Studies of radical–molecule reactions using a rotating cryostat. Reactions of hydrogen atoms with organic substrates at 77 K

Abstract

The rotating cryostat has been used in conjunction with electron spin resonance (e.s.r.) spectroscopy to study directly the reactions of hydrogen and deuterium atoms with a range of organic substrates in the solid state at 77 K. In every case the primary radicals formed by the reaction were trapped and could be observed by e.s.r. For ethylene, allene, ketene and acetaldehyde, free radicals formed by a secondary reaction were also observed.

No primary abstraction reactions were observed for any of the alkenes studied, but hydrogen abstraction occurred almost exclusively for acetaldehyde and allyl alcohol, and both abstraction and addition reactions were observed for substituted benzenes.

The relative amounts of addition to the two ends of the double bond in a series of asymmetric alkenes were measured and are in qualitative agreement with predictions based on quantum mechanical calculations, though for pent-2-ene the experimental selectivity was much greater than expected. A small secondary inverse deuterium isotope effect was observed for the addition of hydrogen atoms to 1,1-dideuteroethylene.

For the substituted benzenes preferential addition to the ortho-position of the ring occurred, with much less addition to the para-position and virtually none at the meta-position.