The Role of Ion−Molecule Pairs in Solvolysis Reactions. Nucleophilic Addition of Water to a Tertiary Allylic Carbocation

The acid-catalyzed solvolysis of 2-methoxy-2-phenyl-3-butene (1-OMe) in 9.09 vol % acetonitrile in water provides 2-hydroxy-2-phenyl-3-butene (1-OH) as the predominant product under kinetic control along with the rearranged alcohol 1-hydroxy-3-phenyl-2-butene (2-OH) and a small amount of the rearranged ether 2-OMe. The more stable isomer 2-OH is the predominant product after long reaction time, K eq = [2-OH] eq/[1-OH]eq = 16. The ether 2-OMe reacts to give 2-OH and a trace of 1-OH. Solvolysis of 1-OMe in 18O-labeled water/acetonitrile shows complete incorporation of 18O in the product 1-OH, confirming that the reaction involves cleavage of the carbon−oxygen bond to the allylic carbon. A completely solvent-equilibrated allylic carbocation is not formed since the solvolysis of the corresponding chloride 1-chloro-3-phenyl-2-butene (2-Cl) yields a larger fraction of 1-OH. This may be attributed to a shielding effect from the chloride leaving group. Quantum chemical calculations of the geometry and charge distribution show that the cation should rather be described as a vinyl-substituted benzyl cation than as an allyl cation, which is in accord with its higher reactivity at the tertiary carbon.