Hammett plots/data

For unknown reasons the point for 2.4a in the Hammett plot for ethanol strongly deviates from the otherwise good correlation. The data for 2.4a in ethanol have therefore not been used in the calculation of p. Instead the data for another compound 2.4 with X = CO2CH3 (k2 = 0.655 M" s" ) were used in the correlation. 

It will be obvious that the treatment thus far includes only substituents that are meta or para to the reaction site. When Hammett plots are made with data for ortho-substituted reactants, scatter diagrams usually result. This failure might be attributed to steric effects, but this is not very helpful, and many attempts have been made... 

A Hammett plot (a) of the apparent first-order rate constants for semicarbazone formation at pH 3.9, where a change in the RCS occurs, and (b) of the denitrosation of A-methyl-A-nitrosoanilines, where the mechanism changes. Data are from Refs. 9 and 10. 

Lucchi , who studied the oxidation of substituted benzaldehyde derivatives found that chlorine atoms in the meta and para position accelerate the reaction and alkyl groups retard the oxidation. A Hammett plot of Lucchi s data yields a good straight line with the slope p = 1.06. These data suggest that the reaction proceeds by way of the chromic ester of hydrated benzaldehyde as intermediate, viz. 

It is, however, important to resist the temptation to introduce new parameters into the Hammett equation merely to achieve a better fit with the experimental data. This is particularly true where, as in some cases, it may be difficult to ascribe real significance, in physical terms, to the new parameter anyway. It is in fact possible, as we shall see, to obtain much highly relevant information about reaction pathways using simple Hammett plots only. 

The conductance of the OPE nitro-16 (X = N02) was monitored by the STM-BJ method, as the nitro group was reduced to NO and NH2, and then protonated to NH3. The resulting data gave an inverse linear Hammett plot with the meta-cr substituent parameter, indicating that substituent electron release into the aromatic core increases conductance [63]. 

Fig. 12.14 Free energy correlation and Hammett plot of chemical shift data of the intermediate dihydrides.

As pointed out above, values of KTS are obtainable from rate data without making any assumptions about the reaction mechanism. Therefore, one may use KTs and its variation with structure as a criterion of mechanism, in the same way that physical organic chemists use variations in other kinetic parameters (Brpnsted plots, Hammett plots, etc.). For present purposes, the value of Kts can be useful for differentiating between the modes of binding in the S CD complex and the TS-CD transition state, between different modes of transition state binding, and hence between different types of catalysis (Tee, 1989). 

Most of the data in Table 12 come from the work of Shvo et al. (78). Careful band-shape analysis and solvent-effect studies permitted evaluation of the rate constants and AG values at 298 K, which renders the discussion of substituent effects more meaningful than usual. The authors obtained reasonably linear Hammett plots when correlating log km with Or (79) for X and Y, holding one of these substituents constant. They also found that the dihydropyridine system may act as an unusually efficient donor, giving a AG of 17.6 kcal/mol with X, Y = H, CN, the only barrier below 25 kcal/mol reported for any donor-substituted cyanoethylene. However, with other acceptor combinations the dihydropyridine moiety is not so outstanding, and this illustrates the difficulty of measuring donor and/or acceptor effects by rotational barriers alone (vide infra). 

MCBA enhances the solubility of the cobalt salts in MeCN solution, thereby ensuring better efficiency to a needed redox decomposition of the hydroperoxide intermediate of the substrate en route to the products". By using the HPI/Co(II)/MCBA/02 system in MeCN solution at 25 °C, competitive oxidations of p-X-substituted benzyl alcohols were run pairwise (Scheme 8). From the amount of the aldehydes produced, the relative reactivity (kx/ h) could be reckoned, and the acquired data provided a p = —0.68 in a Hammett plot vs. <7+. ... 

Figure 8.7 Hammett plots for meta- and para-substituted phenols, phenylacetic acids, and 3-phenylpropionic acids (data from Serjeant and Dempsey, 1979).

