Hammett cr-constants

The value of a for the methyl esters was chosen as 2.48, (not 1.00) so that the or values would be similar to the Hammett cr constants. Once established, the or scale can be used for many such reactions. Table 10-3 presents an abbreviated list of or constants.12... 

HO -Addition to Monocyclic Aromatic Compounds and Biphenyls. The rate constants for HO addition to aromatic rings are simply calculated by using the rate constant for the unsubstituted compound and the Hammett-cr+ constants. Thus, kHO, (Ar) for addition to a substituted benzene ring can be estimated by ... 

The Hammett cr+ constant for the 4(5)-position of imidazole is around -1 for C-2 it is of the order -0.8 (86CHE587). The electrophilic substitutions which do occur at the 2-position invariably involve preformation of an anion at that position. The 2-proton, which should be the least active in a conventional SEAr sense, turns out to be the most labile over a wide pH range, and there is a marked rate acceleration on going from imidazole to imidazolium cation. Any negative charge generated at C-2 is stabilized by the adjacent pyrrole-type nitrogen (see Section 3.4.1.8.2). 

Electron demand at the incipient carbocation is also important in determining whether or not homoallylic T aHicipation Takes place. Gassman and Fentiman have plotted the logs of the rates of solvolysis of both 57 and 58 in dioxane-water vs. the Hammett cr+ constants of the X substituents. They... 

The slope of the empirical relationship correlating substitution in toluene is 1.32. The value of b predicted by the Hammett side-chain constants is 1.7. This observation is further confirmation of the inapplicability of the Hammett cr-constants to aromatic substitution... 

The cr-constants (Jaffe, 1953) are based on benzoic acid ionization data of greatly variable quality (McDaniel and Brown, 1958). In some cases the uncertainty in the pKA values introduces an uncertainty of + 0.1 unit in the Hammett cr-constants. To avoid the inclusion of this doubt in the cr+-constants only the thermodynamically based [Pg.88]

The structure-reactivity relationship of acidic organophos-phorus compounds is well demonstrated by mono-esters of p-substi-tuted phenylphosphonic acids. The acidity of these organic acids increased as the polar nature of the substituents enhanced. A linear free energy relationship exists between the pKa value and the Hammett cr constants in acidic p-substituted phenylphospho-nates. When these structure parameters are plotted either against the t POO" asym. or against the 31p chemical shift of their dicyclo-hexylammonium salts straight lines resulted in both cases. 

Second-order rates for reduction of l-(z-benzyl)nicotinonitrile cations, z-BNN +, by l-(x-benzyl)-l,4-dihydronicotinamides, x-BNAH, in aqueous acetonitrile correlate well with Hammett cr-constants (Bunting and Brewer, 1985). For substituents (z) in the nitrile, p = 0.63 for reductions by BNAH,... 

Using Hammett cr constants, Garg et al. rederived QSAR 1.102 for the same set and QSAR 1.103 and 1.104 for the diverse set of multi-, di", and monophenols, which were se-... 

Values of these parameters are reported in Tables 4-9. Estimated values of Hammett cr constants can be calculated from the relationships ... 

This means that substituents that can withdraw electrons conjugatively will have a greater effect on the acidity of phenols (reaction 3.9) than on the acidity of benzoic acids. The difference will only be appreciable at the para position for the meta position, no through resonance is possible. If we make a plot of logATA for the acidities of mcta-substituted phenols against Hammett cr constants, we find that the points lie on a straight line, the slope of which gives the p value for the reaction as +2.23... 

AG°, A/F, AS° and log Ki values are summarized in Table 7. The Ki values show a strong dependence on Z. The log Ki values for Z = Me, H, Br and CF3 correlate well with the Hammett cr constants from which one calculates p = 2.2 the point for Z = MeO deviates negatively by about 0.9 log units from the Hammett line defined by the other substituents. 

The publication was novel in at least six respects (1) It provided a quantitative relationship between the biological potency and physical properties of a compound (2) It recognized the quantitative importance of hydrophobicity to affinity (3) It put the hydrophobic effect of a substituent, n, on a scale similar to that of the Hammett cr constant for electronic effects (4) It provided a mechanism to identify the optimum value of some property as it relates to potency, for example, the parabolic fit of n with (5) It considered the additive effect of different physical properties on potency (6) The fit to the equation used a computer. 

Enthalpies of activation are 7-lOkcalmor and the negative entropies (-30 to -40 cal cal ) are consistent with the associative kinetics. The linear relationship between log k and either the pK of RCO2H or Hammett cr constants for XC6H4CO2H suggest that the mechanism involves heterolytic 0—0 cleavage and electrophilic insertion of oxygen into the Pd—C bond, in what is proposed to be a concerted process/ ... 

Partition coefficients (w-octanol/water), log P Fragment partition constants, tt Hammett cr constants, cr Field and resonance constants, F, R Taft steric constants,... 

Descriptors molar volume, surface area, Hammett cr constants, molar refrac-tivity, quantum-chemical descriptors (HOMO, LUMO, hardness, electronegativity, heat of formation, ionization potential, dipole moment) and connectivity indices. 

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