Heats of Hydrogenation. V. Relative Stabilities in Certain Exocyclic-Endocyclic Olefin Pairs

Abstract

Stability differences in a number of isomer pairs possessing exocyclic and endocyclic double bonds have been investigated by the hydrogenation method. In all cases that have been examined thus far the endocyclic modification possesses the lower heat of hydrogenation and hence the greater stability. The heats of hydrogenation (kcal./mole) that have been measured in the present study are: methylenecyclopentane (-26.9), 1-methylcyclopentene (-23.0); methylenecyclohexane (-27.8), 1-methylcyclohexene (-25.4); methylenecycloheptane (-26.3), 1-methylcycloheptene (-24.0); ethylidenecyclopentane (-24.9), 1-ethylcyclopentene (-23.6); ethylidenecyclohexane (-26.3), 1-ethylcyclohexene (-25.1). Stability relationships derived from the hydrogenation work have been confirmed in several instances by studies of acid-catalyzed isomerization. © 1958, American Chemical Society. All rights reserved.