Sciencemadness Discussion Board

Reaction: SO2 + 2 Cl2 --> SOCl2 + Cl2O?

AJKOER - 24-1-2013 at 14:04

I have seen the following reaction listed on several test and educational sites (see, for example, http://www.z-answers.com/question/vtvqqt-chemistry-question-... ). To quote:

"Sulfur dioxide reacts with chlorine to produce thionyl chloride (used as a drying agent for inorganic halides) and dichlorine oxide (used as a bleach for wood pulp and textiles) according to the following equation:

SO2(g) + 2Cl2(g) → SOCl2(g) + Cl2O(g) "

I'm not sure why SOCl2 is listed as a gas unless the reaction vaporizes the normally liquid thionyl chloride which boils at 74.6 C.

But Wkipedia (http://en.wikipedia.org/wiki/SO2Cl2 ) quotes the following reaction in the presence of an activated carbon catalyst:

SO2 + Cl2 → SO2Cl2

Note, Sulfuryl chloride boils at 69.4 C.

However, it is still possible that SO2 otherwise reacts with an excess of Chlorine to form thionyl chloride and dichlorine monoxide. Further, as SOCl2 is a liquid at room temperature, separation appears possible, and even if Cl2O dissolves in SOCl2, mild heating (under 50 C) may be successful in separating the products.

So the question is, why is not SO2 + excess Chlorine ever mentioned as preparation route for either Cl2O or SOCl2?

Perhaps, there is a safety issue, but by this statement, it is not explicitly noted. To quote a safety statement on diChlorine monoxide (link http://www.guidechem.com/dictionary/en/7791-21-1.html ):

"The liquid at 2 °C is an unstable spark- and touch-sensitive explosive. The gas may explode when heated above 42 °C. A powerful oxidizing agent. Explodes on contact with alcohols, ammonia, antimony, antimony sulfide, arsenic, barium sulfide, calcium phosphide, carbon, carbon disulfide vapor, charcoal, cork, dicyanogen, ethers, hydrogen sulfide, mercury sulfide, nitrogen oxide, paper, phosphine, phosphorus, potassium, rubber, sulfur, tin sulfide, turpentine, and other oxidizable materials. Self-explodes. Incompatible with carbon, dicyanogen, diphenylmercury, nitrogen oxide, oxidizable materials, and potassium. Explosive reaction when heated above 50 °C with many hydrocarbons (e.g., butadiene, ethane, ethylene, methane, propane)."

Here, for example, is a reference on the production routes for SOCl2 (per Wikipedia http://en.wikipedia.org/wiki/SOCl2 ) to quote:

"The major industrial synthesis involves the reaction of sulfur trioxide and sulfur dichloride:[3]

SO3 + SCl2 → SOCl2 + SO2

Other methods include syntheses from phosphorus pentachloride, chlorine, or phosgene:

SO2 + PCl5 → SOCl2 + POCl3
SO2 + Cl2 + SCl2 → 2 SOCl2
SO3 + Cl2 + 2 SCl2 → 3 SOCl2
SO2 + COCl2 → SOCl2 + CO2 "

where there is no mention of just SO2 + Cl2 path. Similarly for the literature on Cl2O (see attached file). So what is wrong with this synthesis for both thionyl chloride and Dichlorine monoxide? Do all those test makers just copy each others bad case examples? Are the reaction conditions extreme?


Attachment: Cl2O Preparation ScienceMadness.pdf (532kB)
This file has been downloaded 1963 times


[Edited on 25-1-2013 by AJKOER]

AndersHoveland - 24-1-2013 at 14:36

Quote: Originally posted by AJKOER  


"Sulfur dioxide reacts with chlorine to produce thionyl chloride (used as a drying agent for inorganic halides) and dichlorine oxide (used as a bleach for wood pulp and textiles) according to the following equation:

SO2(g) + 2Cl2(g) → SOCl2(g) + Cl2O(g) "

However, it is still possible that SO2 otherwise reacts with an excess of Chlorine to form thionyl chloride and dichlorine monoxide.

I doubt it. I think perhaps they were mistaken. Sulfur dioxide is indeed used as a bleach in the paper industry, because of its reducing powers. And chlorine dioxide is also used as a bleach in the paper industry because of its oxidizing powers, though I have never read of dichlorine oxide being used in the paper industry. In the pulp industry, sulfur dioxide when acidified can reduce chlorate to chlorine dioxide.

