Molecular Modeling to Predict Regioselectivity of Hydration Reactions
In a series of introductory organic chemistry laboratory experiments, students oxidize several isomeric alkenes using acid-catalyzed hydration, oxymercuration/demercuration, and hydroboration to compare the regioselectivity of the different techniques. The product mixtures are subsequently analyzed by GC and IR. To explain the results fully, students use the Spartan 5.0 molecular modeling package to predict the regioselectivity of these hydration reactions. For the acid-catalyzed hydration reaction, they predict the products by calculating the stability of the possible carbocation intermediates using the semiempirical AM1 program. Using semiempirical MNDO calculations, they determine the activation energies for the different possible transition states of the hydroboration reaction. With the activation energies, they can easily predict the preferred products. Semiempirical calculations with a PM3 (tm) basis set to determine the geometry and electrophilicity of the LUMO of the mercuronium ion formed when 1-propene undergoes a reaction with Hg(OAc)2 allow students to investigate the regioselectivity of the oxymercuration reaction. Using this information, they are able to explain the ratio of products obtained in the laboratory experiment. The calculation and visualization of these reaction pathways greatly enhances students' understanding of the oxidation experiments performed in the lab.
Graham, K. J.; Sloglund, K.; Schaller, C. P.; Muldoon, W. P.; Klassen, J. B. Molecular Modeling to Predict Regioselectivity of Hydration Reactions. J. Chem. Educ., 2000, 77 (3), 396.