Carbocation Rearrangement in SNi

Question

In Peter Sykes' A Guidebook to Mechanism in Organic Chemistry regarding the retention of configuration of reaction of alcohols with $\ce{SOCl2}$:

The rate at which the alkyl chlorosulphite intermediate breaks down to $\ce{RCl}$ is found to increase with increasing polarity of the solvent and also with the increasing stability of the carbocation $\ce{R+}$: an ion pair $\ce{R+SOClO-}$ is almost certainly involved.

But it is nowhere mentioned whether a carbocation rearrangement is possible or not.

In "NCERT Chemsitry Class 12" in the section of Preparation of Alkyl halides from Alcohols:

The hydroxyl group of an alcohol is replace by halogen on reaction with concentrated halogen acids, phosphorous halides or thionyl chloride. Thionyl chloride is preferred because the other two products are escapable gases.

If that method is preferred, then I assume rearrangement does not occur.

So, why does not a carbocation rearrangement (like Wagner–Meerwein rearrangement) occur in the $\mathrm{S_Ni}$ pathway eventhough the intermediate is involved? Is it because the intermediate breaks down too quickly before any rearrangement is possible?

Answer 1

The Mechanism of the reaction

The reaction of an alcohol with thionyl chloride under base-free conditions is one of the most significant examples of the S_Ni mechanism (D_N+A_ND_e to give it it’s correct IUPAC designation, which is perhaps more instructive in this case).

There are two steps to the S_Ni mechanism, as shown below (figure taken from Bruckner, Organic Mechanisms ^[1]):

Step 1: The alcohol reacts with thionyl chloride to afford an alkyl chlorosulfite, which can often be isolated.^[2] At this stage, the oxymethine stereocentre hasn’t been touched– it’s the second step that will determine the stereochemistry.

Step 2: The chlorine is delivered intra-molecularly, with extrusion of sulfur dioxide gas. Since the sulfur is being delivered from the same face that the hydroxyl was on, the reaction occurs with retention overall. This can be seen in the dotted lines in the figure above.

Two things are worth noting at this stage:

1. If pyridine is added to the reaction mixture, we see overall inversion via an S_N2 mechanism.^[2] This occurs because the pyridine interacts with the intermediate alkyl chlorosulfite formed, liberating a free chloride (nucleophilic) and hence allowing the substitution to take place inter-molecularly. This can be seen in the solid lines in the figure above.

2. The second step in which I described as happening intra-molecularly doesn’t quite happen intra-molecularly. In reality, the C-O bond begins to fragment in an S_N1 fashion, before the chloride attacks. The reason why we still see retention (rather than racemisation as is common with S_N1) is due to a phenomenon called contact ion pairs.^[#]

Carbocation rearrangement

The simple answer to your question is that at no stage of the reaction is it thought that there is any free carbocation concentration, making rearrangement less likely on kinetic grounds (along with elimination, which could equally be argued by your logic).

The ion pair theory is such that the carbocation remains closely associated with the leaving group, allowing the nucleophilic attack to occur rapidly without the need for an additional collision to take place.

^{[1]: Bruckner, R. Organic Mechanisms- Reactions, Stereochemistry and Synthesis; Springer: Berlin, 2007}

^{[2]: J. Am. Chem. Soc. 1952, 74, 308}

^{[#]: You can read about this in any advanced organic chemistry text such as Carey or March, and is a bit too long of an explanation for me to give satisfactorily here}