The Hammett Correlation

For the purpose of systematizing kinetic and equilibrium data, for literally hundreds of reactions, it is desirable to have a single reference series for all. Hammett adopted as the standard the acid ionization constants for substituted benzoic acids in aqueous solution at 25 °C. This choice was fortunate because the compounds are stable and for the most part readily available. Also, their pA a s can easily and precisely be measured for nearly every substituent. Thus, one constructs a plot according to either of the following equations, in which Eq. (10-4) constitutes a further example  

The quantity on the right-hand side of Eq. (10-8) can be written in simplified form. This expression defines the substituent constant rr in terms of the m- and / -substituted benzoic acids, relative to benzoic acid itself. The value of 7 is given by 

The values of r, characteristic of substituent X in either meta or para positions, are thus available from pX measurements of benzoic acids. One cannot so easily extend the correlation to ortho groups, however, because steric effects are superimposed on the electronic effects under study. Table 10-1 presents a list3 of crm and ap values. Note that the groups considered as electron-withdrawing relative to H have positive 7 values, and vice versa. Thus, we have a(p-CF ) — +0.54 and cr(/n-CHi) = —0.069. 

We consider data for two reactions to illustrate the method. They are the first and second steps in the following sequence of semicarbazone formation and acid-catalyzed dehydration.4 The use of different pH ranges allowed the study of each of the steps [R = NHC(0)NH2]. 

Just the reverse trend characterizes fc2. This reaction is aided by electron donation to the carbonyl carbon, as would be expected if the mechanism consists of electrophilic addition of the proton to the OH group. The reaction constant is p = -1.74. 

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The Hammett correlation of the acid-catalyzed dehydration of 1,2-diarylethanols has been studied. 

Illuminati et al. have also investigated the methoxydechlorination of 4-substituted-2- and 2-substituted-4-chloroquinolines. The relation between the reaction site, the 2- or 4-position, and the substituent in the 4- or 2-position, respectively, is always meta. The authors found the two reaction series well correlated with one another, but diverging quite seriously from the Hammett correlation. They concluded that mesomerically electron-donating substituents, because of the importance of resonance structures like 12 and 13, are more deactivating than expected, while electron-withdrawing substituents, and even the methyl group, seem to follow normal a correlation. 

A number of authors measured the influence of ring substituents on the rate of catalytic reduction of aromatic nitro compounds by hydrogen (5-7,114,128). The series have been composed in such a way as to allow the Hammett correlations, but, with a single exception, scatter diagrams resulted. The successful case by Rflzicka and Santrochova 128) (series 102, 12 points, slopes for three different platinum catalysts 0.24, 0.34, and 0.92, respectively) differs from the others in the use of platinum catalysts, whereas the other authors worked with rhodium (4,5), palladium (5,114), ruthenium... 

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

Styrene and 1,1-diphenylethene are frequently used as target molecules in investigations of the role of reagent structure on its reactivity toward organic compounds. Low-temperature fluorination with trifluoromethyl hypofluorite gives up to six products.32-33 The Hammett correlation value q was established to be — 2.48 and a mechanism suggested in which spin-paired free-radical pairs arc formed, which are then converted by electron transfer into an ion pair.33... 

Estimation of Acidity Constants The Hammett Correlation Illustrative Example 8.2 Estimating Acidity Constants of Aromatic Acids and Bases Using the Hammett Equation... 

In summary, the most important factors influencing the pKa of a given acid or base function are inductive, resonance, and steric effects. The impact of a substituent on the pK3 depends critically on where the substituent is located in the molecule relative to the acid or base group. In one place, a given substituent may have only one of the mentioned effects, while in another location, all effects may play a role. It is quite difficult, therefore, to establish simple general rules for quantifying the effect(s) of structural entities on the pK3 of an acid or base function. Nevertheless, in certain restricted cases, a quantification of the effects of substituents on the pKa value is possible by using LFERs. In the next section, we discuss one example of such an approach, the Hammett correlation for aromatic compounds. First, however, a few comments on the availability of experimental pKiz values are necessary. 

In our discussion of the Hammett correlation, we have confined ourselves mostly to benzene derivatives. Of course, a similar approach can be taken for other aromatic systems, such as for the derivatives of polycyclic aromatic hydrocarbons and heterocyclic aromatic compounds. For a discussion of such applications, we refer to... 

Analysis248 of the UV spectra of the monoprotonated 2-substituted quinoxalines, and the Hammett correlation of the pKa shifts with the substituent constants, give two straight lines, corresponding to two sets of substituents, and so reflecting a change in the position of protonation. Thus, 2-methoxyquinoxaline was found to protonate at N-4, and 2-aminoquinoxaline at N-1. However, the site of protonation of 2-chloro-quinox aline was ambiguous. 

