Hammett-type correlation

Phenyl aryl cyclopropenones16 were cleaved by methanolic KOH to a mixture of cis aryl cinnamic acids (318/319 R = phenyl, R = aryl), whose rates of formation gave rise to a linear Hammett-type correlation with a values in the range of -0.268 to +0.373 and p = 0.75. This also indicates that cleavage yielding the more stable carbanion is preferred. Interestingly, ortho-substituted aryl phenyl cyclopropenones gave only a-phenyl-0-aryl acrylic acids (319 R = phenyl, R = aryl), which was explained in terms of steric interactions. 

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-... 

Advantage has been taken of the ready accessibility of eleven para-substituted trityl and 9-phenylxanthyl cations, radicals, and carbanions in a study of the quantitative relationship between their stabilities under similar conditions.2 Hammett-type correlations have also been demonstrated for each series. Heats and free energies of deprotonation and the first and second oxidation potentials of the resulting carbanions were compared. The first and second reduction potentials and the p/CR values of the cations in aqueous sulfuric acid were compared, as were calorimetric heats of hydride transfer from cyanoborohydride ion. For radicals, consistent results were obtained for bond dissociation energies derived, alternatively, from the carbocation and its reduction potential or from the carbanion and its oxidation potential. 

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... 

Violation of a Hammett-Type Correlation When the Reaction Passes from the Classical to the Ion-Radical Mechanism... 

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 ... 

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. 

NaBH4 reduction of 3-substituted 5,5-dimethylcyclohex-2-enones in alkaline solution of water dioxane occurs exclusively at the 1,2-positions. The rate of reduction is strongly dependent on the 3-substituent. A Hammett-type correlation revealed similar reaction characteristics to those of borohydride reduction of substituted acetophenones. 

The Hammett-type correlation for the rate constants of 5-allyl-5-R-barbiturates has been reported by Carstensen et al. and suggested for use in stability predictions.569 Similar correlations were also found for the hydrolysis of 5-arylidenebarbituric acids.363,567 Linear free energy relationships have also been reported for dissociation constants,45,51 polarographic half-wave potentials,570 fluorescence70 and luminescence phenomena,71 and 13C-NMR chemical shifts129 for different classes of barbituric acid derivatives. Application of the dual substituent parameters method in LFER analysis of barbiturates, using Taft s polar and steric constants for various chemical and physicochemical properties, was also evaluated.571... 

Taft equation. Taft (1956) has extended the Hammett-type correlation to aliphatic systems. Because steric effects of substituents in aliphatic systems cannot be ignored as they were for m- and / -substituted benzene compounds, Taft recognized the need to develop separate terms for the polar and steric effects for substituent constants. Based on the observation that the acid-catalyzed hydrolysis of meta- and para-substituted benzoic acid esters are only slightly affected by the electronic nature of the substituent group (p values are near 0), Taft concluded that the acid-catalyzed hydrolysis of aliphatic esters would also be insensitive to polar effects of substituent groups. Any effect on rate due to substituent groups could therefore be attributed to steric effects. Taft defined a steric substituent constant, E, by ... 

In an attempt to quantify the variation in redox potentials, we have developed a Hammett-type correlation. The Ru(II)/Ru(III) oxidation potential data are concerned with a process in which positive charge is developing in the transition state. Accordingly it is most appropriate to use Hammett 0+ parameters in any correlation. In practice, we obtain similar correlations with the use of either o or parameters. We next defined a mean 0+ value for any specific complex in terms of the parameters associated with the substituents on the Xtpy and Ytpy ligands ... 

Coordination compounds with organonitrile ligands are of considerable interest due to the increased susceptibility of the coordinated nitrile to nucleophilic attack by reagents such as hydroxide ion. Rate accelerations of 10 in base hydrolysis (nitrile-> amide) have been observed. Two new aromatic organonitrile complexes of [Co(NH3)5] with 2- and 4-nitroben-zonitrile have been prepared, and their base hydrolysis to coordinated carboxamides studied. The second-order rate constants for base hydrolysis are 180 4 and 510 90 M s at 25°C and / = 1.0 M for the ortho and para isomers, respectively. Rate constants for the hydrolysis of 11 coordinated aromatic nitriles follow a Hammett-type correlation with log koH = l-93o- + 1.30 at 25 C and / = 1.0 M. Carbon-13 NMR studies of the free and coordinated nitriles indicate similar chemical shifts, so that the variation in the slope of the Hammett plots for free- and coordinated-nitrile hydrolysis is a transition-state effect rather than a ground-state phenomenon. 

The effects of solvent on the reactivity of the solvated electron have been discussed. The rates of reaction of e jj with various compounds have been measured in liquid 2-methyltetrahydrofuran, and a comparison with data in other solvents suggests that neither a Hammett-type correlation of rate constants with substituent constants, a, nor the correlation with electron absorption coefficients in the gas phase can be used to describe the reactivity of these solutes towards the electron in solution. 

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