constants have been formulated for situations in which the substituent enters into resonance with the reaction center in an electron-demanding transition state (cr+) or for an electron-rich transition state (cr ). cr constants give better correlations in reactions involving phenols, anilines, and pyridines and in nucleophilic substitutions. Values of some modified sigma constants are given in Table 9.4. 

Hansch-Leo hydrophobic substituent parameter Hammett sigma constants Modtried Swam Lupton constants... 

This phenomenon is not possible in p-nitrobenzoic acid hence, p-nitrophenol is a stronger acid with respect to p-nitrobenzoic acid than is expected on the basis of a comparison of substituents in which this resonance delocalization is not an important factor. It was, therefore, recommended that Op = 1.27 be used for p-nitro derivatives of phenols and anilines, rather than the Op = 0.78 given in Table 7-10. These enhanced sigma constants, symbolized a, apply primarily to electron-withdrawing groups in reactions aided by low electron density at the reaction site. 

Values subject to slight corrections to conform to certain relationships between sigma constants. 

Hammett-Taft sigma constants Electron density TT-Bond reactivity Electron polarizability Dielectric constant Dipole moments Ionization potential Electron affinity... 

It was found that certain substituents render nitrobenzene inert to reduction by potassium borohydride, whereas other substituents activate the reduction to azoxy compounds. The substituents which favored this reduction all had positive Hammett sigma constants (e.g., p-C 1, p-Br, p-I, p-COOH, m-C 1, m-Br, m-1, m-CHO reduced to m-azoxybenzyl alcohol and m-OC2H5). Among the by-products of the reaction of a p-halogen compound, when carried out in ethanol solution, was p-nitrophenetole. The reduction ofp-fluoronitrobenzene afforded only p-nitrophenetole [48]. 

Substituted aromatic nitro compounds having a substituent with a positive Hammett sigma constant can be reduced with potassium borohydride to the azoxy stage. 

Various oxidation and reduction reactions of substituted metallocenes have already been discussed. A large number of substituted metallocenes have been oxidized chronopotentiometrically at a platinum foil in acetonitrile solution (39, 46). Electron-withdrawing substituents decrease the ease of oxidation, while electron-donating substituents increase the ease of oxidation with respect to the parent metallocenes. A plot of chronopotentiometric quarter-wave potentials, El, vs. Hammett para-sigma constants shows a definite linear relationship. The Ei s for ruthenocene and osmocene indicate these metallocenes are more difficultly oxidized than ferrocene, in agreement with earlier qualitative observations (18). 

In some related studies, Little and Eisenthal have found that a plot of similarly obtained pKA values for substituted ferrocenecarboxylic acids vs. Hammett para-sigma constants yields a satisfactory straight line (52). They have also recently measured the second-order rate constants for the esterification of a series of heteroannular substituted ferrocenecarboxylic acids with diphenyldiazo-... 

Alkyllithium-transition metal halide catalysis is completely different from the sodium ketyl and alfin catalysis. Natta, Danusso, Scanesi and Macchi (36) have found that the polymerization of styrene and substituted styrenes by titanium tetrachloride-triethyl aluminum catalysts was different from the above anionic systems. A plot of the log of the rate of the polymerization against Hammett s sigma constant produced a straight line with a rho value of —1.0. Electron releasing groups facilitated this polymerization. 

Comparison of acidities and sigma constants for three benzoic acids... 

Values of Hammett sigma constants are listed in Table 7.1. Taft sigma (tr ) values may be used similarly with respect to aliphatic and alicyclic systems. Values of a constants are also listed in Table 7.1. 

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