Carbocations three main reactions

The addition of an alkene to formaldehyde in the presence of an acid catalyst is called the Prins reaction.Three main products are possible which one predominates depends on the alkene and the conditions. When the product is the 1,3-diol or the dioxane, the reaction involves addition to the C=C as well as to the C=0. The mechanism is one of electrophilic attack on both double bonds. The acid first protonates the C=0, and the resulting carbocation attacks the C=C ... 

Until recently, knowledge about absolute and relative rates of reaction of alkenes with carbocations was very limited and came almost exclusively from studies of carbocationic polymerizations [119-125]. The situation changed, when it became obvious that reactions of carbocations with alkenes do not necessarily yield polymers, but terminate at the 1 1 product stage under appropriately selected conditions (see Section III.A). Three main sources for kinetic data are now available Relative alkene and carbo-cation reactivities from competition experiments, absolute rates for reactions of stable carbocation salts with alkenes, and absolute rates for the reactions of Laser-photolytically generated carbocations with alkenes. All three sets of data are in perfect mutual agreement, i.e., each of these sets of data is supported by two independent data sets. 

Carbocation intermediates are formed in three main types of reactions additions to pi bonds, unimolecular eliminations, and unimolecular nucleophilic substitution. 

The exclusive formation of re-butane at short contact time in the ethane-ethylene reaction indicates that a mechanism involving the addition of ethyl cation to ethylene cannot be operative here, since in excess superacid ethylene is completely protonated. Even if this reaction could occur, trivalent butyl cations for conventional acid-catalyzed alkylations cannot participate in this process. The involvement of the trivalent butyl cation should produce isobutane as the main product the 1- and 2-butyl cations would preferentially isomerize to the tert-butyl cation and thus yield isobutane (eq. 52). Since there is no free ethylene present, the ethyl cation will attack ethane via a pentacoordinate (three-center, two-electron) carbocation (15) to produce re-butane (eq. 53). Mechanistic studies with labeled compounds also demonstrated the validity of this mechanism (79). 

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