Concerted front-side displacement mechanism

Racemization, however, does not alwiys accompany isotopic scrambling. In the case of 5ec-butyl 4-bromobenzenesulfonate, isotopic scrambling occurs in trifluoroethanol solution witiiout any racemization. Two mechanisms are possible. Scrambling may involve an intimate ion pair in which the sulfonate can rotate with respect to the caibocation without allowing migration to die other face of the caibocation. The alternative is a concerted mechanism, which avoids a caibocation intermediate but violates the prohibition of front-side displacement. ... 

The Sni mechanism (substitution-nucleophilic-internal), as originally proposed, postulated a front-side displacement occurring via a four-center transition state. It is now clearly established that such a process has never been observed, and that all the reactions proposed to proceed by SnI mechanisms involve ion pairs. The Sni mechanism was initially suggested to explain the observed stereochemistry of chlorosulfite decomposition. Chlorosulfite esters are formed by reaction of alcohols with thionyl chloride, and are the key reactive intermediates in the conversion of alcohols to chlorides with thionyl chloride. Nucleophilic attack by the chloride of the chlorosulfite ester was considered to be concerted with cleavage of the C-O bond ... 

A concerted reaction must be stereospecific. The mechanism described by Fig. 5.2 requires inversion of configuration. An alternative direct displacement mechanism involving front-side attack would also exhibit second-order kinetics and respond similarly to structural and medium effects, but would require retention of configuration as the stereochemical course. As we shall see, the available data support fully the mechanism involving back-side displacement. Additionally, SCF-MO calculations of the hypothetical transition states for hydride displacement of hydride from methane indicate that the inversion geometry is 14.9 kcal/mol more favorable than the retention geometry ... 

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