Reactivity effects substituents, positive charge interaction

Tidwell and coworkers (Chwang etai, 1977 Koshy etai, 1979) focused their interest on the reactivity-structure relationship of the hydration rates. Additivity of substituent effects of the four substituents in the ethylene has been studied. As the interaction of substituents with the positive charge in the intermediate [36C "] is analogous to that in the electrophilic aromatic substitution intermediate, the substituent effects on the hydration of [36] may... 

The precise reactivities of various types of carbenium ions (free, paired, and aggregates) are not known, but it seems that the differences in reactivities are not very large. This may be due to solvation effects, which are similar for all types of carbocations, and also due to the large size of the counterions which interact weakly with the cations. A positive charge in carbocations is located partially on the sp2-hybridized C atom and is widely delocalized over the /3-protons and substituents, especially in the case of aromatic and alkoxy a-substituents. 

Reactivity and selectivity are largely determined by the position of the TS on the reaction coordinate. With highly reactive electrophiles, the TS will come early on the reaction coordinate as in Figure 9.7a. The TS then resembles the reactants more closely than the intermediate. The positive charge on the ring is small, and, as a result, the interaction with the substituent group is relatively weak. However, the substituent also effects electron distribution in the reactant, which can cause position selectivity. 

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