I am a novice in chemistry. While reading a magazine, I stumbled on some information about phosphorus pentachloride and sulfuric acid. Phosphorous pentachloride and sulfuric acid are both good dehydrating agents. What is the reaction between them (at standard temperature and pressure).

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    $\begingroup$ An invoice? Do you mean a novice? $\endgroup$
    – f''
    Commented Jul 4, 2016 at 15:19
  • $\begingroup$ Are you sure that you want to ask about phosphorous pentachloride $(\ce{PCl5})$ rather than phosphorus pentoxide $(\ce{P2O5})$ as a dehydrating agent? $\endgroup$
    – user7951
    Commented Jul 4, 2016 at 15:19
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    $\begingroup$ Now that would be an epic battle. I guess $\ce{PCl5}$ would wrestle some water from sulfuric acid and drive it to $\ce{SO3}$, or maybe even all the way to $\ce{SO2Cl2}$. $\endgroup$ Commented Jul 4, 2016 at 15:28
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    $\begingroup$ @IvanNeretin Brace yourselves! We need meme pictures! ;) $\endgroup$
    – Jan
    Commented Jul 5, 2016 at 0:17

3 Answers 3


First phosphorus pentachloride reacts with sulfuric acid to form the intermediate(thanks @bon) chlorosulfonic acid. See here. Also Wikipedia

$$\ce{H2SO4 + PCl5 → POCl3 + HSO3Cl + HCl~~~~~~~~~~~~~~~~~~(1)}$$

Chlorosulfonic acid is actually an intermediate between sulfuryl chloride and sulfuric acid. After the formation of the intermediate, the reaction will proceed forward to form stable sulfuryl chloride. (Here)

$$\ce{HSO3Cl + PCl5 → POCl3 + SO2Cl2 + HCl~~~~~~~~~~~~~~~~~~~(2)}$$

Adding (1) and (2), we get the final reaction:

$$\ce{H2SO4 + 2PCl5 ->2POCl3 + SO2Cl2 + 2HCl}$$

My reaction is same as that of Vishnu JK's reaction and bon.


This seems to be a much more complicated reaction than one might initially guess. A variety of products may be obtained depending on the temperature and the relative proportions of the reactants.

Chlorosulfuric acid, $\ce{HSO3Cl}$, was first discovered by Williamson in 1854[1] from the reaction of $\ce{PCl5}$ and $\ce{H2SO4}$. He states that (emphasis mine):

as a result of numerous experiments with the most varied proportions of pentachloride and acid ... the first action of the pentachloride consists in ... forming the compound $\ce{HSO3Cl}$, which is strictly intermediate between the hydrated acid and the final product $\ce{SO2Cl2}$.

A more recent source[2] suggests that chlorosulfuric acid decomposes at higher temperatures and at 170 °C an equilibrium exists (also mentioned here[4]): $$\ce{2HSO3Cl <=> H2SO4 + SO2Cl2}$$

It also says that:

chlorosulfonic acid, by boiling in the prescence of ... catalysts, decomposes quantitatively into sulfuryl chloride and sulfuric acid.

It would seem that formation of $\ce{SO2Cl2}$ by this route is favoured by using excess $\ce{PCl5}$.

Other recent works suggest that it is possible to prepare $\ce{HSO3Cl}$ by careful distillation of the product. One book[3] suggests that it may be distilled at 161 °C and gives the following two reactions (the first is also mentioned by another source[4]): $$\ce{H2SO4 + PCl5 -> HSO3Cl + POCl3 + HCl}$$ $$\ce{POCl3 + 2H2SO4 -> 2HSO3Cl + HCl + HPO3}$$

It mentions that the 'calculated quantity of phosphorous pentachloride' should be used but declines to say what this is but from the equations one would expect that this route is favoured by using excess sulphuric acid.


[1] http://www.jstor.org/stable/111726?seq=4#page_scan_tab_contents

[2] https://books.google.co.uk/books?id=roI_CwMEcHMC&pg=PA1&lpg=PA1&dq=phosphorus+pentachloride+and+sulphuric+acid&source=bl&ots=FeM9kqKg8-&sig=BRMAXCcVormdf6nAvy8dI0yP4Do&hl=en&sa=X&ved=0ahUKEwiFqsCbu-TNAhUBuhoKHREeD244ChDoAQg2MAU#v=onepage&q=phosphorus%20pentachloride%20and%20sulphuric%20acid&f=false

[3] https://books.google.co.uk/books?id=IDsXBQAAQBAJ&pg=PA401&lpg=PA401&dq=phosphorus+pentachloride+and+sulphuric+acid&source=bl&ots=M4Gpbcn4tO&sig=bvYuWQ9i2hmGbanLt9x8sFt443s&hl=en&sa=X&ved=0ahUKEwiFqsCbu-TNAhUBuhoKHREeD244ChDoAQgzMAQ#v=onepage&q=phosphorus%20pentachloride%20and%20sulphuric%20acid&f=false

[4] http://sulphur.atomistry.com/chlorosulphonic_acid_cl.html


The reaction occur as follows

enter image description here

thus the products are sulfuryl cholride,phosphoryl chloride and hydrochloric acid. This might clear all the doubt between Nilay Ghosh and my answer.

enter image description here

And Thank@bon for pointing out the mistake


first link

2nd link and also my text book

4th question pdf

  • 2
    $\begingroup$ That is not thionyl chloride, it is sulfuryl chloride. Also, I don't think either of those sources are reliable. $\endgroup$
    – bon
    Commented Jul 8, 2016 at 18:06

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