Taft constant

Hammett and Taft constants, S, 107 homolytic substitution, S, 5 hydrogen exchange, S, 57, 69-70 hydroxy... 

Voltametric studies (34, 37) have revealed the electron transfers Mo(R2c/fc)4 Mo(R2C fc)4° MoCRjdfc) Mo(R2half wave potentials for the processes 0 +1 and + - + 2 depend upon the nature of R. The dependency can be described by the Taft relation =p2a where a is the Taft constant for the N-bonded substituents R. The rather low values of p indicate that the redox orbitals have mainly a metal character, the mixing of these orbitals with those of the ligands is rather small. This conclusion is in accordance with the interpretation of the electronic spectra and the results of Extended Hiickel MO calculations (37). Only the compound with R = Ph does not fit into the Taft relation, the Ej 2 value is lower than... 

Although c is an e irical paraseter, it can be correlated to sore general solvent strength scales, such as the Kavlet-Taft constants [251] and Reichardt s E,(30) scale of solvent polarity [252]. 

Nuclear Quadrupolar Resonance. - A variety of dichlorophosphanes have n.q.r. 35C1 resonances whose average for each compound correlate linearly uith the total Taft constants.103 The spectra of a number of po1yf1uoroary1 hexachlorophosphazenes have been studied.104 A n.q.r. study of pentafluorophenyl-... 

The interactive properties of the derivatives of benzene vary widely, depending upon the nature of the substituent and its influence upon the aromatic ring. The effects of substituents have been categorized and quantified through the introduction of first the Hammett and then the Taft constants, which were obtained through the analysis of linear free-energy relationships (Exner 1988). The electrostatic potentials of benzene derivatives provide another means of ascertaining how the substituents affect the interactive behavior of the aromatic systems (Murray, Paulsen, and Politzer 1994). 

The Hammett equation has been further extended to include substituents of aliphatic systems too. In this case the constants that quantify the inductive effects, which are represented as o, have different values compared to those concerned with aromatic systems and are defined as Taft constants. Some values of these latter are also reported in Table 1. 

These complexes undergo two consecutive one-electron reductions with features of chemical and electrochemical reversibility both in MeCN and CH2C12 solution.2 Table 2 summarizes the formal electrode potentials for the first reduction process and the sums of the Taft constants of the various substituents for each complex. 

Table 2 Formal electrode potentials ( V vs. SCE) for the first reduction process of [Fe3(CO)s(RC2R)2] complexes together with the relative Taft constants...

Figure 2 The dependence of the reduction potential of the series [Fe3(CO)8(RC2R)2] on the sum of the Taft constants of the different substituents...

The linear dependence of the sum of the four substituents Taft constants suggests that the electron density due to the substituents is delocalized along the entire butadiene system. 

Examples for compounds are given in Figure 8.1, and the regression analysis equation is provided below for the QSAR of triazine derivatives in photosynthesis (Draber, 1992). The inhibitory potency expressed as a pl50 value is equal to a lipophilicity parameter tt (log of the partition coefficient P), an electronic substitution parameter a (the Hammett constant) and to a lesser degree to a steric component Es (the Taft constant). 

Substituent Hammett constants Inductive constants Swain-Lupton constants ... 

The exchange of equation 175, proceeds nearly quantitatively at 80-90 °C in both aprotic and polar protic solvents, including water, in the presence of metallic copper or copper(I) compounds368 in over 90% yield. The exchange reaction is first order both in the copper mono-iodide and in the iodo-organic compound. The reactivity of the compounds investigated correlated with the Hammett-Taft constants. The exchange proceeds probably by the mechanism illustrated by equation 176 367,371. 

The reactivities of the R groups in the alcohols correlated with the combined Taft constants, Iff, of the substituents in the Alkoxy groups. When the Zo values for X and Y in the dialkoxy tellurium tetrafluorides were greater than + 1.75 further reaction with alcohols occurred, however, when Zo was less than + 1.75 no reaction was observed1. 

It is instructive to consider N-substituted azoles in reverse, i.e., the azole ring as the substituent linked to some other group. Hammett and Taft -constant values for azoles as substituents are given in Table 11. The values show that all A-azolyl groups are rather weak net resonance donors, imidazol-l-yl being the strongest. They are all rather strong inductive acceptors, with pyrazole considerably weaker in this respect than imidazole or the triazoles. 

Table 11 Hammett and Taft -constant values for azoles as substituents in a benzene ring <1981 JCR(S)364>...

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