One internet site I found said claimed that, "If Chlorine reacts with sulfur dioxide, thionyl chloride and dichlorine monoxide can be obtained", and gave a reference:
Kotz, John C. and Purcell, Keith F. Chemistry & Chemical Reactivity 2nd ed.; Saunders College Puiblishing: Orlando, FL, 1991; p 130.
I have not actually checked the reference. It does appear to be just a course educational book, however, so this reference may be unreliable. I suspect the reaction equation is just being used as a practice problem.

I have severe doubts that this alleged reaction is correct, but it may nevertheless be worth investigation. The authors probably just got confused about another common reaction, then fabricated a balanced equation for their students to practice with. I do not think this is even deserving of speculation, but since I seem to enjoy speculating so much...
Quote:

I suppose it might be possible, if SO2 is initially attacked by chlorine, it might initially form the intermediate radical ClSO2• which could then oxidize chlorine. But then it would result in the isomer ClSO2Cl, which I do not think would be very chemically stable. If this intermediate species was able to react with chlorine, I would think it would have to simultaneously react with a Cl2 molecule through an intermolecular resonance, such that the hypochlorite group would simultaneously break off and bond to one of the chlorine atoms at the same time. Again, I find all this very unlikely, but if this reaction can proceed, this is the best explanation I can think of.

Another unlikely possibility is that SO2Cl2 could oxidize excess Cl2 to Cl2O, but if this were possible one would think SO3 could also oxidize chlorine (although this might produce something more complicated than Cl2O because SO3 is such a strong Lewis acid)
-OSO2-O-Cl+-Cl



Usually, Cl2 only reacts with SO2 in sunlight, or in the presence of a catalyst (activated carbon, or red hot coppric sulfate), and only sulfuryl chloride forms, SO2Cl2. I have never seen any of these sources (describing SO2Cl2 formation) make any reference to a side reaction that also produces SOCl2.

[Edited on 24-1-2013 by AndersHoveland]

kristofvagyok - 24-1-2013 at 14:42

Reaction of Cl2 and SO2 will end up with SO2Cl2 and not OCl2. OCl2 is made via HgO and chlorine gas, a pretty awesome reaction, I have made both long ago.

AJKOER - 25-1-2013 at 06:32

OK, I came across some interesting research that literally takes us to Venus. Per "Chemical kinetic model for the lower atmosphere of Venus" by Vladimir A. Krasnopolsky (full link: http://yly-mac.gps.caltech.edu/venus/Venusicarus07.pdf ). Some interesting cited reactions in the Venian atmosphere:

44 Cl+ SO2 +M→ClSO2 + M
45 ClSO2 +M→Cl+ SO2 + M
46 ClSO2 +ClSO2→SO2Cl2 +SO2
47 SO2Cl2 +SO2→ClSO2 + ClSO2
48 ClSO2 +Cl→SO2 +Cl2
49 ClSO2 +S→SO2 + SCl
50 ClSO2 +H→SO2 + HCl
51 SO2Cl2 +Cl→ClSO2 + Cl2

Also, per a work by Yung (https://docs.google.com/viewer?a=v&q=cache:xL7YsYdeCHUJ:... ) to quote:

"(ii) Reactions involved. Possible reactions in this system follow:

CI2 + hv --> 2C1

CI + SO2 + M --> CISO2 + M

2CISO2 --> S02C12 + SO2 --> 2 SO2 + Cl2

CI + C1SO2 --> SO2C12 --> Cl2 + SO2

In conclusion, the author notes:

"net 2CI + SO2 --> SO2CI2

and it is removed primarily by photolysis. The lifetime of SO2C12 is of the order 10^4 sec; "

The author also notes in the presence of Oxygen, the formation of SO3, which with H2O forms the Sulfuric acid which has been observed in Venus' atmosphere.

My take on all of this, is that SOCl2, with respect to photolysis is either non-existent (or a very transient) species with only a cited formation of SO2Cl2. There is no Cl2O formed absence water.

The threads questioned reaction, "SO2 + 2 Cl2 --> SOCl2 + Cl2O?", in agreement with comments so far, is increasing looking like pure dribble.

Unfortunately, I am not so forgiving on the propagation of bad science by so called teaching professionals. I would speculate that the problem arises as teachers prefer to rely on each other (keep it in the profession), and not experienced scientists, as their sources. To be fair, this is not an ill isolated to teachers alone, but still, in my opinion, not entirely excusable.

