Addition of acetic acid to norbornadiene. Mechanisms of addition reactions
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The addition of acetic acid to norbornadiene (I) has been studied under a large variety of conditions and the extents of both homoconjugate addition and Wagner-Meerwein rearrangements accompanying addition have been scrutinized in some detail. The work is described in this and in the following three papers. In this paper we have discussed our interpretation of the data; the second paper describes details of the acid-catalyzed additions; the third describes uncatalyzed additions; and the fourth describes details of mass spectral analysis. The data which are discussed herein include the nature of acid catalysis in acetic acid, salt effects, and medium effects on catalyzed and uncatalyzed reactions as noted by reaction rates and by amounts of rearrangement accompanying addition. The data require a multiplicity of mechanisms. endo-Protonated pi complexes are not utilized, and exo-protonated pi complexes are not product-determining intermediates. The data are accommodated by assuming competitions between cyclic (noncationic, but polar) processes and processes involving ion pairs, ion dipoles, and solvated ions as intermediates.