Activation energies substituent effects

Table 2.10 shows the effect of substituents on the endo-exo ratio. Under homogeneous conditions there is hardly any substituent effect on the selectivity. Consequently the substituents must have equal effects on the Gibbs energies of the endo and the exo activated complex. 

This involves a more uniform distribution of charge because of the identical substituents and thus lacks the stabilizing effect of the polar resonance form. The activation energy for this mode of addition is greater than that for alternation, at least when X and Y are sufficiently different. 

There are several compilations and reviews of fluorocarbon physical properties [4, 5, 6 9, 10, II 12], and only the boihng points, surface energies and activities, and solvent properties are discussed in this section to illustrate the characteristic fluonne substituent effects... 

The second possible cause of nonlinearity is a change in mechanism. Within a reaction series any change in mechanism must be such as to provide a smaller free energy of activation for the reaction (otherwise the mechanism would not change). If a substituent effect can produce a change in mechanism, the result must therefore be curvature that is concave upward. Figure 7-2 is a per plot for the S l solvolyses... 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

Nitrobenzenediazoate can be considered as an azo compound comparable to an azobenzene having one electron acceptor and one donor on each side of the azo group the acceptor-donor relationship is more dominant in the (Z) -> (E) diazoate pair than in the diazohydroxide pair. The N=N rotation mechanism of the diazoate pair is therefore the favored process (E = 84 kJ mol-1 Lewis and Hanson, 1967). On the other hand, 4-C1 is not a substituent with a —M effect therefore it does not reduce the double-bond character of the N = N bond and the mechanism involving inversion at the N((3)-atom becomes dominant. The activation energy of the latter process (E = 104 kJ mol-1 Schwarz and Zollinger, 1981) is higher than that of the N = N rotation mechanism for the 4-nitro derivative, but it is reasonable to assume that it is lower than that for N = N rotation in the 4-chloro derivative. Furthermore, one can conclude that N-inversion is more favorable in the diazohydroxide than in the diazoate. ... 

However, measurements of substituent effects supported the hypothesis that the aryl cation is a key intermediate in dediazoniations, provided that they were interpreted in an appropriate way (Zollinger, 1973a Ehrenson et al., 1973 Swain et al., 1975 a). We will first consider the activation energy and then discuss the influence of substituents, as well as additional data concerning the aryl cation as a metastable intermediate (kinetic isotope effects, influence of water acitivity in hydroxy-de-di-azoniations). Finally, the cases of dediazoniation in which the rate of reaction is first-order with regard to the concentration of the nucleophile will be critically evaluated. 

Ab initio calculation of Diels-Alder reactions of a series of 5-heteroatom substituted cyclopentadienes Cp-X (65 X = NH, 50 X = NH, 64 X = NH3, 67 X = O", 54 X = OH, 68 X = OH3% 69 X = PH, 51 X = PH, 70 X = PH3% 71 X = S, 55 X = SH, 72 X = SH/) with ethylene at HF/6-31++G(d)//HF/6-31-i i-G(d) level by BumeU and coworkers [37] provided counterexamples of the Cieplak effect. The calculation showed that ionization of substituents has a profound effect on the n facial selectivity deprotonation enhances syn addition and protonation enhances anti addition. The transition states for syn addition to the deprotonated dienes are stabilized relative to those of the neutral dienes, while those for anti addition are destabilized relative to those of the neutral dienes. On the other hand, activation energies for syn addition to the protonated dienes are similar to those of the neutral dienes, but those for anti addition are very much lowered relative to neutral dienes (Table 6). 

The effect of a conjugating substituent in the monomer may be summarized by observing that its influence is much greater in the product radical than in the monomer. In the activated complex, which is intermediate in character between reactants and product, resonance stabilization is substantially greater than in the monomer reactant, though less than in the product radical. The substituent therefore lowers the activation energy for the process, and enhances thereby the reactivity of the monomer. 

It is seen that the rate constant ks is lower for compounds with electron-accepting substituents than with electron-donating substituents, which implies a dependence of the rate of Ar20 and R02 recombination on the electron density at the para- and ort/zo-positions of the benzene ring of the phenoxyl radical. The activation energies of this reaction vary from -33 to 10 kJ mol-1 however, the concurrent variation in the pre-exponential factor from 103 to 1010 L mol-1 s-1 causes a strong compensatory effect. It can also be seen that phenoxyl radicals readily react with peroxyl radicals k= 10s—109 L mol-1 s-1), whereas the disproportionation of peroxyl radicals is sufficiently slower (see Chapter 2). Hence, during the oxidation of hydrocarbons in the presence of phenols when k7[ArOH] > /c2[RH], the recombination reaction of ArO with R02 is always faster than the reaction of disproportionation of peroxyl radicals. 

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