The direct and sensitized photochemical behaviour of cyclic and acyclic olefins is compared. Under either set of conditions cyclo-hexenes and -heptenes undergo photoprotonation in hydroxylic media, perhaps via a trans intermediate. By contrast acyclic, exocyclic, and larger-ring cyclic olefins fail to undergo photoprotonation on either direct or sensitized irradiation. However, both cyclohexenes and cycloheptenes, as well as larger-ring cyclic and acyclic olefins, undergo slow isomerization to

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... ositional and skeletal isomers on direct irradiation in either protic or aprotic media. Unique behaviour has been observed for tetraalkyl-substituted olefins, which exhibit behaviour on direct irradiation in alcoholic solvents suggesting the intervention of a cation radical intermediate. Mechanistic implications of these various types of photochemical behaviour are discussed.