Other Steric Effects on Reactivity

The results mentioned are best demonstrated on secondary nitramines [7], which in their molecular structure are relatively simple polynitro compounds, and the mechanism of primary homolysis of their molecules is well understood [11,14] (Scheme 1). Polynitro arenes, on the other hand, have a more complex structure and intramolecular effects in their molecules here the mesomeric, inductive and steric effects on reactivity operate simultaneously. This fact makes the problem of their primary fragmentation somewhat complicated too [6,9] (Schemes 2, 3a, 3b, 7 and 8). If a molecule of these compounds contains several types of substituents, it can contain several potential reaction centres (e.g., the PYX and TMPM molecules, see Schemes 7 and 8). The initiation proper can then be realized by the molecule simultaneously participating by several centres or always by a single centre in a given type of initiation (the initiation of PYX by impact or shock versus its initiation by electric spark, see Figs. 12 and 26) [6]. 

The influence of steric effects on the photocyclization process is clearly discernible for the ortho and meta substituted stilbenes. In these cases steric repulsion intervenes as the two C atoms forming the new bond approach. This steric repulsion opposes the stabilizing interactions which promote the cyclization process. As a result the reactivity is decreased. Other consequences of such superimposed steric... 

Comparisons between R- and T-state hemoglobins on the one hand and a variety of synthetic model compounds on the other have allowed an evaluation of the possible occurrence and importance of electronic, proximal-base tension, and distal-side steric effects on the kinetics of ligation of CO and 02. Although all of these effects could influence the reactivities of hemoproteins, we conclude that hemoglobin reactivity and cooperativity are controlled predominantly by the presence or absence of proximal-base tension. 

Anthracene dimers as well as dihydroanthracene have been identified as initial reaction products in all pyrolysis studies of anthracene. As shown in Chart I, 11 dimers from anthracene are possible. Because the 9-position is the most reactive, one might expect a predominance of the 9,9 -dimer. However the 2,9-dimer was reported as the major product in one study (18). Many of the other possible dimers were also obtained, depending on the reaction conditions employed. Both steric effects and reactivity factors must, therefore, be taken into account for considering the possible reaction products in aromatic hydrocarbon pyrolysis. The results for anthracene show how the lack of a functional group and the nonspecificity for molecular recombination lead to complex product mixtures in aromatic pyrolysis. 

In summary, the stability of the propagating radical in free-radical polymerization, as well as other relevant radical intermediates, is profoundly affected by the nature of its primary substituents and, to a lesser extent, the nature of its more remote substituents. These effects, which have been widely characterized both experimentally and theoretically for small model radicals, can be readily understood in terms of the relevant orbital interactions and the impact of steric effects and other direa interaaions (such as hydrogen bonding). In subsequent sections, we shall examine how these effects on stability translate into effects on reactivity but in order to do this, we need to first charaaerize some of the other key properties of propagating radicals and the other reagents involved. 

Examples of effects of reactant stmcture on the rate of nucleophilic substitution reactions have appeared in the preceding sections of this chapter. The general trends of reactivity of primaiy, secondary, and tertiaiy systems and the special reactivity of allylic and benzylic systems have been discussed in other contexts. This section will emphasize the role that steric effects can pl in nucleophilic substitution reactions. 

The low yields of 6,6 -disubstituted-2,2 -bipyridincs recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine. On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one 71-electron would be smaller in a quinoline than in a pyridine derivative. 

With reactive aldehydes an early transition state is probably involved and therefore the steric demands of the aldehyde substituents are not highly influential. On the other hand, with less reactive ketones, the carbon-carbon bond formation is established further along the reaction coordinate, permitting the steric effects to play a greater role in the determination of the transition stale structure. 

Diels-Alder cycloadditions are sensitive to steric effects of two major types in the diene. Bulky substituents on the termini of the diene hinder approach of the two components to each other and decrease the rate of reaction. This effect can be seen in the relative reactivity of 1-substituted butadienes toward maleic anhydride.19... 

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