Under what condition do the reaction proceed with sole removal of oxygen in contrast with the product also ending up with chlorine substituent?

Deoxygenation of pyridine N-oxides with PCl3

I'm familiar with both reaction mechanism, but can't seem to figure out when will a certain pyridine end up with or without chlorine. I guess one factor would be electron richness of the ring system, chlorine being deactivating subsituent ends up only on the electron rich pyridines.

  • 3
    $\begingroup$ POCl3 is phosphorus oxychloride. What is POCl? $\endgroup$ Feb 2, 2016 at 18:41
  • 1
    $\begingroup$ Indeed there's no such thing like POCl - second reaction looks bad $\endgroup$
    – Mithoron
    Feb 2, 2016 at 19:03
  • $\begingroup$ So, the answer should be corrected..... $\endgroup$ Feb 2, 2016 at 19:05
  • $\begingroup$ @Mithoron You mean the whole reaction is not feasible? (omitting now proposed side product) $\endgroup$
    – wuschi
    Feb 2, 2016 at 19:27
  • $\begingroup$ dx.doi.org/10.1016%2FB978-008096519-2.00027-8 looks possible but art. is behind paywall $\endgroup$
    – Mithoron
    Feb 2, 2016 at 19:57

2 Answers 2


I'm not sure whether $\ce{PCl3}$ can lead to the installation of a chlorine atom at C-2. I performed a cursory Reaxys search and in the first page saw several examples of deoxygenation without chlorination, but no examples of concomitant chlorination.

As far as I know, treatment of pyridine N-oxides with $\ce{PR3}$ simply leads to deoxygenation, with $\ce{P(III)}$ being oxidised to $\ce{P(V)}$.

Deoxygenation by PCl3 without further functionalisation

However, if you were to treat pyridine N-oxide with phosphorus oxychloride $\ce{POCl3}$, then you would see chlorination at C-2.

Conversion of pyridine N-oxide to 2-chloropyridine

If you have a source that says that $\ce{PCl3}$ can deoxygenate and chlorinate, please do inform us. Otherwise as far as I can tell, you need $\ce{POCl3}$ to effect a chlorination.


  1. Joule, J. A.; Mills, K. Heterocyclic Chemistry, 5th ed.; Wiley: Chichester, U.K., 2010; pp 121, 155.
  2. Weickgenannt, A. Pyridine N-Oxides, Baran Group Meeting, June 9, 2012; p 3.
  • $\begingroup$ In practice, phosphorus oxychloride is the byproduct after removing the oxygen from the N-oxide; therefore would the parent N-oxide be chlorinated and deoxygenated as in reaction #2 if removing the byproduct from the reaction mixture is not effective? In other words, can we use another deoxygenating agent? Off the top of head, I would imagine use a alkali metal sulfite $\ce{Na2SO3}$, can the byproduct alkali metal sulfate $\ce{Na2SO4}$ can be disposed off and does not halogenate. $\endgroup$
    – Kav
    May 21, 2021 at 18:23

I would argue that electron withdrawing groups will favour the oxidation to POCl3 while electron donating groups will favour the POCl path.

$$\ce{HPyN+-O- + PCl3 <=> HPyN+-O-PCl2 + Cl- <=> Cl-HPyN-O-PCl2}$$

With electron withdrawing groups the oxygen atom is more positively polarized than with electron donating groups, so let the bonds flip to the left:

$$\ce{Cl-HPyN-O-PCl2 + Cl- -> Cl- + HPyN + POCl3}$$

In the opposite case let the bonds flip to the right:

$$\ce{H-ClPyN-O-PCl2 -> H+ + ClPyN + POCl + Cl-}$$

  • $\begingroup$ That's what OP says - you should say sth more than he, shouldn't you? $\endgroup$
    – Mithoron
    Feb 2, 2016 at 19:07
  • $\begingroup$ You're right. Now my answer looks pretty redundant to me. $\endgroup$
    – aventurin
    Feb 2, 2016 at 19:18
  • $\begingroup$ What Mithoron said in the comment section above is that "there's no such thing like POCl". So, you should check your answer....... $\endgroup$ Feb 2, 2016 at 19:54
  • $\begingroup$ POCl does not exist. However, Chlorine in PCl3 can easily substituted by other groups. I guess "POCl" is used to represent such an intermediate product in an informal way. $\endgroup$
    – aventurin
    Feb 2, 2016 at 20:13
  • $\begingroup$ Hope the edit clarifies what I tried to say. $\endgroup$
    – aventurin
    Feb 2, 2016 at 20:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.