ARBOCATIONS

A reaction energy diagram for an S l reaction. The rate-limiting step is spontaneous dissociation of the alkyl halide to give a arbocation intermediate. 

In the concerted process, the benzene ring reacts with the alkyl halide-aluminum chloride complex to form the alkylated benzene. In this mechanism, there is no carbocation rearrangement because there is no free carbocation stage. In the two step process, however, there is a free carbocation stage. The alkyl halide complexes with the aluminum chloride. This complex breaks down to form the free carbocation, which can then undergo rearrangement to form a more stable species. The benzene ring can then react with the carbocation to form the alkylated benzene. Therefore, reactions (a), (b) and (c) must have followed the two step mechanism that is, the c-arbocation mechanism. 

As also shown in Scheme 7.48, the alternative possibility of migration of the methide at the same time the iodide is leaving, is indistinguishable from the formation of a secondary arbocation followed by migration of the methide (on the basis of the evidence presented). 

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