Influence of Substituents

Rate constants for the Diels-Alder reaction of 2.4b-e have also been determined. The results are shown in Table 2.3. These data allow an analysis of the influence of substituents on the Lewis-acid catalysed Diels-Alder reaction. This is interesting, since there are indications for a relatively large... 

In order to obtain more insight into the local environment for the catalysed reaction, we investigated the influence of substituents on the rate of this process in micellar solution and compared this influence to the correspondirg effect in different aqueous and organic solvents. Plots of the logarithms of the rate constants versus the Hammett -value show good linear dependences for all... 

The electronic theory provides by these means a description of the influence of substituents upon the distribution of electrons in the ground state of an aromatic molecule as it changes the situation in benzene. It then assumes that an electrophile will react preferentially at positions which are relatively enriched with electrons, providing in this way an isolated molecule theory of reactivity. 

According to the nature of nuclei in the case of asymmetrical dyes, it has been shown that the influence of substituents in the meso position is neither identical nor in the same direction. 

The 2-position of thiazole reacts in the same conditions as with aldehydes previously discussed. In this case, the carbinol is obtained in poor yields with zinc chloride and with improved yields in acetic anhydride. But, in this reaction, the 2-position seems to be more sensitive to the influence of substituents at the 4- or 5-positions, or both as exemplified in the following case. 

The reaction constant p is a quantitative measure of the sensitivity of the reaction to the influence of substituents. Three factors combine to determine the value of p ... 

Although the UV spectra of a large number of substituted thiophenes have been recorded by many authors during recent years, the first systematic studies of the UV spectra of monosubstituted thiophenes designed to obtain information about differences in the influence of substituents in 2- and 3-substituted thiophenes were carried out independently by Andrisano and Pappalardo and by the present author. ... 

As seen from Table I, no systematic data obtained under identical conditions are available regarding the influence of substituents in the ring on the... 

The main conclusion on the influence of substituents in the imidazole ring on the state of the tautomeric equilibria 14a 14b is that electron-withdrawing groups favor the 4-position, i.e., the tautomers 14a with = Hal, NO2, and so on, are the energetically preferable species. Application of Charton s equation, Kt = [4-R Im]/[5-R Im] = 3.2 was discussed in detail [76AHC(S1) 96CHEC-II(3)77]. The equation was found to be in a qualitative agreement with the experimental data presented in Table III. 

The influence of substituents R in the five-membered ring of 27 on the position of the tautomeric equilibria is illustrated by the data collected in Table IV. 

Another analogy with the behavior of the azidoazoles is the influence of substituents in various positions of the azine rings on the tautomeric equilibria. Electron-withdrawing substituents favor azidoazine and electronreleasing substituents favor tetrazoloazine forms (Table X). 

The scheme shows that the influence of substituents on CH acidity in ethynylpy-razoles is not additive as compared with benzene derivatives (84IZV923), and its value depends, for each substituent, on the nature of other groups in different positions of the azole. 

The influence of substituents (jP-Cl, g-OMe) on kxJkK was investigated by Ayrey at al. w They found disproportionation was favored by the p-OMe substituent and that the extent of disproportionation increased with increasing temperature. This result is contrary to the model studies (Section 5.2.2.1.1) that show kJkK has little dependence on substituents and, indeed, suggest the opposite trend. 

Several authors studied the influence of substituents on activation parameters. Bad-dar et al.315 who studied the polyesterification of y-arylitaconic anhydrides and adds with 1,2-ethanediol found that in the non-catalyzed reaction a p-methoxy substituent decreases both the activation enthalpy and the entropy whereas an increase is observed with a p-chloro substituent. On the other hand, Huang et al., who studied the esterification of 2,2-dimethyl-l,3-propanediol with benzoic, butanedioic, hexanedioic, decanedioic and o-phthalic add found the same values since the activation enthalpy is 64 kJ mol-1 for the first reaction and 61 kJ mol-1 for the others. 

Bagal (1974) studied the influence of substituents on the ground and first excited states of arenediazonium ions. With regard to compounds in which mesomeric structures such as 4.1b are important, these authors are skeptical about the validity of the PP method. Later, Bagal et al. (1982) used CNDO/2. The calculated 7r-electron densities at all the nitrogen and carbon atoms were similar to those in the earlier PP results. 

The investigation by Mustroph et al. (1981) was made using SCF-CI with the PPP approximation to calculate the influence of substituents on the longest wavelength band. This study is interesting because the authors come to the conclusion that 591 nm is the theoretical limit that can be obtained with substituted benzenediazo-nium ions. As far as we are aware, that limit had not been reached by the early 1990s. 

Lewis and Suhr (1959 b) measured the influence of substituents on the rate of formation and the equilibrium of (jE )-benzenediazocyanide. Ritchie and Wright (1971 b) used the stopped-flow technique to determine the rate constants of formation and the equilibria of substituted (Z)-benzenediazocyanides in water, and also later in methanol. 

Influence of Substituents on the Addition of Nucleophiles to Arenediazonium Ions... 

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. 

The influence of substituents on the solvolysis of benzenediazonium ions in water and in TFE is very similar. This fact suggested that the mechanism is the same in both solvents. Exchange of 3-15N-labeled 4-methoxy- and 2,4,6-trimethyl-benzene-... 

We will conclude this section on theory with such a case. In Section 8.3 it was shown that the influence of substituents on the rate of dediazoniation of arenediazonium ions can be treated by dual substituent parameter (DSP) methods, and that kinetic evidence is consistent with a side-on addition of N2. We will now discuss these experimental conclusion with the help of schematic orbital correlation diagrams for the diazonium ion, the aryl cation, and the side-on ion-molecule pair (Fig. 8-5, from Zollinger, 1990). We use the same orbital classification as Vincent and Radom (1978) (C2v symmetry). 

Gilbert et al. (1992, see also Galli, 1988) showed that a dual substituent parameter treatment (DSP, see Sec. 7.2) gave the best fit to the influence of substituents as measured by Doyle et al. (1987 a) with Fe(CN)6, as well as for reactions with the ferro complex of ethenediaminetetraacetic acid (Fen-EDTA). 

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