[Edited on 25-1-2013 by AJKOER]

woelen - 25-1-2013 at 07:04

I also think that SO2 plus Cl2 only gives SO2Cl2 under the right conditions and no Cl2O and SOCl2. Maybe trace amounts of the latter are formed and maybe these exist as transient species.

I have tried reacting SO2 with Cl2 myself some time ago and I found that in the absence of water this reaction is VERY slow. As soon as a few drops of water are added to the mix of gases, then water becomes very hot and a lot of fumes are produced (HCl). I never tried the adding of active charcoal.

I also tried this reaction with Br2 and I2 and the result is very similar.

AndersHoveland - 25-1-2013 at 16:29

Here is another thought I had...

If it is moist chlorine gas reacting with SO2Cl2, it is well known that chlorine can reversibly disassociate in water.

Cl2 + H2O <==> HCl + HOCl

It is also true that SO2Cl2 is a very strong dehydrating agent.

SO2Cl2 + H2O --> HOSO2Cl + HCl

In fact, there was another thread where we debated what this actual reaction really is when only a limited quantity of water is present, as we suspected that both SO2Cl2 and very concentrated sulfuric acid might be able to oxidize hydrogen chloride, and that the limited hydrolysis of SO2Cl2 may result in some other products.
http://www.sciencemadness.org/talk/viewthread.php?tid=17661

Reaction rates may be very important here also. Even though the SO2Cl2 is very strong dehydrating agent, it might potentially be possible that the reaction rate of Cl2 reversibly disassociating with water is significantly faster. This could potentially greatly alter the dynamics of the reaction.

So the intermediate reactions may be:

Cl2 + H2O --> HCl + HOCl

2 HCl + SO2Cl2 --> SOCl2 + Cl2 + H2O
(where the water is being pulled out by the other more powerful dehydrating agent, since otherwise it would further react)

If HOCl is able to transiently form, it would quickly be dehydrated to Cl2O, while the HCl may be able to reduce SO2Cl2 to SOCl2.

2 HOCl + SO2Cl2 --> HOSO2Cl + HCl + Cl2O

the HCl would be oxidized by another molecule of SO2Cl2 before it could react with the Cl2O. We cannot get rid of the HCl just by oxidizing it with more HOCl and dehydrating out the water, because 1 molecule of HCl is released for every 1 H2O dehydrated out. (Actually, solutions of hypochlorous acid in water have a very small equilibrium constant with Cl2O.)

And so the net reaction could indeed be, as AJkoer originally suggested,

(1.) SO2 + 2 Cl2 → SOCl2 + Cl2O

but also along side another (possibly lesser) simultaneous reaction:

(2.) 3 SO2 + H2O + 3 Cl2 --> 2 SOCl2 + 2 HSO3Cl

Again, a small ammount of water may be necessary for first the reaction, but it would also be consumed. The ratios of these two reactions would likely not be a 1 to 1 ratio.

This would also raise the question of whether a catalyst would be necessary in the first place to form the SO2Cl2 in the first place. I would suggest that it might be possible a catalyst is not necessary if SO2Cl2 is not actually being produced in the net overall reaction, even though it could be produced as an intermediate. The moisture may be serving as the catalyst, but this water catalyst would be unsuitable if one actually desired SO2Cl2 to build up in any ammount.

Again, I suggest this all as merely an unlikely possibility. I do not think anything other than sulfuryl chloride can result from the reaction of SO2 with Cl2.

We still do not know whether AJkoer's reaction would actually happen. I would be very interested if someone would do an experiment. But at least I have now presented 3 different potential reaction routes that such a reaction might proceed. The chemistry of this is very complex. :(

*Just to note, many of these hypothetical reactions presented in this post can only be viewed in the context of the entire overall proposed reaction. Many of these indivual reaction equations cannot stand alone, since they are only describing intermediates here, not the net overall reaction.

[Edited on 26-1-2013 by AndersHoveland]

barley81 - 25-1-2013 at 19:00

If you reacted wet SO2 and Cl2, you might get a small amount of chlorosulfonic acid or sulfuric acid along with some HCl, but I can't see how Cl2O or thionyl chloride would be made in appreciable quantities. As far as I know, thionyl chloride reacts far faster than sulfuryl chloride with water (another reason why it cannot be made with moist reactants).

[Edited on 26-1-2013 by barley81]

AndersHoveland - 25-1-2013 at 20:06

Quote: Originally posted by barley81  
As far as I know, thionyl chloride reacts far faster than sulfuryl chloride with water...

