# Stereochemical dichloride treatment [duplicate]

In this question there is no inversion in A to B, but there is one in C to D. I feel like this must be due to the lone pairs on the $$\ce{S}$$ (in A) being able to open the ring, whereas this isn't possible in C to D so this can only occur via an $$\mathrm{S_N2}$$ reaction, but I just can't seem to make the mechanisms work for either.

• Remember S is a significantly larger atom than O, and episulfonium ions are well known – Waylander May 20 '19 at 12:25
• Instead of opening (i.e. breaking) rings, think about forming rings, maybe – orthocresol May 20 '19 at 12:30
• So is the sulphur lone pair able to attack via SN2 to form an episulfonium ring, which is then attacked by SN2 with the CN ions, meaning there is no inversion? – J. Deans May 20 '19 at 16:45
• That is exactly so – Waylander May 20 '19 at 17:48
• Brilliant thank you very much – J. Deans May 20 '19 at 21:55

Because of the absence of a lone pair, C undergoes usual $$\mathrm{S_N2}$$ transformation under the conditions. However, the availability of a lone pair in A undergoes the nucleophilic substitution of cyanide ion via neighboring-group participation (see above comments by Waylander and J. Deans) by the sulfur atom under the conditions (Ref.1). A very reliable mechanism is given in Ref.1 for the substitution of azide ion (work the same way for cyanide ion):