8 - C7H6−C7H12

Abstract

This chapter presents hydrocarbon isomerizations, which have been at or near the center of experimental and theoretical efforts by physical organic chemists, such as C7H6, C7H8, and C7H10. Deuterium labeling studies indicate that the course of the pyrolysis of the bicyclic triene involved cleavage of the C1, C7 bond followed by a vicinal hydrogen shift that gave a biradical in which 1,5-hydrogen shifts could give an appropriate cyclopentadienyl radical, which is the fulvenallene. Norcaradiene is accessible from tropilidene as evidenced by the formation of Diels–Alder adducts of it when the monocycle is treated with various dienophiles. The low activation enthalpy and negative activation entropy for the cis isomer strongly suggests a concerted 3,3-shift, necessarily via a boat-like transition state with the vinyl groups over the ring. But the trans isomer apparently isomerizes to the cis compound in the rate-determining step judging by the small positive activation entropy and an activation energy comparable to the bond dissociation energy of the doubly allylic cyclopropane bond.