Steric substituent constants for

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. 

Table 3 Polar and steric substituent constants for ortho groups...

R.W. Taft, Jr., Polar and steric substituent constants for aliphatic and o-benzoate groups from rates of esterification and hydrolysis of esters, J. Am. Chem. Soc. 74 (1952), pp. 3120-3128. 

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. 

Fig. 3. Correlation of AHq - AHgr for the halogen transfer from CCI4 and BrCCl3 to radicals) and the steric substituent constants S f84, 85 ...

Hansch, C., Leo, A. (1979) Substituent Constants for Correlation Analysis in Chemistry and Biology. Wiley-Interscience, New York. Hansch. C., Leo, A. J., Hoekman, D. (1995) Exploring QSAR, Hydrophobic, Electronic, and Steric Constants. ACS Professional Reference Book, American Chemical Society, Washington, D.C. 

Values of steric effect substituent constants for typical groups are given in Table 4. 

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)... 

Because the electronic nature of substituents has little effect on the rate of acid-catalyzed hydrolysis of meta- or para-substituted benzoates (e.g., p for the acid hydrolysis of XC6H4COOR esters is close to zero), Taft suggested that the electronic nature of substituents will also have little effect on acid-catalyzed hydrolysis of aliphatic esters (Lowry and Richardson, 1987). Nevertheless, a strong electronic effect occurs in basic hydrolysis, as can be examined from the large p values for base-catalyzed hydrolysis of meta- or para-substituented benzoates. Hence, the effect of X on acid hydrolysis is purely steric but is a combination of steric and electronic effects in basic hydrolysis. Taft defined Es, a steric substituent constant, by Equation (5.11) ... 

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). 

These efforts were guided by the study of quantitative structure-activity relationships (QSAR) following the Hansch approach. In this method linear free-energy related and other electronic, hydrophobic, and steric substituent constants are used for a quantitative analysis of the possible ways in which substituents may modulate bioactivity in a congeneric series. In the QSAR studies of benzoylphenyl ureas the electronic Hammett a-constants and the hydro-phobic Hansch n-constants were used. To measure the steric influences, steric substituent constants of a new type (B1,B2,B3,B4, and L) were applied which had recently been introduced by us and which give improved correlations in comparison with the steric Es constants used in the literature hitherto (21, 22). The constants B- toBj are measures of the widths of substituents in four rectangular directions. The L-constant accounts for the length of a substituent ... 

The symbols r, o-, and Es refer to the substituent constants for partition, polar, and steric factors (1). 

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