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Question

Which of the following reactions of alcohol (A), proceeds by SN2 mechanism?

(I) (A)+SOCl2
(II) (A)+COCl2
(III) (A)+SOCl2Pyridine
(IV) (A)+COCl2Pyridine


A
(I),(III)
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B
(I),(II)
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C
(III),(IV)
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D
(I),(IV)
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Solution

The correct option is B (III),(IV)
SOCl2 and COCl2 both replace OH of alcohols by SN1 mechanism. They coordinate to the alcohol, with loss of HCl and formation of a good leaving group (“chlorosulfite”). But when pyridine (nucleophile) is present, it can attack the chlorosulfite, displacing chloride ion and forming a charged intermediate. Now, if the leaving group departs, forming a carbocation, there’s no lone pair nearby on the same face that can attack. So chloride attacks the carbon from the backside, leading to inversion of configuration and formation of a CCl bond. This proceeds in the SN2 reaction.

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