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I just learnt that alcohols can undergo $\mathrm{S_N}i$ with $\ce{SOCl2}$ (without pyridine) to achieve retention:

However, what I really do not understand is the 'internal return' step (forming 'intimate ion pair'). What drives the departure of oxygen + attack from chloride? Electronegativity of oxygen is more negative than chlorine, and bond strength of $\ce{C-O}$ is $\pu{358kJ/mol}$, $\ce{C-Cl}$ is $\pu{339kJ/mol}$. It seems to me that it is more stable to stay at $\ce{C-O}$ bond then forming $\ce{C-Cl}$ bond.

Moreover, what causes $\ce{Cl}$ to attack the carbocation on the same face? Like $\mathrm{S_N1}$, it can attack from the front and back (both sides of empty $\mathrm{p}$-orbital), what cause this to differ from $\mathrm{S_N1}$? Also, is primary/methyl alcohol unable to undergo $\mathrm{S_N}i$ due to carbocation formation?

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    $\begingroup$ Here's another link, en.wikipedia.org/wiki/SNi which suggests that part of the driving force is the formation of sulphur dioxide, this is thought to occur at the step you lable chlorosulfite with a greater degree of concertedness than you draw. "what causes Cl to attack the carbocation on the same face?" This is the idea of the intimate ion pair, the species are bound togethre with their ionic charges which restricts their ability to migrate. It would be interesting to model ... $\endgroup$ – user1945827 Jul 28 '20 at 15:54
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    $\begingroup$ The solvent cage effect. sites.google.com/site/mechanismmordor/first-year-mechanisms/… $\endgroup$ – user55119 Jul 28 '20 at 15:59
  • $\begingroup$ I see, so SNi (using SOCl2) is unavailable for methyl / primary alcohol since it forms a carbocation, unlike the usual route SN2 (using HCl)? $\endgroup$ – user96067 Jul 30 '20 at 9:06

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