Taft relationship

The reactivity of aliphatic compounds, where steric hindrance near the substituent site is not important, is aceommodated by the Taft modification of the Hammett equation  

---

As expected, deviations from (16) are found for alkenes bearing one or several branched groups. In order to take into account the steric effects, the authors applied the extended Taft relationship, log(/c//c0) = p a + S Es. The resulting correlation is (17) where p, now —5.4, is considerably more... 

The series 3-6 in Table II constitute a strong support for this approach to elimination mechanisms. The slopes of the Taft relationships p ) change with the nature of the catalysts and could be correlated with their other intensive properties, which were determined independently (Fig. 3). The... 

The structure effects on the hydrogenation rate of ketones also have been used for comparisons of catalysts. Simonikova, Ralkova, and Kochloefl 113) have pointed out that the slopes of the Taft relationships for series 91-93 for copper, platinum, and rhodium catalysts, together with the similar results of Iwamoto, Yoshida, and Anouma 112) for a nickel catalyst (series 90), exhibit an opposite trend from the d character of the metals. 

These and other investigators have shown, for many systems where a biological response can be measured quantitatively, that relative biological response can be expressed as functions of various molecular properties by using equations of the same forms as Hammett or Taft relationships ... 

The dehydration rate depends very strongly on substitution on Ca. Large differences in reactivity of primary, secondary and tertiary alcohols over solid catalysts were reported as early as in 1931 by Dohse [90]. Also, substituents on Cp affect the rate. Both influences can be quantitatively described by the Hammett and Taft relationships the published correlations are summarised in Table 4. Of special interest is the extensive set of alcohols of the type R R2R3COH [56], which includes primary, secondary and tertiary alcohols and gives a single Taft correlation with an excellent fit. The values of p and p which can give information about the mechanism and catalyst nature will be discussed in the following sections. 

The hydroxide-catalysed hydrolysis of aliphatic amides is generally first-order in hydroxide ion. Bruylants and Kezdy30 showed that, for a series of alkyl amides RCONH2 ranging from R = CH3 to R = C13C, the rates of alkaline hydrolysis obey the Taft relationship... 

Based on the o value for the HOCH2 group and the Hammett-Taft relationship governing the pKa values of tertiary amines (see Reference 46a, p. 163), one can estimate that carbinola-mines are ca 1.8-2.0 pK units weaker bases than the related tertiary amines. 

These arguments do not invalidate the immense array of linear free energy relationships gathered over the past 70 years or so. Class II systems such as Hammett or Taft relationships can be excluded from these considerations because the two variables are independent. The following arguments can be employed to demonstrate experimentally that a Class I correlation such as a Bronsted or a Leffler Equation does not arise from a statistical artifact in a system under investigation. Rearranging the Leffler Equation (Equation 27) yields Equation (29). 

Table 4 Hammett and Taft relationships for some equilibria...

Following the original work of Hammett [47] several other linear Gibbs energy relationships have been derived. The most important of these is the Taft relationship [50] which allows consideration of aliphatic as well as aromatic systems and also of steric effects. The Taft equation has the same form as equation (7.11.4) but the substituent constants are defined in another way. More information about this relationship and other linear Gibbs energy relationships can be found in the monograph by Hammett [48]. 

Many reactions may be correlated by a simple Taft relationship (Eqn. 71) and the success is due to the minimal effect of steric interactions in these reactions. When steric effects compete with electronic the extended Pavelich-Taft equation (Eqn. 65) is successful [53b,55j. Steric parameters may be defined by Eqn. 72 for the hydrolysis of ethyl esters. 

Figure 3.12. Taft relationship of alkyl substituents in acids in esterification of methanol ( ), ethanol (A), (x) -1-propanol and 2-propanol ( ).

A similar influence of the pH was seen in haloacetylphosphonic acids. Using the Taft relationship for ketone hydration, the equilibrium constants for hydration of the dianionic... 

For an extensive list of Taft relationships for predicting the acidities of organic acids and protonated bases, see reference 12, pp. 126-135. 

The solvent dependency of Katrp was also examined in terms of Kamlet-Taft parameters for 11 solvents and then extrapolated to cover catalyst activity in a total of 28 solvents, including water. The log(KATRp) values measured in this work for Cu Br/l,l,4,7,10,10-hexamethylttiethylenetetramine (HMTETA) + MBriB are plotted against values predicted by the Kamlet-Taft relationship the line representing values predicted by the Kamlet-Taft relationship is shown in Figure 8. Predicted values of Katrp for 16 organic solvents and water are also provided, based on these solvent-independent coefficients and the appropriate solvatochromic parameters. 

Fig. 3.11 Taft relationship of alkyl substituents in alcohols in esterification of acetic (A), propanoic ( ), and pentanoic ( ) acids.

Fig. 3.13 Taft relationship for perhydrolysis of carboxylic acid, based on the steric part only.

---