Taft s constant

The charge at the vanadium in the usual dithiolene complexes may be estimated49 as less than 2, and these complexes are therefore included in the section on low valence states. Various tris(dithiolene) complexes have been isolated and one-electron oxireductions studied by polarographic and voltammetric techniques. Table 3 summarizes methods of preparation and electrochemical behaviour. Schrauzer and co-workers50 correlated electrochemical data with Taft s constants the observed linear correlation reflected the ligand n orbital origin of the orbitals involved in the redox process. 

Here, k and k0 are the rate constants for the reaction initiated by a tertiary amine and a standard tertiary amine, respectively, q is the reaction constant, a are the substituent parameters, 8 is the steric parameter of the reaction and En(En) are steric parameters of the substituents (identical with the Taft s constants ES(E9)). 

The aliphatic systems are well correlated by Taft s constant (a ). Halogenated alkanes show a negative linear relationship with four datapoints (trichloromethane, 1,1,2-trichloroethane, 1,2-dichloropropane, and 1,2-dibromo-3-chloropropane), as shown in Figure 7.4. 

Correlation between the rate constants of substituted alkanes and Taft s constants. 

Taft s constants were used as molecular descriptors to correlate with activation energy and kinetic rates, respectively. They were used because they describe the inductive effects of the substituents in aliphatic compounds. Taft s constant showed a poor correlation (r2 = 0.68) for aliphatic compounds however, the correlation between activation energy and Taft s constant was... 

Chemical Kinetic Class Parameters n Ehomo ft Elumo ft Hammett s Constant ft Taft s Constant... 

Figure 5 gives a plot of the values of log of taken from Table 10 against Taft s constants aT (Taft and Lewis, 1958) which are also given in this table. As can be seen, there is a satisfactory linear relation which... 

The Eu(III) absorption band due to the transition 7Fo —> 5Do and the shifts in this band due to the nephelauxetic effect were studied for various fi-diketones. A correlation between the energy of 7Fo —> 5Do transition and the sum of Taft s constants [186] aRI + Rll has been established as shown in Fig. 8.42. 

The quantitation of steric effects is complex at best and challenging in all other situations, particularly at the molecular level. An added level of confusion comes into play when attempts are made to delineate size and shape. Nevertheless, sterics are of overwhelming importance in ligand-receptor interactions as well as in transport phenomena in cellular systems. The first steric parameter to be quantified and used in QSAR studies was Taft s constant (157). is defined as... 

The fundamental assumption of SAR and QSAR (Structure-Activity Relationships and Quantitative Structure-Activity Relationships) is that the activity of a compound is related to its structural and/or physicochemical properties. In a classic article Corwin Hansch formulated Eq. (15) as a linear frcc-cncrgy related model for the biological activity (e.g.. toxicity) of a group of congeneric chemicals [37, in which the inverse of C, the concentration effect of the toxicant, is related to a hy-drophobidty term, FI, an electronic term, a (the Hammett substituent constant). Stcric terms can be added to this equation (typically Taft s steric parameter, E,). 

Katritzky and Topsom have reviewed the information available, largely from infrared and n.m.r. studies, concerning the distortion of the tt-electron system in the benzene ring brought about in the ground state by substituents. Of particular interest is the observation that both n.m.r. studies (of m- F and chemical shifts) and infrared investigations (of the intensities of bands due to certain skeletal vibrations) suggest that the value of Taft s [Pg.226]

A good correlation between log fc/fcp is the rate constant for methylthiazole), and the Taft s parameter (261J has been found for this series. Table IIT51 indicates this correlation. 

Hammett and Taft Benzene Ring (sijcr(S)364)... 

In this definition ko is the rate constant for CH3COOR and k is the constant for RCOOR thus = 0 for R = CH3. Table 7-11 lists some values. Taft s Es steric constants are in some instances based on averages of several different reactions, so MacPhee et al. have defined a steric constant Es by Eq. (7-52) for a single reaction, namely, the acid-catalyzed esterification of carboxylic acids in methanol at 40°C. Es values are also given in Table 7-11. Additional Es and Es values are available. 

Oj and aR are respectively the inductive and resonance constants of Taft s analysis of ordinary composite Hammett constants (values obtained by Charton were used) and v is the steric substituent constant developed by Charton161,162. The intercept term, h, notionally corresponds to log k for H as an orfho-substituent, but is not found in practice always to agree closely with the observed value of log k for the parent system. 

Taft s Terminal olefins, synthesis of 629 Tertiary alcohols, allylic, epimerization of 736... 

Finally, in this account of multiparameter extensions of the Hammett equation, we comment briefly on the origins of the a, scale. This had its beginning around 1956 in the a scale of Roberts and Moreland for substituents X in the reactions of 4-X-bicyclo[2.2.2]octane-l derivatives. However, at that time few values of o were available. A more practical basis for a scale of inductive substituent constants lay in the o values for XCHj groups derived from Taft s analysis of the reactivities of aliphatic esters into polar, steric and resonance effects . For the few o values available it was shown that o for X was related to o for XCHj by the equation o = 0.45 <7. Thereafter the factor 0.45 was used to calculate c, values of X from o values of XCH2 . ... 

In the nineteen sixties, there was some confusion about the value of p, the reaction constant for polar effects of alkyl groups on bromination rates, as calculated according to Taft s equation, log (k/k0) = p a. For 22 alkenes substituted by one, two or three linear alkyl groups, there is a satisfactory relation (16) between their reactivities, log k, in methanol and the sum of their... 

Fig. 1. Relationship between oxidation peak potentials (Ep) of fluoroalkyl sulfides and Taft s substituent constants (

The van der Waals volume can be related to the hydrophobicity of the solutes, and retention of molecular compounds can be predicted from their van der Waals volumes, 7r-energy, and hydrogen-bonding energy effects [72-74], It should be noted that the isomeric effect of substituents cannot be predicted with good precision because this is not simply related to Hammett s a or Taft s other hand, the hydrophobicity is related to enthalpy [75], Retention times of non-ionizable compounds were measured in 70 and 80% acetonitrile/water mixtures on an octadecyl-bonded silica gel at 25-60°C and the enthalpy values obtained from these measurements. 

The rate of acid-catalyzed degradation of the penicillins also depends largely on the nature of their acylamido side chain. Structure-activity-relationship studies undertaken for the rational design of orally active penicillins have shown that the stability in gastric juice increases with the sum of Taft s inductive substituent constants (of values) of the 6-amino side chain [95]. 

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