[azmanam]

QUESTION: Explain the stereochemical results for the following transformation

[Dan]

Initially S_N2 displacement of mesylate by iodide, then S_N2 displacement of that iodide by iodide (eventually) racemises the system?

[azmanam]

Yup.  Two quick ones in a row... I guess I'll have to start making them harder  

Iodide is a great nucleophile and a great leaving group.  Initial formation of the optically pure iodide is soon attacked by the good nucleophile I^- to liberate the good leaving group I^- to give the iodide with the opposite configuration.  Repeated attack of I^- to liberate I^- eventually provides the racemate.  No S_N1 mechanism is proposed.

I've updated the LAH reduction with a different focus.  Take a look at that if you want another challenge this week. http://www.chemicalforums.com/index.php?topic=33985.0