Factors That Influence Equilibrium Position

Factors That Influence Equilibrium Position (Section 8.8)... 

Acidity and basicity depend on the various factors that influence the position of the equilibrium between an acid HA and its conjugate base A. These include orbital effects, electronegativity, inductive effects, resonance effects, solvation hydrogen bonding, steric effects, and aromaticity. 

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

Nitrogen, N2, is very unreactive. The Haber process is the economically important industrial process by which atmospheric N2 is converted to ammonia, NH3, a soluble, reactive compound. Innumerable dyes, plastics, explosives, fertilizers, and synthetic fibers are made from ammonia. The Haber process provides insight into kinetic and thermodynamic factors that influence reaction rates and the positions of equilibria. In this process the reaction between N2 and H2 to produce NH3 is never allowed to reach equilibrium, but moves toward it. 

Factors which affect the oxepin-benzene oxide equilibrium positions are similarly expected to influence the thiepin-benzene episulfide distribution at equilibrium. Since however the thianorcaradiene tautomer has not to date been detected, the main evidence for this form is based upon the thermal instability and reactions of the thiepin system. Thus it is assumed that where the thianorcaradiene isomer is present, a spontaneous thermal decomposition involving extrusion of a sulfur atom will occur. Substitution at the 2,7-positions in the oxepin-arene oxide system leads to a preference for the seven-membered ring form and this effect was further enhanced by bulky substituents (e.g. Bu ). A similar effect was observed in thiepins and thus the remarkable thermal stability of (49) (2,7-r-butyl groups) and (51) (2,7-hydroxyisopropyl groups) contrasts with the behavior of thiepin (55)(2,7-isopropyl groups), which was thermally unstable even at -70 °C (78CL723). The stability of thiepin (49) results from the 2,7-steric (eclipsed) interactions which obtain in the thianorcaradiene form but which are diminished in the thiepin tautomeric form (relative to the episulfide tautomer). 

As in the oxepin-arene oxide system, the resonance effect will also influence the position of equilibrium in the analogous organosulfur series. Compounds (46) and (53) thus appear to exist exclusively in the thiepin form. Since the resonance factor would favor the 1,2-episulfide of naphthalene over the thiepin tautomer (54) it is highly improbable that this thiepin will be detectable at ambient temperature. Both thiepins (46) and (55) have been isolated as thermolabile compounds (78JOC3379, 81MI51700). 

Other factors (charge repulsion, solvation factors, etc.) could influence the position of the equilibrium in favor of enolate dianion 216. It is also possible that there is a kinetic preference for the formation of dianion 216 and that this species would undergo protonation more rapidly than equilibration. This rule of axial protonation" of 216 has been found to be widely applicable in many cases. However, in systems in which a significant amount of strain must be introduced in order for protonation to occur axially on 216, protonation of conformer 217 (and even conformer 218) becomes important (60). 

Although the presence of mixed bridging ligands may favour the formation of high nuclearity assemblies, it remains difficult to pinpoint the factors that give rise to these unusual cyclic products. In accordance with the previous discussion, it seems likely that these multi-component species may exist in solution in equilibrium with related oligomeric species their isolation will thus be influenced by both the position of the equilibrium as well as by the relative solubilities of the respective species in the chosen reaction solvent. 

This latter assumption is not necessary when the reaction is analyzed by ESR spectroscopy since one is observing the radical directly. Table 13 lists the structures of the thermodynamically (ESR) or chemically more favored cyclopropyl radicals. There are a number of factors which will influence the position of the equilibrium. Among them are steric effects and electronic effects. As can be seen in Table 13, entries 5, 6 and 13-21 are examples in which the position of the equilibrium is influenced by steric interactions. Entry 5 shows that the (t orbital containing the odd electron prefers to be cis to the phenyl group... 

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