Hydrogen abstraction by the OH radical may result in the formation of a positive transition state complex in nature because of the negative slope of the Hammett correlation for substituted alcohols, as shown in Figure 5.22. 

A correlation for chlorinated alkanes was also established using a as a descriptor. As discussed earlier, cf can be used to describe the behavior of aliphatic compounds. Figure 5.23 demonstrates the Hammett correlation for chlorinated alkanes. 

The Hammett correlation for substituted carboxylic acids is demonstrated in Figure 5.26. The Hammett constant om fails for aliphatic compounds, and the derived constant o must be used to predict accurate Hammett correlations. The least-substituted carboxylic acid, formic acid, was used as the reference compound. The Hammett correlation for substituted carboxylic acids (CAs) demonstrates that the CAs substituted by electron-withdrawing substituents, such as Cl, oxidize the fastest. CAs substituted by electron-donating groups, such as CH3 and NH2, oxidize more slowly than those substituted by electron-withdrawing substituents. The reaction pathway for substituted carboxylic acids is shown in Figure 5.27. These trends are different for phenols and alkanes, because the reaction site is at the election pair located at the oxygen atom. 

The Hammett correlation was also established for substituted benzenes and halogenated benzenes. A good correlation could not be established using halogenated and substituted benzenes in the same equation. Halogenated... 

The reaction pathway for substituted alkenes proceeds by hydrolysis. The OH radical attacks the C=C bond to form a positive transition state complex. This positive transition state complex corresponds to the negative slope of the Hammett correlation in Figure 5.30. The reaction proceeds until both carbons of the C=C bond have been hydroxylated. 

The Hammett correlation for nitrobenzenes is almost identical to the correlation at pH 9 therefore, the degradation of substituted benzenes at pH 3 can also be described by the same hole oxidation mechanism. Figure 9.21 demonstrates the oxidation of nitrobenzene by positive hole. At pH 3, substituted benzenes are oxidized by the formation of a positive hole. An electron transfer from nitrobenzene to Ti02 creates this positive hole. 

A Hammett correlation was also established for chlorinated alkanes studied by UV/Ti02. The least substituted compound, ethane, was used as a reference compound. Figure 9.23 demonstrates the Hammett correlation for chlo-... 

The apparent first-order degradation rate constants of six dichlorophenols and three trichlorophenols were studied by D Oliviera et al. (1993). ft was determined that the first-order rate constant, k, correlated with the Hammett s constant o and the octanol/water partition coefficient. The Hammett correlation for the cholorophenols was developed as follows ... 

The correlation coefficient for the Hammett correlation is 0.987 if the value for 2,4,6-trichlorophenol is omitted. This experimental value is too small and should be disregarded. This correlation successfully demonstrates that some... 

A comparison of the kinetic rate constants for elementary hydroxyl radical and UV/Ti02 is provided in Figure 9.24. The slopes of the Hammett correlations for the hydroxyl radical data and UV/TiOz are both negative. This suggests a similarity in the reaction mechanism that is supported by a comparison of these correlations. 

The Hammett correlations for substituted benzenes were similar for elementary hydroxyl radical reaction and UV/Ti02 because both correlations have negative slopes. The similarity of the correlations may be due to the same oxidation species, such as hydroxyl radical. For both reactions, the benzenes substituted with electron withdrawing groups were difficult to degrade. 

Quantitative structure-activity relationship models can be used to predict kinetic rate constants for compounds similar in chemical structure under UV /TiOz photocatalytic oxidation. In formulating the Hammett correlations, the descriptors, such as on s and o, proved to be successful in deriving equations for aromatic and aliphatic compounds respectively. For the case of aromatic compounds, the ores descriptor formulated better Hammett correlations than the om descriptor. 

Rate constants have been measured for the capture of para-substituted phenylchloro-carbenes by chloride ions to form aryldichloromethide carbanions and for the additions of these carbanions to acrylonitrile.144 A conventional interpretation of the Hammett correlations has suggested that the reactions of carbenes with Cl- traverse early transition states. 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

This leaves unexplained the anomalously high reactivities for both mercuriation and protiodemercuriation. For the latter, it may be relevant that with thiophene and substituted thiophenes kinetics were second order for only 20% of reaction, and there were marked differences in entropies of activation for the heterocyclic and phenyl derivatives. Even for benzenoid compounds the reaction is not well behaved as (1) the Hammett correlation shows marked curvature, suggesting that reactive compounds undergo a different mechanism to unreactive ones (2) the reaction is sensitive to oxygen and (3) it is sensitive to added chloride ion, due possibly to salt effects and reaction via hydrogen chloride ion pairs (65AJC1521). A reexamination of both mercuriation and protiodemercuriation of thiophene would seem desirable. 

Can the Hammett correlation be applied to pyridines using the a values for benzenes What equilibrium p value does it give and how do you interpret it Why are no 2-substituted pyridines included in the list ... 

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