Studies on the mechanism of olefin-alcohol interconversion. The effect of acidity on the O-18 exchange and dehydration rates of t-alcohols

Evidence from the acidity and temperature dependence of the rates of 018 exchange and dehydration of t-alcohols has been obtained which indicates that these two reactions proceed by common initial steps but involve different rate-determining steps. The results appear to eliminate a mechanism in which a '' free '' carbonium ion isomerizes to a pi-complex in the rate-determining step of the alcohol-olefin interconversion. However, present results are found to be consistent with mechanisms which involve (in the formation of olefin from alcohol) either a rate-determining reaction of a free carbonium ion with water or the isomerization of an encumbered carbonium ion intermediate. R+...OH2, to a pi-complex. The encumbered carboniurn ion classification is proposed for intermediates which possess substantial carbonium ion character and involve measurable interactions between the cationic center and leaving or entering groups of either neutral or various charge types. The mechanism involving the encumbered carbonium ion intermediate is favored.