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Grignards don't couple with alkyl halides. I know that thionyl chloride reacts with alcohols and other $\ce{-OH}$ group containing molecules to replace the $\ce{-OH}$ with a $\ce{-Cl}$. So we probably aren't making an alkyl halide from the alcohol with the thionyl chloride.

The book says that the starting product, which is an alcohol, attacks the sulfur in the $\ce{S=O}$ bond of $\ce{SOCl2}$. I got that the starting product should be an alcohol, but I did not expect the alcohol to attack the thionyl chloride.

Nevertheless, this generates an $\ce{R-O-SOCl}$ molecule, and the resonance-stabilized $\ce{O-SOCl-}$ ion is a good leaving group. This is the substrate that the Grignard can perform an SN2 type reaction on.

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My question is: how common is it that alcohols attack thionyl chloride in this pattern? I'm guessing "not very common" because this would make thionyl chloride an undesirable reagent for making alkyl halides from alcohols. Is this reaction synthetically useful?

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    $\begingroup$ 1. Maybe not Sn2, maybe addition-elimination instead.. 2. Grignard reagents do react with alykl halides and can couple, but yields are low and elimination byproducts are common. It certainly isn't the answer in this case. 3. ~SO2Cl is a decent leaving group, but it will decompose into SO2 and ~Cl. SO2 gas being liberated will drive equilibrium. 4. It's a great reaction for alcohol conversion. 5. I do not like this question from the pedagogical perspective. $\endgroup$
    – Lighthart
    Commented Jun 3, 2016 at 21:15

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This generates an R−O−SOClR−O−SOCl molecule, and the resonance-stabilized O−SOCl−O−SOCl− ion is a good leaving group. This is the substrate that the Grignard can perform an SN2 type reaction on.

I could be wrong about this, because I always doubt everything that I do, but I don't think that's quite right.

Here is a good discussion about thionyl chloride works. It should turn your alcohol into an alkyl chloride, and now that chloride is your leaving group for the SN2 reaction.

Knowing that, we can work retrosynthetically.

Your product is octane. You get to the octane by reacting something with ethyl magnesium bromide (a great nucleophile), so that's a strong indication that you did a substitution reaction. If you add on an ethyl group and get octane, then your substrate was an ethyl group shorter, so it would have contained a hexyl chain. You got to that hexyl species by treating an alcohol (as it was described in the initial question) with SOCl2, and you know that SOCl2 turns alcohols into chlorides, so your hexyl species that you treated with EtMgBr was 1-chlorohexane, which you obtained by treating hexan-1-ol with SOCl2.

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The use of Thionyl Chloride is a very good way to prepare Alkyl Halides, especially because it is a "clean" reaction where the byproduct $\ce{SO2}$ is a gas so purification of your product is much easier.

You shouldn't put in the Grignard without working-up the reaction because the HCl and/or any alcohol that remains in solution will destroy the Grignard. However, working up the reaction also means that your R-OSOCl intermediate will not survive and will become R-Cl. The original reaction you show is also clear about this given how it drew two arrows instead of just one.

And finally, is this type of reaction synthetically useful? Why yes it is! This type of reaction allows you to control the stereochemistry of the product. This particular sequence (Thionyl Chloride followed by Grignard) allows you to invert the stereocenter of the final product, whereas using thionyl chloride with pyridine or $\ce{PCl5}$ first will retain the stereocenter.

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  • $\begingroup$ @K_P yes, but if coupled with the next step which is SN2 then you get inversion. I guess that part wasn't clear and I'll edit the answer now! $\endgroup$
    – IT Tsoi
    Commented Jun 3, 2016 at 0:55
  • $\begingroup$ There are so many better ways to invert stereocenter than this lousy sequence, that would give you low yields and lots of byproducts. You can use Mitsunobu reaction or easily mesylate the alcohol and then attack with some good nucleophile/weak base like alkyne, cyanide, malonate, etc. Stereospecifity is usually very good and there are rarely any elimination byproducts. Tosylates and mesylates Mitsunobu reaction $\endgroup$
    – Jan Rzymkowski
    Commented Jan 17, 2018 at 14:16

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