SN2 with thionyl chlorides
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.