Reactions of ethyl bromide and aluminium bromide. II. Isomerisation of ethyl‐[1‐¹⁴C] bromide

Abstract

The isomerisation of ethyl-[ l-"C] bromide to ethyl-[ ZWC] bromide in the liquid phase is catalysed by aluminium bromide. From measurements of the reaction rate at temperatures between +25" and +50° the activation energy of this reaction appears to be 19.1 kcal/mole (2 2 %).The isomerisation of alkyl halides was first studied a long time ago *) and shortly after the discovery of the reaction of FriedeZ and Crafts it was noted that these rearrangements are catalysed by the aluminium halides. The mechanism of these isomerisation reactions under the influence of Lewis acids is of considerable interest with respect to our knowledge of the mechanism of the Friedel-Crafts alkylation.As part of a series of investigations into problems connected with the mechanism of Friedel-Crafts reactions we have investigated the isomerisation of some simple alkyl halides under the influenceof aluminium bromide. Isomerisation reactions of this type have been known since the investigations of KekuZ6 8 ) and Gustauson s) which showed that propyl bromide is isomerised readily to isopropyl bromide by the action of aluminium bromide.From later investigations it appeared that the reaction tends to an equilibrium mixture4) which at 2 6 ' for example contains 91 % of isopropyl bromide. For some time it was believed that isomerisation reactions of this type occurred by elimination and subsequent readdition of hydrogen bromide, but this hypothesis has been definitely disproved by Nash, Taylor and Doerings), who effected the rearrangement of propyl chloride to isopropyl chloride under the influence of aluminium chloride in an atmosphere of deuterium chloride and found that no hydrogen was exchanged for deuterium during the reaction.Many related isomerisation reactions have been investigated 6 ) and in more recent years the results usually have been interpreted in terms of rearrangements of carbonium ions. Reactions of ethyl bromide and aluminium bromide. 11. 75 (1956) RECUEIL 171 us to communicate some results which we obtained in our study of the reaction between ethyl bromide and aluminium bromide. Aluminium bromide readily dissolves in ethyl bromide and therefore determinations of the velocity of the isomerisation reaction are possible.The present investigation forms the first part of a series of quantitative studies of the rearrangement reactions of organic bromides under the influence of aluminium bromide. Our program involves the study of the transfer of hydride, alkyl and aryl groups and the comparison of these reactions with the rates of bromine exchange on the one hand and of the Friedel-Crafts reaction on the other.