Taft steric substituent constants

There is a considerable body of evidence both from polymerization studies and from gas-phase addition kinetics that radical reactivity is influenced by steric effects (see Section 3). Taft steric substituent constants Es are available for only a very few halogenoalkyl radicals (Charton, 1971), but it was found that log Or correlates well with E% for the few that are known (Ashton et al., 1975). 

Unfortunately, Taft steric substituent constants Eg are available for only a few radicals (47), but for these few log OR... 

Free energy parameters - the Taft steric substituent constants Eg, have been employed to quantify this idea. Eg values represent solely steric parameters and are directly proportional to van der Waals radii. A parabolic function was developed to express the relationship of LD50 to ... 

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

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 has also introduced the steric substituent constant E, which is used separately in an expression of the type shown in Eq. (3) or together with polar (T constants in a four-parameter equation. 

Fig. 15. Taft correlation with steric substituent constants (Es) in the vapour phase esterification of carboxylic acids with ethanol over Na-poisoned silica—alumina at 250°C ( ) [126] and in homogeneous acid-catalysed esterification at 40°C (°) [419], Acids 1, acetic, 2, propionic, 3, butyric, 4, isobutyric, 5, isovaleric, 6, pivalic, 7, 2-ethylbutyric.

Charton used statistical methods to investigate the contributions to Taft s s values of the size of the groups concerned and their polar effects. As a measure of group size he took the van der Waal s radius, rv, and he used o-, and crR as measures of inductive and mesomeric effects. Correlations of steric substituent constants were then carried out250 using the equation (for group x)... 

In the 1950s Taft devised a method of extending linear free-energy relationships to aliphatic systems.16 He suggested that, since the electronic nature of substituents has little effect on the rate of acid-catalyzed hydrolysis of meta- or para-substituted benzoates (p values are near 0, see Table 2.3), the electronic nature of substituents will also have littie effect on acid-catalyzed hydrolysis of aliphatic esters. All rate changes due to substituents in the latter reactions are, therefore, probably due to steric factors.17 Taft defined Es, a steric substituent constant, by Equation 2.16... 

The most popular substituent constants are electronic substituent constants, - Hansch-Fujita hydrophobic substituent constants and -> steric substituent constants such as - Taft steric constant, - Charton steric constant, -> substituent front strain, and - steric density parameter. 

Taft, R.W. (1952). Polar and Steric Substituent Constants for Ahphatic and o-Benzoate Groups from Rates of Esterification and Hydrolysis of Esters. J.Am.Chem.Soc., 74,3120-3128. 

In the early 1950s Taft (10) outlined a sound quantitative basis for the estimation of steric effects and for separating them from polar and resonance effects. This derivation follows the standard extrathermodynamic approach and is therefore empirical. The definition of steric substituent constants is closely related to polar substituent constants, for they are obtained from the same reference system (10). The polar substituent constants, however, have been shown to arise from electronic effects, and they strongly correlate with the inductive substituent constants (10, 43, 61). 

Taft equation Various equations are associated with R.W. Taft, but the term is most often used to designate the family of equations that emerged from Taft s analysis of the reactivities of aliphatic esters and that involved the polar substituent constant a and the steric substituent constant Es... 

An examination of Table 1 shows that the value of is decreased by the presence of electron-attracting substituents, and increased by bulky groups. The former effect can be attributed to de-stabilization of the carbonyl compound, and the latter to steric strain in the diol. It is therefore of interest to compare the observed values of with the polar and steric substituent constants a and derived by Taft (1952, 1953,... 

Since the lipophilic character was found important for physiological dispositions of barbiturates, several workers investigated the relationship between partition coefficient and structural parameters (Taft s polar and steric substituents constants, number of C-atoms) of these compounds.548,549 Partitioning of barbiturates was also proved to be a significant factor for their... 

Table 1.46 Taft s steric substituent constants Es for aliphatic substituents in RC02Et...

For alifatic systems an equation proposed by Taft is often applied that involves the polar substituent constant a and the steric substituent constant E... 

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