Ionization-structure-reactivity

To sum up, the rate retardation attributed to steric effects of bulky alkyl groups can arise from substituent-electrophile, substituent-substituent and substituent-solvent interactions in the first ionization step of the reaction and also from substituent-nucleophile interactions in the product-forming step. It is therefore not surprising that the usual structure-reactivity correlations or even simpler log/log relationships cannot satisfactorily describe the kinetic effects of alkyl groups in the electrophilic bromination of alkenes. 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents, and the reactivity of the peroxy acids is increased by EWG substituents." These structure-reactivity relationships demonstrate that the peroxy acid acts as an electrophile in the reaction. Low reactivity is exhibited by double bonds that are conjugated with strongly EWG substituents, and very reactive peroxy acids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds. " Strain increases the reactivity of alkenes toward epoxidation. Norbornene is about twice as reactive as cyclopentene toward peroxyacetic acid." trani-Cyclooctene is 90 times more reactive than cyclohexene." Shea and Kim found a good correlation between relief of strain, as determined by MM calculations, and the epoxidation rate. ° There is also a correlation with ionization potentials of the alkenes. Alkenes with aryl substituents are less reactive than unconjugated alkenes because of ground state stabilization and this is consistent with a lack of carbocation character in the TS. 

The study of structure-reactivity relationships by the organic chemist Hammett showed that there is often a quantitative relationship between the two-dimensional structure of organic molecules and their chemical reactivity. Specifically, he correlated the changes in chemical properties of a molecule that result from a small change in its chemical structure that is, the quantitative linear relationship between electron density at a certain part of a molecule and its tendency to undergo reactions of various types at that site. For example, there is a linear relationship between the effea of remote substituents on the equilibrium constant for the ionization of an acid with the effect of these substituents on the rate or equilibrium constant for many other types of chemical reaction. The relative value of Hammett substituent constants describes the similarity of molecules in terms of electronic properties. Taft expanded the method to include the steric hindrance of access of reagents to the reaction site by nearby substituents, a quantitation of three-dimensional similarity. In addition, Charton, Verloop, Austel, and others extended and refined these ideas. Finally, Hansch and Fujita showed that biological activity frequently is also quantitatively correlated with the hydrophobic character of the substituents. They coined the term QSAR, Quantitative Structure-Activity Relationships, for this type of analysis. 

Both compounds react by an SnI mechanism, and their relative rates reflect their- activation energies for carbocation fonnation. Because the allylic chloride is more reactive, we reason that it ionizes more rapidly because it for-rns a more stable car bocation. Structurally, the two carbocations differ in that the allylic carbocation has a vinyl substituent on its positively charged carbon in place of one of the methyl groups of ten-buXyl cation. 

The present results show that the first step of the interaction between olefins and Bt2 is the formation of CTCs, whose Kf are highly sensitive to structural effects. Both Kf ratios and reactivity ratios of olefins are scarcely affected by the solvent. An increase by two in number of alkyl substituents on the double bond increases both Kf and kobsd roughly by a factor of 103. Therefore, at variance with the expectation for an AdgCl mechanism, substituent effects are not much more influential on k tsd than on Kf. This suggests that the rates of CTC ionization be actually reduced by reversal. 

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