Effects of Structure and Medium on Reactivity

The effects of structure on reactivity can be divided into three major types field, resonance (or mesomeric), and steric. In most cases, two or all three of these are operating, and it is usually not easy to tell how much of the rate enhancement (or decrease) is caused by each of the three effects. 

Tor a monograph, see Klumpp, G.W. Reactivity in Organic Chemistry, Wiley, NY, 1982. For a general theoretical approach to organic reactivity, see Pross, A. Adv. Phys. Org. Chem. 1985, 

March s Advanced Organic Chemistry Reactions, Mechanisms, and Structure, Sixth Edition, by Michael B. Smith and Jerry March Copyright 2007 John Wiley Sons, Inc. 

On the other hand, groups that have a multiple-bonded electronegative atom directly connected to an unsaturated system are —M groups. In such cases, we can draw canonical forms in which electrons have been taken from the unsaturated system into the group, as in nitrobenzene, 1. Table 9.1 contains a list of some +M and —M groups. 

TABLE 9.1. Some Groups with +M and —M Effects, Not Listed in Order of Strength of Effect  

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Gray, P. and Williams, A., The thermochemistry and reactivity of alkoxyl radicals, Chem. Rev., 59, 239. 1959 Bacha, J.D. and Kochi, J.K., Polar and solvent effects in the cleavage of t-alkoxyl radicals, /. Org. Chem., 30, 3272,1965 Walling, C. and Padwa, A., Positive halogen compounds. VI. Effects of structure and medium on the 5-scission of alkoxyl radicals,/. Am. Chem. Soc., 85,1593, 1963, and the references cited in these papers. 

In the end of 1960s, Nikolaev et al.29 and Ito et al.30 independently demonstrated an appreciable effect of the reaction medium on the reactivity ratios in the copolymerization of methyl methacrylate and styrene (Table 19). Ito et al. found that the relative reactivity of methyl methacrylate toward the polystyryl radical is correlated with the transition energies ET for the longest wavelength absorption band for pyridinum TV-phenolbetaine in solvents. They suggested that the polarized structure of methyl methacrylate monomer becomes important in the transition state. Bonta et al.32 also demonstrated that there is an appreciable solvent effect on the reactivity ratio in the styrene-methyl methacrylate copolymerization in non-... 

A radical solution to all of the above-mentioned difficulties is to eliminate the solvent medium entirely and to measure structural effects on heteroaromatic reactivity in the gas phase. During the last decade, a revolution has occurred in the experimental and theoretical approaches to understanding gas-phase ion chemistry. This has occurred as the result of the simultaneous development of several experimental methods for studying organic ion-molecule kinetics and equilibria in the gas phase with precision and range of effects equivalent to or even better than that normally obtained in solution and by very sophisticated molecular orbital calculations. The importance of reactivity studies in the gas phase is twofold. Direct comparison of rates and equilibria in gaseous and condensed media reveals previously inaccessible effects of ion solvation. In addition, reactivity data in the gas phase provide a direct evaluation of the fundamental, intrinsic properties of molecules and represent a unique yardstick against which the validity of theoretical estimates of such properties can be adequately assayed. 

Any understanding of the kinetics of copolymerization and the structure of copolymers requires a knowledge of the dependence of the initiation, propagation and termination reactions on the chain composition, the nature of the monomers and radicals, and the polymerization medium. This section is principally concerned with propagation and the effects of monomer reactivity on composition and monomer sequence distribution. The influence of solvent and complcxing agents on copolymerization is dealt with in more detail in Section 8.3.1. 

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