Hammett reaction constants polar effects

The symbol k or K is the rate or equilibrium constant, respectively, for a side-chain reaction of a meta- or para-substituted benzene derivative, and or Kf> denotes the statistical quantity (intercept term) approximating to k or K for the parent or unsubstituted compound. The substituent constant o measures the polar (electronic) effect of replacing H by a given substituent (in the meta- or para-position) and is, in principle, independent of the nature of the reaction. The reaction constant p depends on the nature of the reaction (including conditions such as solvent and temperature) and measures the susceptibility of the reaction to polar effects. Hammett chose the ionization of benzoic... 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

The effects of polar substituents on the alkaline hydrolysis of esters are well-established. Since the rate of the reaction is determined largely by the rate of addition of hydroxide ion to the carbonyl group of the ester, any substituent which withdraws electrons from the carbonyl group will increase the reactivity of the ester. The most accessible quantitative measure of the effect is the Hammett or Taft reaction constant, and a large number of measurements are available. Taft et al.2i0 found p = 2.53 for the base-catalyzed methanolysis of meta- and para-substituted (/)-menthyl benzoates, closely similar to the known value of p = 2.37 for the alkaline hydrolysis of substituted ethyl benzoates. Jones and Sloane s value239, obtained with five esters, of p = 2.41 for the methoxyl exchange reaction of substituted methyl benzoates in methanol, is almost identical. 

In principle, extrathermodynamic relationships that deviate from the simple Hammett equation (equation 8) can be treated by equation 14. The major problem is the determination of the different sets of o s, (e.g., set and 0 set) in a way that will indeed reflect their relation to independent properties. An example of such a procedure is the separation of polar and steric effects (10). The need for such a separation arose when a nearly complete lack of correlation was observed between substituent effects represented by the Hammet a constants and the rates for alkaline hydrolysis of aliphatic systems (12). Inspection of the structures indicated that the proximity of the substituents to the reaction site was a common feature. The steric interaction between R and X had to be considered separately from the electronic effects. Polar substituent constants were thus defined as the difference between the rate constants of base and acid catalyzed hydrolysis of esters. From the structural similarity of the transition states for these reactions (equation 15) it was assumed that the difference in their charge reflects only the polar effect of the substituent... 

Inductive o values for substituents may be obtained by fitting Hammett Equations to a standard series of reactions which are considered to possess no resonance interaction the average values of a from these correlations provides a set of coefficients (a") " which correspond to purely inductive transmission of the polar effect. The coefficients derived from the ionisation of XC6H4CH2CO2H, XC6H4CH2CH2CO2H and the alkaline hydrolysis of XC6H4CH2CO2C2H5 and XC6H4CH2CH2CO2C2H5 (a°) are regarded as purely inductive and field effect constants. 

The Hammett equation in the form Equation (3.44) or (3.45) is free of complications owing to steric effects because it is applied only to meta and para substituents. The geometry of the benzene ring ensures that groups in these positions cannot interact sterically with the site of reaction. The ct values for many substituents have been determined, and some are shown in Table 3.26. Substituent constants are available for a much wider range of substituents. The ct value for any substituent reflects the interaction of the substituent with the reacting site by a combination of resonance and polar interactions. Table 3.26 lists some related substituent constants such as ct+, (T, (Tj, and CT. We diseuss these shortly. Table hll shows a number of p values. The p value reflects the sensitivity of the particular reaction to substituent effects. The examples that follow illustrate some of the ways in which the Hammett equation can be used. 

As we mentioned earlier, not all reaction series can be correlated by a Hammett equation. An underlying reason for the inability of Hammett and values to correlate all reaction series is that the substituent effects used to assign ct are a mixture of resonance and polar components. When direct resonance interaction with a reaction site is possible, the extent of the resonance increases and the substituent constants appropriate to the normal mix of resonance and polar effects fail. There have been various attempts to develop sets of a values that take extra resonance interactions into account. In addition to the and values used with the classical Hammett equation Table 3.27 lists substituent constants and ct . These are substituent constant sets that reflect enhanced resonance participation. The values are used for reactions in which there is direct resonance interaction between an electron donor substituent and a cationic reaction center, whereas the set pertains to reactions in which there is a direct resonance interaction between an electron acceptor and an anionic reaction site. In these cases, the resonance component of the substituent effect is particularly important. 

The slope of the Hammett plot (p) does not bear a simple relationship with the transition-state structure (relative to ground state) because the model and the reaction or equilibrium in question are usually entirely dissimilar. Even the phenylacetic acid dissociation constants are not strictly comparable with those of benzoic acids because the polar effect is transmitted via different routes. 

Comparison of the Hammett p values for phenylacetic acid ( + 0.47) and benzoic acid (-I-1.00) dissociation constants provides us with a measure of the transmission of the methylene group (0.47) compared with direct coupling. The ratio (0.47) is remarkably constant for attenuation of the polar effect down a methylene side chain and essentially the same value is obtained for the reaction of phenylacetic and... 

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,... 

Waltman et al. [325] found a linear correlation when the peak oxidation potentials, py, of 3-substituted thiophene monomers were plotted versus their Hammett substituent constants, o-" [324], as shown in Fig. 15. values, which are normally plotted versus cr", were not available in this case because these reactions were irreversible in nature. Regardless, the values should be similar to the values as determined for other families of polymers, such as substituted fluorenes [325]. Three parameters—polar, steric, and mesomeric effects—control the change in the Ep values [326]. The Hammett-Taft equation describes the behavior as... 

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