The Elimination-Addition Mechanism of Nucleophilic Aromatic Substitution

Author: Panar, Manuel

Year: 1961

Degree: Dissertation (Ph.D.)

Advisor: Roberts, John D.

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/5JAK-A186

Abstract

Part I: The elimination-addition (benzyne) mechanism postulated for nucleophilic substitutions of non-activated halobenzenes provided the first coherent explanation of the rearrangements which frequently accompany these reactions. However, several less likely alternative mechanisms were able to explain the available data. These alternatives have been disproven by the finding that the amination of iodobenzene-l-^(14)C-2,4,6-^2H_3 gives the same extend of ^(14)C rearrangement in the resulting aniline as does iodobenzene-l-^(14)C. Part II: Substituent effects in aminations of halobenzenes have been explained by inductive control of both the direction of formulation of benzyne and the manner of addition of the nucleophile to the reactive benzyne bond. Although the intermediacy of benzyne in non-rearranging aminations of halobenzenes having electron-withdrawing substituents in the 3-position has been rationalized, the possibility of direct displacement remained. The intermediacy of benzyne has been confirmed by a determination of the isotope effect in reactions of 3-bromoanisole-2,4,6-^2H_3. Part III: While leaving groups such as non-activated halogens react with amide ion in liquid ammonia via a benzyne intermediate, others react entirely or primarily via direct substitution. In order to obtain needed data on the leaving group characteristics which lead to benzyne formation, attempts have been made to find further non-activated substituted benzenes which react with amide ion.

Part IV: While all alternatives to the elimination-addition mechanism for amination of non-activated halobenzenes have been disproven one cannot rule out the possibility that benzyne exists as a c_6H_4-complex with halide ion or metal ion. In order to demonstrate the ability of benzyne to exist as a stable C_6H_4 hydrocarbon it would be desirable to pass benzyne through the vapor-phase. Several experiments of this nature have been attempted.

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