Hammett o- values

In subsequent studies attempting to find a correlation of physicochemical properties and antimicrobial activity, other parameters have been employed, such as Hammett O values, electronic distribution calculated by molecular orbital methods, spectral characteristics, and hydrophobicity constants. No new insight on the role of physiochemical properties of the sulfonamides has resulted. Acid dissociation appears to play a predominant role, since it affects aqueous solubiUty, partition coefficient and transport across membranes, protein binding, tubular secretion, and reabsorption in the kidneys. An exhaustive discussion of these studies has been provided (10). 

Bamford, Jenkins and coworkers131157 concluded that many of the limitations of the Q-e scheme stemmed from its empirical nature and proposed a new scheme containing a radical reactivity term, based on experimentally measured values of the rate constant for abstraction of benzylic hydrogen from toluene (Ay i), a polar term (the Hammett o value) and two constants a and J which are specific for a given monomer or substrate (eq. 57) 146... 

They have also demonstrated that a linear energy relationship exists between pKa values of sulphoxides and the hydrogen bonding ability which was determined by IR-stretching shift of the OH band in PhOH in CC14. Oae and coworkers173 determined the pKa values of diaryl sulphoxides and found that the substituent effect does not correlate with the Hammett o values but approximately with the a values. The pKa values thus obtained are summarized in Table 19. 

For Z-a-benzoylaminocinnamic acid and methyl ester substrates, with various donor and acceptor substituents in the phenyl ring, there was no correlation between the Hammett o--values of para substituents and optical purity of the product (239). The DIOP systems hydrogenate Z isomers more rapidly than E isomers, and induce a greater optical bias. N-Acetyl substrates always gave higher optical purities than the JV-benzoyl substrates, and similarly acid substrates were better than the corresponding methyl esters (239). 

A comparison of the rate constants for the [Cun(FLA)(IDPA)]+-cata-lyzed autoxidation of 4/-substituted derivatives of flavonol revealed a linear free energy relationship (Hammett) between the rate constants and the electronic effects of the para-substituents of the substrate (128). The logarithm of the rate constants linearly decreased with increasing Hammett o values, i.e. a higher electron density on the copper center yields a faster oxidation rate. 

The electrophilic character of n-perfluoroalkyl radicals was confirmed by a correlation of the rates of addition of the n-C8F17 radical to a series of para-substituted styrenes with Hammett o values, as shown in Fig. 3. The p value is negative (-0.53), as would be expected for an electrophilic reactant [115]. 

Improved understanding and treatment of the phenyl case was developed from the recognition that the derived o-si (C6H5-) is almost identical with the "standard Hammett o- value, a parameter containing both mesomeric and inductive contributions. Since published data on the krei of ozonation of substituted phenyldimethylsilanes were available (11), and included the common point, phenyldimethylsilane, simple mathematical manipulation could bring these and the present data on to the same scale. When the Hammett o- for the substituted phenyls was... 

Substituent effects on the polarographic reduction of l-amino-4-aryl-1,2,3-triazoles and lif-4-aryl-1,2,3-triazoles of the general formula (67) have been reported. Cathodic halfwave potentials ( 1/2) determined in acetonitrile at 20 °C were between -2.1 and -2.6 V. Linear relations were found between E1/2 and the tt-electron density at C-5, as well as Hammett o--values (76ZC280). 

Ewing, D.F. (1978). Correlation of nmr Chemical Shifts with Hammett o Values and Analogous Parameters. In Correlation Analysis in Chemistry (Chapman, N.B. and Shorter, J., eds.). Plenum Press, New York (NY), pp. 357-396. 

Attempts have been made to correlate displacements of the absorption bands of parent aromatic compounds by substituents with Hammett o-values derived from the effects of the substituents on ground state kinetic and equilibrium properties. For example,... 

Table 5.9 Hammett o Values and CASPT2 and BPW91 Singlet Energies (kcal/mol) Relative to the Triplet State for R-Substituted Phenylnitrenes ... 

The use of a constants in QSAR studies started with the early finding of Hansen [13] that the toxicities of substituted benzoic acids against mosquito larvae are correlated with Hammett o values (Table 10) eq. 38 was derived after the p-nitro analog had been excluded (all analogs n = 14 r = 0.711 s = 0.427). 

Nuclear aromatic substitution does not respond to treatment with Hammett o values. A new set of substituent constants, designated a, have been derived for this purpose. The work leading up to the establishment of these constants has been reviewed and a list of these values is given [6]. The success of o values and... 

The effect of substituents on the benzene ring of the nucleophile, the substrate, or the leaving group of 5 2 reactions has been investigated by determining the 8AH, 8 AS, and 8 AG values obtained from equations such as AH = 8AH a + AH where AEI is AH for the reaction of the unsubstituted compound and 8 AH represents the difference in the AH for the reaction with a substituent with a Hammett o value on... 

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