Quantitative Aspects of the Reactivity Data

The rate data collected in the foregoing sections are gathered, in terms of r+ values, in Table 8.10. The main features of these results are summarized in the following subsections. 

It should be stressed that there is nothing exceptional about the benzo[f ] compounds in not providing constant r+ values. Proper analysis of the literature data for practically every aromatic shows the ct+ values to be variable. For some molecules that are not very polarizable, the variation in ct+ is quite small, leading to the erroneous conclusion of their constancy. The presently described molecules are particularly polarizable and show that satisfactory treatment of reactivity data can generally be achieved only through the use of a multiparameter equation, notably the Yukawa-Tsuno equation (59BCJ971). 

Molecules Containing One Heteroatom Benzo/c/selenophene, Isoindole, and Indolizine 

The benzo[c] derivatives (8.2) of the five-membered heterocycles do not contain a true Kekule benzenoid ring and so tend to undergo other reactions under the conditions for electrophilic substitution. Like indolizine, however, they are very reactive toward electrophilic substitution for a reason that does not appear to have been noted previously, namely that in passing to the transition state, a true benzenoid ring is generated (e.g., 8.60 and 8.61 for benzoic]selenophene and indolizine, respectively) [87JCS(P2)591]. 

The effect of the high reactivity of indolizine also shows up in substituent effects that are very small, as they should be according to the reactivity-selectivity principle. The exchange-rate coefficients for deuteriation in DzO/dioxan at 50°C are given in Table 8.12 (71T4171), and from these data the methyl substituent effects may be calculated. Notable features are [87JCS(P2)591] summarized below. 

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The research we describe in this paper addresses the fundamental nature of the active catalytic species in ionically supported systems. Structure and reactivity are probed using a variety of techniques, in order to make a detailed comparison of the supported and homogeneous catalysts. An important aspect of the study was to develop an in situ technique to follow the key organometallic reactions of the polymer-supported catalyst, and to obtain quantitative kinetic data for these reactions. 

Some data recently obtained on high pressure ethylene copolymerizations illustrate the quantitative aspects of an ethylene-based Q-e scheme (6). In Figures 3 and 4 copolymer composition curves for the ethylene-vinyl chloride and the ethylene-vinyl acetate copolymerizations are given. The monomer reactivity ratios for these two systems are tabulated in Table III along with Q values and e values for vinyl chloride and vinyl acetate calculated using ethylene as the standard (Q = 1.0 and g = 0). These Q and e values may be compared with those obtained using styrene as the standard. 

The first generalization is illustrated by the behavior of the 2- and 4-vs. the 3-derivatives of pyridine, the second by the reactivity of 4- vs. 2-substituted pyridines, the third by the relation of 4- vs. 2-derivatives of pyrimidine, and the fourth by the appreciable reactivity of 3-substituted pyridines or 5-substituted pyrimidines compared to that of their benzene analogs. Various combinations of azine-nitrogens in other poly-azines supply further examples. Theoretical aspects of (1), (2) and (3) are discussed in Section II, B, 2. The effect involved in (4) is believed to be more the result of the inductive stabilization of an adjacent negative chaise in the transition state (cf. 251) than of the electron deficiency created in the ground state (cf. 252). The quantitative relation between inductive stabihzation and resonance stabilization is not precisely defined by available data. However, a... 

The fact that none of these reports has emphasized the physical aspects of electrophilic substitution in the series reflects the paucity of quantitative studies, and the low reactivity of these compounds in the presence of electrophiles. Few kinetic studies have been reported and the regio-chemical effects of substituents have seldom been quoted in quantitative form. The present chapter brings together those quantitative results that are available, and collates data on substituent effects. One worthwhile field of study would appear to be the application to the azines of Taylor s method involving thermolysis of esters [75JCS(P2)277, 75JCS(P2) 1783]. 

Qulfur atoms and the atoms of the group Via elements undergo facile addition to olefins and acetylenes. These reactions represent the simplest examples of a cycloaddition reaction. Among them, the sulfur atom reactions stand out with their experimental simplicity, cleanliness, and unique reactivity. Therefore the sulfur atom-olefin system is a model for studying some of the little explored aspects of cycloaddition reactions. Since the first report on sulfur atom addition reactions which appeared in 1962, a considerable amount of quantitative data and relevant information have accumulated on the subject, which are summarized briefiy here. 

One of the most important and general trends in organic chemistry is the increase in carbocation stability with additional alkyl substitution. This stability relationship is fundamental to understanding many aspects of reactivity, especially of nucleophilic substitution. In recent years, it has become possible to put the stabilization effect on a quantitative basis. One approach has been gas phase measurements which determine the proton affinity of alkenes leading to carbocation formation. From these data, the hydride affinity of the carbocation can be obtained. 

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