Reactivity of cyclic and acyclic olefinic hydrocarbons in acid-catalyzed hydration

Abstract

The rates of hydration of ethylene, propene, 1-hexene, cyclohexene, 1-methylcyclohexene, 1-methylcyclopentene, and norbornene catalyzed by H/sub 2/SO/sub 4/ and D/sub 2/SO/sub 4/, and the hydration of cyclohexene catalyzed by HClO/sub 4/ and DClO/sub 4/, all at 25/sup 0/C, were measured. Ethylene is only a factor of 24 less reactive than p-nitrostyrene at H/sub 0/ = -7.37, but has a very steep slope of log k/sub obsd/ vs. H/sub 0/ of -1.54 so that ethylene rates extrapolated to low acidities appear quite small relative to other alkenes. All of the substrates are interpreted to react by rate-determining protonation on carbon to give the corresponding carbonium ions (the A/sub SE/2 mechanism). Solvent isotope effects k/sub H/sup +///k/sub D/sup +// range from 2.82 for propene to 1.06 for cyclohexene, and are only tenuously related to absolute reactivity. Norbornene is 770 times more reactive than cyclohexene, and at least part of this difference is attributed to the unsymmetrical ..pi.. bond in this molecule endowing it with enhanced nucleophilicity on the exo face.