極低温固相中での有機反応 水素原子引き抜き反応の制御因子

Abstract

Studies on hydrogen-atom abstraction from organic molecules in cryogenic solids are reviewed here. Hydrogen atoms generated in cryogenic solids are shown to abstract hydrogen atoms from organic molecules even at 4.2 K due to quantum-mechanical tunneling. The location of a hydrogen-abstracted carbon atom in an alkane molecule is determined not only by the activation energy for hydrogen abstraction but also by the degree of steric hindrance which prevents the deformation of the molecule accompanied by the abstraction. The decrease in the deformation slows down the dissipation of the heat of hydrogen abstraction to the product molecule. The rate of hydrogen abstraction thereby decreases with increasing steric hindrance which increases with increasing number and length of alkyl chains bonded to a carbon atom. The tertiary carbon atom of 3-methylpentane is therefore not hydrogen-abstracted, though it has the lowest activation energy for the abstraction and is the easiest to be hydrogen-abstracted in liquid.