Acid-Catalyzed Hydrolysis of Bridged Bi- and Tricyclic Compounds. 25. Comparison of the Hydrations of 2-Methyl-2-norbornene and 2-Methylenenorbornane with Those of 1-Methylcyclohexene and Methylenecyclohexane

Article abstract of DOI:10.1021/jo00359a026

Hydration rates of 2-methyl-2-norbornene, 2-methylenenorbornane, 1-methylcyclohexene, and methylenecyclohexane were measured spectrophotometrically in aqueous perchloric acid.The activation parameters and solvent deuterium isotope effects are in all cases in agreement with the slow proton transfer to an olefinic carbon atom.The free energy diagrams show that the Gibbs energy of the transition state of protonation (hydration) is higher for methylenecycloalkanes than for methylcycloalkenes.The energy difference is small (0.8 kJ mol^-1) in the case of the bicyclic olefins and large (11.5 kJ mol^-1) in the case of the monocyclic olefins mentioned.Thus, no marked difference in the energies of the transition states caused by a possible distortion of the ?-orbitals of 2-methyl-2-norbornene can be seen in the hydrations of the bicyclic olefins.An explanation for the latter large difference is evidently a change of conformation during the protonation of 1-methylcyclohexene, which possibly also causes an exceptionally low isotope effect (k_H/k_D=1.13).