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I have read that $\mathrm{S_Ni}$ reactions involving the attack of $\ce{SOCl2}$ on alcohols proceed without the formation of discrete carbocations, and hence there is no rearrangement involved. But that doesn't explain this reaction:

Conversion of (E)-but-2-en-1-ol to 3-chlorobut-1-ene with thionyl chloride

Here, is the allylic carbocation formed, or is there a different explanation for this reaction?

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    $\begingroup$ It's not that cations don't form, it's that the formation of cations might be so fleeting that there's no time for loss of stereochemistry. In your particular example, there's no reason why we can't have a concerted $S_{\mathrm{N}}2'$ reaction though. $\endgroup$ – Zhe May 11 '17 at 12:20
  • $\begingroup$ @Zhe The thing that I don't get is why there will be the need for Sn2'? Why not simply Sn2? $\endgroup$ – Apocalyptic Warrior May 13 '17 at 17:46
  • $\begingroup$ It can't be $S_{\mathrm{N}}2$. That would imply that the nucleophile is on the same carbon as where the leaving group was. $\endgroup$ – Zhe May 13 '17 at 21:19
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As a disclaimer, it is worth noting that the reaction of an alcohol with thionyl chloride can proceed via multiple different mechanistic pathways (Wikipedia has a pretty good overview), depending on the conditions.

In the $\mathrm{S_Ni}$ mechanism a free carbocation is not formed; instead the key intermediate is an intimate ion pair $\ce{R+...^-OSOCl}$. This is not the same as a typical $\mathrm{S_N1}$ mechanism, where the leaving group is fully dissociated.

Following the formation of the intimate ion pair, the anionic component breaks down to form $\ce{SO2}$ and chloride ion, the latter of which attacks the cationic component $\ce{R+}$ to form $\ce{R-Cl}$. The reaction proceeds with retention of configuration, because the nucleophilic chloride ion originates from the leaving group $\ce{-OSOCl}$, and hence must attack from the same face as the leaving group left from.1

SNi mechanism

If the carbocation $\ce{R+}$ is allylic, then it is electrophilic at both the α-position (i.e. where the hydroxyl group started) and γ-position (i.e. two carbons away from the hydroxyl group). Therefore, following breakdown of the $\ce{-OSOCl}$ leaving group, the chloride can choose to attack either position. The attack at the γ-position is sometimes called the $\mathrm{S_Ni'}$ mechanism.

enter image description here

While the example above proceeds with extremely high selectivity for the rearranged product, this is not always the case. The above reaction was reported by Young in 1962,2 and carried out in dilute ether solution, which strongly favours the $\mathrm{S_Ni'}$ pathway. According to Gu and Zakarian in Comprehensive Organic Synthesis II (Chapter 6.16),3

Thionyl chloride has been used traditionally as the reagent of choice to ensure a high degree of selectivity in favor of allylic transposition, however, the outcome is often less predictable than desired. [...] The reaction, however, is often more complicated and dependent on reaction conditions and, to a greater extent, the structure of substrate. This is illustrated in the total synthesis of codeine reported in 2011. When allopseudocodeine was treated with thionyl chloride, a nearly equimolar mixture of regioisomeric products was produced.

Reaction of allopseudocodeine with SOCl2

References

  1. Smith, M. B. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th ed.; Wiley: Hoboken, NJ, 2013, pp 408–409.

  2. Young, W. G. Unexpected rearrangement and lack of rearrangement in allylic systems. J. Chem. Educ. 1962, 39 (9), 455–460. DOI: 10.1021/ed039p455.

  3. Gu, Z.; Zakarian, A. Functional Group Transformation via Allyl Rearrangement. In Comprehensive Organic Synthesis II; Knochel, P., Molander, G. A. Eds.; Elsevier, 2014; Vol. 6, pp 636–754. DOI: 10.1016/B978-0-08-097742-3.00624-8.

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