SOCl2 reaction stereochemistry

[ptran]

For a test, I was flipping through some other Organic Chemistry I websites to find some extra practice problems for a test and I noticed at one it said that the stereochemistry of SOCl2 is retained when used with a nonpolar solvent like DMSO but when a polar solvent is used the stereochemistry is changed to the opposite. I really didn't think too much about it and come test time, I wrote in what I remembered from said website. My professor marked it wrong but didn't explain why. Anyways, I really can't remember what website it was but I was wondering if anyone could confirm this and/or perhaps send a link proving how it would work.

[Tartaglia]

You need to be clearer in your question. I think you mean the stereochemistry of the alcohol which you are reacting with SOCl2 not the stereochemistry of SOCl2 which is planer. Also DMSO is a polar solvent not a non polar solvent

[hell_ever]

In an inert solvent, retention of configuration of the substrate is observed, whereas in the presence of certain other compounds, like pyridine for instance, inversion of configuration is observed.

This is because the pyridine complexes with the HCl formed initially, and this makes the chloride ion free, and the chloride attacks from the back side of the leaving group (SO2 in this case)

In the first case, after elimination of HCl, the Cl directly attacks the electrophilic carbon from the same side as the leaving group.

[vedmecum]

In an inert solvent, retention of configuration of the substrate is observed, whereas in the presence of certain other compounds, like pyridine for instance, inversion of configuration is observed.

This is because the pyridine complexes with the HCl formed initially, and this makes the chloride ion free, and the chloride attacks from the back side of the leaving group (SO2 in this case)

In the first case, after elimination of HCl, the Cl directly attacks the electrophilic carbon from the same side as the leaving group.

 

why not Cl- attacks from the same side (in second case) as in case 1 ? i think the probable ans. is in 1st case , Cl of SOCl2 acts as nucleophile , these leads to cyclic intermediate and hence retention of configuration

 

while in second case , pyridine or any base generate strong nucleophile Cl- which can attack only from back side as front side is blocked by cyclic intermediate .