Most likely true, but that is exactly the reason why the overall reaction I suggested may be able to work.
Quote: Originally posted by barley81  
...(another reason why it cannot be made with moist reactants).

Paradoxically, it may be just the opposite! Thionyl chloride may "react", but it may just be an equilibrium within the context of the overall reaction. Sulfuryl chloride (SO2Cl2) is the stronger dehydrating agent, but it is also slower. Consider what happens when thionyl chloride (SOCl2) reacts with water:

SOCl2 + H2O --> SO2 + 2 HCl

and now we have SO2 that could potentially react all over again (in AJKOER's reaction). Meanwhile, it would be the sulfuryl chloride that is driving the net reaction. In fact, I would imagine that SO2 and HCl could be dehydrated back to SOCl2 in the presence of SO2Cl2.

SO2 + 2 HCl + SO2Cl2 --> SOCl2 + HSO3Cl (possibly)
Quote: Originally posted by barley81  
I can't see how Cl2O or thionyl chloride would be made in appreciable quantities.

Because the hydrolysis rate of SO2Cl2 is slower, giving Cl2 a chance to first react with any moisture. Then the SO2Cl2 could oxidize the HCl, leaving behind HOCl which could easily be dehydrated to Cl2O.

Again, this is only just a theory I have put forth, and we do not know if AJKOER's original reaction actually works.
Quote: Originally posted by AJKOER  

SO2(g) + 2Cl2(g) → SOCl2(g) + Cl2O(g)


[Edited on 26-1-2013 by AndersHoveland]

AndersHoveland - 25-1-2013 at 20:36

A quick enthalpy of formation calculation may be helpful:

dichlorine monoxide +80.3 kJ/mol
thionyl chloride -212.5 kJ/mol
sulfur dioxide −296.8 kJ/mol
chlorine (elemental) 0
sulfuryl chloride -354.8 kJ/mol, or -394.1 ?


SO2 + 2Cl2 → SOCl2 + Cl2O

−296.8 + (2)0 => -212.5 + +80.3

−296.8 => -132.2

This quick calculation suggests that the overall reaction is NOT favorable. Feel free to do check the values and do the calculation yourself.

It is not completely beyond possibility that it could be an endothermic reaction. There is a big difference in heat of formation values on either side of the equation. Also, we are not taking into account that this unfavorable reaction could be driven by the hydrolysis of SO2Cl2 with water.

We should not forget that chlorine does have some very neglibable equilibrium with Cl2O in water, even though that reversible reaction is very unfavorable also.

Could this reaction be a rare case where reaction dynamics result in a products which are entropically unfavorable? Where the Cl2O gets "trapped" outside the equilibrium, since those chlorine atoms will not be coming off the thionyl chloride, preventing it from being oxidized ?

I am just not sure. :(

One way to test the hypothesis may be to search through the literature to see what can oxidize SOCl2. If an oxidizer with around the same strength as Cl2O can oxidize it, then we will know the reaction could not proceed. I think however that thionyl chloride is fairly resistant to oxidation.

We shlould always approach science with an open mind, but not so open our brains fall out. :D
Sometimes it is difficult to know when to stop speculating, when to give up hope. This seems to be an instance where only actual experimentation will reveal the truth with any certainty.

One investigation studied the photolyzed reaction between SO2 and Cl2, both in the presence of oxygen and without. They did not make any mention to any thionyl chloride.
"Laboratory studies on the reactions between chlorine, sulfur dioxide, and oxygen - Implications for the Venus stratosphere"
W. B Demore, M.T. Leu, M.T.; R. H. Smith
But then again, it may be possible the invetigators were not specifically looking for thionyl chloride, or that the photolyses could have just caused the trace products to recombine, since Cl2O is rather unstable.

I think it is reasonable to conclude from the evidence that AJKOER's reaction is unlikely to be correct. But I would still be interested in absolute certainty.

[Edited on 26-1-2013 by AndersHoveland]

woelen - 29-1-2013 at 00:36

Thionyl chloride is indeed quite resistant against oxidation. I did an experiment in which I added a small amount of CrO3 (a strong oxidizer) to SOCl2. When this is done, then the CrO3 dissolves in the thionyl chloride and gives a red solution. As soon as some water is added, a violent reaction occurs and immediately the red color of the hexavalent chromium disappears and the green color of chromium(III) appears.