Fig. 5. Hammett plot for the hydrolysis of substituted methyl (O) and ethyl ( ) benzoates at 45°C in 99.9% sulphuric acid. Data from ref. 33.

One of the characteristics of the acid-catalyzed hydrolysis of esters, that is shared by ester formation also, is that substituent effects on the rate coefficients are small, and not simply related to a values (see below, p. 131). The data in Table 14 show that this is also true for the, sO-exchange reaction of substituted benzoic acids. This is borne out by the relative constancy of the ratio khyJkexch for the different substituted acids it was not possible to obtain a meaningful p value from the data of Table 14, because of the small number of points and the large amount of scatter evident on the Hammett plot. Mesitoic acid is highly unreactive, compared with the m- and p-substi-tuted esters used, as is its methyl ester towards alkaline hydrolysis138, and presumably reacts by the seriously hindered Aac2 route. 

Similarly, those reactions that are strongly assisted by withdrawal of electrons from the reaction site, such as nucleophilic aromatic substitution, give a poor fit to a Hammett plot for the substituents that are capable of withdrawing electrons by delocalization (—N02, —N2 , —C=N, and so on). An example is Reaction 16 in Table 26-7. To correlate reactivity data with structures where strong resonance effects operate, different sets of substituent constants are required.1... 

Table 6. Rate constants for the acid catalyzed methanolysis reactions at 25 C. Data are used to prepare a Hammett plot ...

These data were plotted according to the Hammett equation to give the plot in Figure 5.17. The first thing to note is that the Hammett plot is linear. The linearity of die plot implies that the substituent constants determined for the ionization of... 

A Hammett plot of the rate data according to equation 83, where kx is the absolute rate constant for reaction of the X-substituted phenol and k is that for reaction of the parent phenol (Figure 13), shows clear evidence for different addition mechanisms depending on whether the phenol is substituted with electron-donating or electron-withdrawing substituents. 

The relative rate data of Table II were subjected to Hammett plots of log kx/kh versus o and o+. In the nitrobenzene oxidations, a better correlation was found with +(p+ = -0.48, r = -0.994). The preferred dependence on <7+ was also found in the copper(II) oxidations when the kinetic point for the methyl... 

For the compounds obeying the Hammett equation, the distribution pattern for the OH attack can be estimated by assuming that the position on the ring with the maximum C7 value has the least probability, while the one with the minimum has the greatest probability. In our recent work > on cinnamate derivatives where the addition of the OH radical to the olefmic double bond is an additional pathway, a value of-0.3 was estimated from the Hammett plot (Fig. 5). This value is in agreement with those reported earlier for several substituted benzenes (p" = -0.52 to -0.4). However, the Hammett treatment for reactions with rate constants close to diffusion-controlled ones may not be satisfactory due to limited variation in kinetic data and large experimental uncertainty. In contrast, better correlation is expected in the corresponding gas phase reactions. 

Substituent effects in the decomposition of benzoyl peroxide have been thoroughly investigated. Data in dioxane solvent with added 3,4-dichlorostyrene to prevent induced decomposition are presented in Table 72. A Hammett plot using the sum of the substituent constants gave p = —0.38 (ref. 339). The observation that electron-releasing substituents increased the rate of decomposition was explained in terms of a dipolar interaction between the two aroyloxy groups , viz. 

From the data in Table 100, a Hammett plot with a gave p = —0.68. The value of the reaction constant is similar to that reported for benzoyl peroxides (p = -0.38). 

Figure 3.6. Hammett plot for the reduction of substituted phenyl azides (RCfH4Nf by dithiothreitol. Data were taken from Table 3.8.

Solvolyses data for 5-substituted 2-furylmethanol derivatives have been correlated.446 Results for arylation fit Hammett plots using a+ values provided that the aryl radical contains strongly polar substituents such as nitro or halogen.4461 Another correlation involves eliminations in 1 -(2-furyl)-ethyl acetate to give furyl alkenes.447 The furan ring itself has been assigned a a value.4-146... 

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