The mechanisms of double-bond migration and the exchange of D2 with the hydrogen atoms of small cyclic olefins (methylenecyclobutane, methylenecyclopentane, cyclopentene, S-methylcyclopentene, cyclohexene, and bicyclo-(2,2,1)-hepta-2,5-diene) were investigated over a pure alumina catalyst. At temperatures below lOO", only those hydrogen atoms which were initially vinyl, or which could become vinyl by isomerization of the olefin, underwent exchange. A primary kinetic isotope effect of about 2.8 was found, indicating that cleavage of the C-H bond was probably the slow step in the exchange reaction. Rapid intermolecular scrambling of all vinyl hydrogen atoms was observed, and the presence of the olefins greatly reduced the usually fast rate of H2 +D2 equilibration.Double-bond migration below 100" was sensitive to the geometry of the olefin. Only molecules having a three carbon chain, including the double bond, which could appear concave when viewed from outside the molecule underwent isomerization ; those which did not fulfill this requirement did not isomerize. Nearly pure 1 ,2-cyclopentene-d2 could be prepared, suggesting a dissociative mechanism for the exchange reaction. Hence, exchange and double-bond migration are independent processes, although both reactions may have involved the same sites and may have had a common intermediate. Poisoning experiments using radioactive C 0 2 indicated an active site density of about 1.4 x 1013 cm-2 for the exchange reaction after 530" pretreatment.Larson, Hightower, and Hall found that perdeuteriocyclopropane (and small non-cyclic olefins) could be prepared without skeletal rearrangement or extensive saturation by exchange of their hydrogen atoms with D2 over an alumina catalyst. Double-bond migration and cis-trans isomerization occurred simultaneously with the exchange, but further work showed that the exchange and isomerization reactions were independent. Moreover, the vinyl C-H bonds in these molecules exchanged preferentially below 100". The data suggested that the chemisorption was dissociative, involving reaction with strained A1-0-A1 linkages. These deuterium tracer studies have now been extended to include cyclic olefins, and the same general reaction characteristics hold. In addition, the rigid configuration of the small cyclic olefins allowed some of the geometric aspects of double-bond migration to be investigated. The present work was undertaken to provide additional insight into the nature of alumina as a catalyst using the structure of the olefin as a probe. The chemical nature and surface density of active sites for exchanges were measured in experiments in which known amounts of specific compounds were used to " titrate '' the reaction sites.
EXPERIMENTAL CATALYSTThe same 0.40 g sample of GA-48 alumina, used in some previous work,2 was reactivated and used for most of the experiments reported herein. In some cases, however, a 0.20g