Alcohols reaction with thionyl chloride, stereochemistry

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 ... 

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