KINETICS AND MECHANISM OF ACID-CATALYZED REARRANGEMENT OF 1-PHENYL-3-METHYLALLYL ALCOHOL AND HYDRATION OF 1-PHENYL-1,3-BUTADIENE

The kinetics of the acid-catalyzed rearrangement, racemization, and 18O exchange of l-phenyl-3-methyl-allyl alcohol (1) have been studied in 40% aqueous dioxane at 25.0°. The three reaction rates were found to be identical in a given acid solution, and were further shown to be proportional to the acid concentration with the same second-order rate constant, 6.8 × 10-2 1. mol-1 sec-1. The kinetics of racemization and 18O exchange of 3-phenyl-1-methylallyl alcohol (2) were also studied and the rates shown to be the same within experimental error, with second-order rate constant 1.2 × 10-3 1. mol-1 sec-1. Finally, the rates of dehydration of 2 and hydration of 1-phenyl-1,3-butadiene (3) were followed and shown to correlate with the acidity of the medium, a plot of In k vs. H0 being linear with slope -1.49 in 40% dioxane and -1.22 in water. The mechanism of rearrangement of 1 thus involves protonation of 1 followed by unimolecular dissociation of the protonated species to form the intermediate carbonium ion, which is partitioned almost exclusively in favor of 2. The hydration of 3 proceeds via a rate-determining protonation of 3 to form the same carbonium ion, and the rate of the dehydration of 2 is governed by the corresponding deprotonation of the carbonium ion. © 1969, American Chemical Society. All rights reserved.