Medium Effect on Chemical Reactions. Hydrogen Abstraction from Alkanes in Liquid and Solid Phases.
Tsuneki Ichikawa, Hiroshi Yoshida · Proceedings of the Japan Academy Series B · 1992
The location of hydrogen-abstracted carbon atoms in alkane molecules has been found to depend strongly on the physical phase of the medium. In liquid branched alkanes at ambient temperature, hydrogen atoms preferentially abstract C-H hydrogen from the tertiary carbon atoms. In the amorphous solid of the branched alkanes at 77K, on the other hand, the hydrogen abstraction takes place very selectively at the penultimate secondary carbon atoms. In the crystalline solid of n-alkanes at 77K, hydrogen atoms abstract C-H hydrogen on penultimate or another internal secondary carbon atoms of the n-alkanes. In the amorphous solid of deuterated alcohols, the hydrogen abstraction takes place only at the penultimate carbon atoms of n-alkanes. These results indicate that the rate of chemical reaction in a condensed medium is determined not only by the steric factor and the activation energy of the reaction but also by the relaxation rate of the product molecule. A preliminary theory shows that the relaxation accelerates the reaction by enhancing transitions from an initial reactant state to many final states with different energies.