Reactant stability

Besides differences in reactant stability as well as differences in the stability of the intermediate ions, there may also be other reasons, such as differences in solvation and differences in backside nucleophilic assistance, for the large discrepancy in reactivity between alkyl halides and vinyl halides. 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

Experimental work is often hampered by limited reactant stability. Any redox process that is sufficiently energetic to produce electronic excitation necessarily involves very potent redox agents. Only in . ew cases is it feasible to isolate the reactants in bulk and react them by a Cow method. More often they are electrolytically generated as needed from parent substances such as neutral aromatics. Electrochemical generation offers several advantages ... 

In the interests of improved electrochemical background limits and reactant stability, it is important to employ solvents that are as free as possible of nucleophiles and proton sources. Special attention always goes to the removal of water. The most important media are carefully purified acetonitrile, dimethyl-formamide, benzonitrile, and tetrahydrofuran. Popular supporting electrolytes are tetra-n-butylammonium perchlorate (TBAP) and fluoroborate (TBABF4). Solutions are usually prepared by vacuum-line methods (Chap. 18) or in a dry box (Chap. 19) to exclude oxygen from the systems and to avoid contamination by water. 

Product stabilizing factor Reactant stabilizing factor... 

Product or reactant stabilizing factors that have been studied thus far include resonance/charge delocalization, solvation, hyperconjugation, intramolecular hydrogen bonding, aromaticity, inductive, jr-donor, polarizability, steric, anomeric, and electrostatic effects, as well as ring strain and soft-soft interactions. Product or reactant destabilization factors are mainly represented by anti-aromaticity, steric effects in some types of reactions, and, occasionally, electrostatic effects. What makes the PNS particularly useful is that it is completely general, mathematically provable,4 and knows no exception. 

There exists substantial evidence that in reactions that involve oxyanions or amines as bases or as nucleophiles, their partial desolvation, as they enter the transition state, typically has made greater progress than bond formation. In the context of the PNS, this partial loss of solvation represents the early loss of a reactant stabilizing factor and hence reduces the intrinsic rate constant. As discussed at some length in our 1992 chapter,4 for strongly basic oxyanions this desolvation effect often manifests itself in terms of negative deviations from Br Ansted plots and/or in abnormally low p or pnuc values.58,188 In fact, a number of cases have been reported where the pnuc value was close to zero or... 

Table 26 The effect of product/reactant stabilizing/destabilizing factors on AG and ka... 

Probably both reactant stabilization and the already evaluated relative instability of the cationic transition state contribute to the slowness of the solvolysis of vinyl components, but other factors are certainly involved. The most obvious experimental problem is whether the compounds compared react by a unimolecular mechanism or nucleophilic attack by the solvent is involved to a certain extent. In the case of vinylic systems, for instance, nucleophilic solvation from the rear is in general much more hindered than in the case of saturated compounds and the transition state is likely to be stabilized only by electrophilic solvation of the leaving group (Rappoport and Atidia, 1970). The low m values observed in the case of vinyl halides or sulphon-ates may be taken as a strong indication of poor solvation of the transition state in solvolytic reactions of vinyl derivatives. These and other complications, such as differences in hyperconjugation, differences in electronegativity of the -—C= and —- bonds (Jones and Maness,... 

Intramolecular nucleophilic substitution to form thiiranes was studied by means of ab initio MO computations based on the 6-31G basis set <1997JCC1773>. Systems studied included the anions SCH2CH2F and CH2C(=S)CH2F which would afford thiirane and 2-methylenethiirane, respectively (Equations Z and 3). It was important to include electron correlation which was done with the frozen-core approximation at the second-order Moller-Plesset perturbation level. Optimized structures were confirmed by means of vibrational frequency calculations. The main conclusions were that electron correlation is important in lowering AG and AG°, that the displacements are enthalpy controlled, and that reaction energies are strongly dependent on reactant stabilities. 

R-X —> LR - -Xj —> R f X Reactant Transition state Products More polar than reactant stabilized more by solvation... 

Figure 6.1 Reactant stabilization relative to the intermediate increases the reaction barrier.

AhydH was measured in n-hexane solution. The sequence -28.5, -27, and -22.5 for Ahyd7/ of cyclohexene, cyclopentene, and 1,2-dimethylcyclopentene shows the influence of hydrogen atom crowding in the cyclopentane ring relative to cyclohexane (product destabilization) and reactant stabilization of the double bond in 1,2-dimethylcyclohexene by two a methyl groups. 

Reactex. [Ivax Industries] Reactant stabilizer for textile finishes. 

Theoretical Consideration The decomposition temperature is an important, if not the most important kinetic parameter used in studies of the decomposition processes. It defines the upper limit of reactant stability and the onset of a decomposition reaction. However, temperature is most commonly used only as an additional factor in determination of the Arrhenius parameters. (For instance, Galwey [1] used an average decomposition temperature in his estimations of A values basing on E parameters, known for various substances.) No quantitative definition of the concept of an initial decomposition temperature has been developed, based on a certain specified value of the decomposition rate J, or on parameters related to it (the rate constant k, or the equilibrium pressure of gaseous products Pb)- The detailed interrelation between the decomposition temperature and the molar enthalpy,... 

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