Taft steric constants, relative rates

Figure 3. The relationship between the log of relative rates of reaction of 9-alkyl-10-lithio-9,10-dihydroanthracenyl anions with hexyl bromide, and the Taft steric constants for the alkyl groups...

If the assumptions underlying the Taft treatment of the separation of electronic and steric effects are valid, then the relative rates of acid-catalyzed reactions of esters should be a measure of the steric effect. Taft " accordingly defined a steric constant Es by Eq. (7-52). 

Data listed in Table 2 include the substituent constants R1 of trialkylchlorosilanes and the relative rate constants fc(R1Me2SiCl)/A (Me3SiCl) for the reactions of the two chlorides with lithium silanolates and isopropylate (equation 39)57. The reaction rates of silanes are influenced almost exclusively by the steric effects of the alkyl groups attached to the silicon atom. The log(A rei) values of the compounds with various R1 groups give a satisfactory correlation with Taft s Es values151. Thus the steric hindrance of silyl groups follows the order listed in entry 4457 of Table 1. 

Taft (1956) used the relative rate constants of the acid catalysed hydrolysis of a-substituted methyl ethanoates to define his steric parameter because it had been shown that the rates of these hydrolyses were almost entirely dependent on steric factors. He used methyl ethanoate as his standard and defined Es as ... 

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. ... 

Taft [109] postulated that the rate of aliphatic ester hydrolysis is a function of substituent type. Using methyl as a standard substituent, he compared, for example, the effects of larger alkyls, and interpreted the results by means of two constants characterizing the steric and polar behaviour of the alkyls. In copolymerization theory, the Taft equation was used many times to describe the relative reactivities (See Chap. 5, Sect. 5.2) in homologous polymer series... 

The linear plot in Figure 7 of log( —j<—) (see Table 2, footnote g) and the Taft-Hammett (23) ap parameter for these complexes Ru(bpy)2(4 -X-stilb)2 + arises because the increasing ap parameter reflects the more anodic redox potential for the vinyl ligand, which results in a greater concentration of initiator, a more rapid polymerization reaction, and hence a greater amount of polymer deposition on the electrode surface. Such a correlation is indeed remarkable because it implies a greater surface coverage per unit time only if factors such as steric bulk, polymer conformation and rate of polymer precipitation remain relatively constant for the substituted trans-stilbazole series. 

Taft proposed a substituent constant, polar effect of alkyl substituents in aliphatic systems. This method is based on the assumption that resonance is unimportant in aliphatic systems and that steric effects are the same for ester hydrolysis whether in acid or base, so only the polar effect of the substituent is different under the two reaction conditions. The value of a for a substituent, R, was based on the rate constants for acid-catalyzed and base-promoted hydrolysis of the ester RCO2R relative to those for CH3CO2R. A factor of 2.48 was used to relate cr values to the Hammett cr values. Thus,... 

The study of structure-reactivity relationships by the organic chemist Hammett showed that there is often a quantitative relationship between the two-dimensional structure of organic molecules and their chemical reactivity. Specifically, he correlated the changes in chemical properties of a molecule that result from a small change in its chemical structure that is, the quantitative linear relationship between electron density at a certain part of a molecule and its tendency to undergo reactions of various types at that site. For example, there is a linear relationship between the effea of remote substituents on the equilibrium constant for the ionization of an acid with the effect of these substituents on the rate or equilibrium constant for many other types of chemical reaction. The relative value of Hammett substituent constants describes the similarity of molecules in terms of electronic properties. Taft expanded the method to include the steric hindrance of access of reagents to the reaction site by nearby substituents, a quantitation of three-dimensional similarity. In addition, Charton, Verloop, Austel, and others extended and refined these ideas. Finally, Hansch and Fujita showed that biological activity frequently is also quantitatively correlated with the hydrophobic character of the substituents. They coined the term QSAR, Quantitative Structure-Activity Relationships, for this type of analysis. 

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