EFFECT OF SOLID-PHASE ON THE SELECTIVITY OF HYDROGEN ABSTRACTION FROM N-ALKANES - AN ELECTRON-SPIN-RESONANCE AND ELECTRON-SPIN ECHO STUDY

The effect of medium phase on the formation of alkyl radicals from n-alkanes in solids at 77 K due to hydrogen abstraction by deuterium atoms has been examined by means of electron spin resonance and electron spin echo spectroscopies. In perdeuterated crystalline n-alkane, radiolytically produced deuterium atoms abstract any hydrogen atom in the solute molecule of the protiated analogue. In the glassy solid of perdeuterated alcohols, a deuterium atom selectively abstracts a hydrogen atom on the penultimate carbon atom of solute n-alkane. The reaction proceeds through quantum-mechanical C-H hydrogen tunneling. The difference of the selectivity of hydrogen abstraction arises from the effect of solid phase on the bending motion of the alkane C-C-C bond assisting the abstraction of hydrogen. The bending motion is impeded in the glassy solid by two alkyl chains which are bonded to the carbon atom and piled into the solid, whereas it is not impeded in the crystals because of the in-plane accordion motion of the carbon skeleton.