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I already know that a cyclic trimer and polymer of formaldehyde is formed at lower temperatures, but would chloromethanol and dichloromethane be possible at higher temperatures?

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  • $\begingroup$ I think the question is OK, even if what OP suggests probably doesn't work. $\endgroup$ – Mithoron Sep 23 '17 at 22:53
  • $\begingroup$ @Mithoron title suggests there is a specific reaction but body does not mention which. $\endgroup$ – Kenny Lau Sep 24 '17 at 8:50
  • $\begingroup$ @KennyLau Reaction leading to chloromethanol and dichloromethane... $\endgroup$ – Mithoron Sep 24 '17 at 12:52
  • $\begingroup$ @Mithoron Question ask for mechanism in the title but whether it is possible in the body. $\endgroup$ – Kenny Lau Sep 24 '17 at 13:02
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We can both immediately agree on the first step of the reaction between formaldehyde and hydrochloric acid which is the protonation of the carbonyl oxygen. The mechanism of formaldehyde oligo- or polymerisation then suggests the attack of the oxygen atom of another formaldehyde moiety to form a hemiacetal. This would be a nucleophilic addition onto a π system.

One possible course this species can now take is the back reaction to reform formaldehyde. Another one is a subsequent attack of the newly-formed hydroxy moiety of the hemiacetal function onto another carbonyl group to form a full acetal. The latter reaction creates a product which is inert enough to not undergo back reactions.

You are indeed correct that chloride, which is also a nucleophile, can equally attack the π $\ce{C=O}$ bond of formaldehyde. And this attack would indeed result in chloromethanol. However, this reaction is also reversible as is the attack of oxygen. Furthermore, this molecule has no real way of stabilisation: chloride will always be a good leaving group while a formal alcoholate is not. Thus, while the attack of chloride may occur it is not productive and it is rapidly reversed.

Therefore, we do not see any chlorinated product — which would decompose on isolation at the latest anyway.

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