One‐Step Paths of the Alkene Hydration Revealed by a DFT Study

Abstract DFT calculations were carried out for alkene hydration reactions. A model, alkene + H 3 O + (H 2 O) 13 → alcohol + H 3 O + (H 2 O) 12 , was adopted to assure proton transfer routes. The second and larger one, alkene + H 3 O + (H 2 O) 21 → alcohol + H 3 O + (H 2 O) 20 , was also employed for some key reactions. Alkene substrates adopted here are ethylene, propylene, isobutene, styrenes (α‐methyl, para Z‐C 6 H 4 ‐CH=CH 2 , Z=NO 2 , CN, CF 3 , Cl, H, Me, OMe, OH, NH 2 and NMe 2 ). Transition‐state geometries were determined, and the subsequent intrinsic reaction coordinate (IRC) analyses were made. In contrast to the traditional scheme, the stepwise path including the carbocation is quite limited (only for the styrene with Z=OMe, OH, NH 2 and NMe 2 ). Even for the isobutene substrate, the tert ‐Bu + cation intermediate was not found and a one‐step path was obtained. Calculated activation energies, +32.52 kcal/mol (a) for ethylene, +27.10 kcal/mol (b) for propylene, +21.33 kcal/mol (c) for isobutene, +19.79 kcal/mol (d) for styrene and +23.79 kcal/mol (e) for para ‐nitrostyrene, are in good agreement with experimental values, +32.4±1 kcal/mol (a), +26.4±1 kcal/mol (b), +21.1±0.2 kcal/mol (c), +19.2±0.5 kcal/mol (d) and +23.2 kcal/mol (e), respectively. The solvent isotope effect k(H 2 O)/k(D 2 O) was calculated and was compared with the experimental data.