Mechanism of Hydration of Simple Olefins in Aqueous Solution. cis- and trans-Cyclooctene

Article abstract of DOI:10.1021/ja00308a033

Rates of hydration of cis- and trans-cyclooctene and 2,3-dimethyl-2-butene to the corresponding alcohols have been measured in concentrated and dilute aqueous perchloric acid, and those of the latter two olefins in bisulfate ion and phosphoric acid buffer solutions as well.The systems examined in buffers show general-acid catalysis.The reaction of trans-cyclooctene is not reversible, but those of cis-cyclooctene and 2,3-dimethyl-2-butene are; for cis-cycloctene, K=/=1.8 and for 2,3-dimethyl-2-butene, K ca. 4.For hydration of trans-cyclooctene, ΔH*= 22 kcal mol^-1, ΔS*= 1 cal K^-1 mol^-1, and k_H(1+)(25 deg C)= 5.2x10^-4 M^-1 s^-1; for the hydration of cis-cyclooctene, ΔH*= 24 kcal mol^-1, ΔS*= -10 cal K^-1 mol^-1, and k_H(1+)( 25 deg C)= 2.1x10^-7 M^-1 s^-1; and for the rate of approach to equilibrium in the 2,3-dimethyl-2-butene system, k_H(1+)( 25 deg C)= 2.9x10^-4 M^-1 s^-1.The lifetime of tertiary carbocations such as that formed by protonation of 2,3-dimethyl-2-butene is estimated to be τ ca. 10^-10 s in dilute aqueous solution, which allows this ion to be a viable, solvationally equilibrated intermediate in the hydration reaction.The secondary cyclooctyl cation is likewise judged to be a solvationally equilibrated species in concentrated aqueous acids, with τ ca. 5x10^-8 to 5x10^-9 s in the 45-55 wt percent HClO4 solutions used for the hydration of cis-cyclooctene.In dilute aqueous solution, however, carbocation lifetimes are shorter, and τ ca. 5x10^-12 s is estimated for the cyclooctyl cation in dilute acids such as those used for the hydration of trans-cyclooctene.Species as short-lived as this can probably still be reaction intermediates, but they are not solvationally equilibrated and may have to react by preassociation mechanisms; an argument is presented that shows that such a mechanism is likely not to be required in the hydration of trans-cyclooctene.