[curiouscat]

What's the reaction between H₂O₂ and HCl?

Do I get free chlorine like so?

H₂O₂ (aq.) + HCl  (aq. H₂O + HOCl (aq.)

HOCl (aq.) + HCl (aq.)  Cl₂    + H₂O

We use a H₂O₂ / HCl mix for making a chlorohydrin and I'm confused as to what species actually takes part in the reaction.

    

Is it the hypochlorite ion that reacts?

HClO (aq.) OCl⁻ + H⁺

Or nascent Chlorine? Or something else?

If it matters, HCl and H2O2 are both ~30%  and temp. is between 60 to 90 C.

If someone has a guess about the mechanism, I'd love to hear it! I've a guess that the Chloronium Ion is involved somewhere but not sure.

[opsomath]

I am pretty sure that you can generate chlorine directly from Cl2/H2O2.

Here is an interesting discussion of the reaction, which apparently leads mostly to HCl-catalysed decomposition to O2 at low concentrations, but yields substantial chlorine concentrations in the interim.

https://sites.google.com/site/unusualchemistry/halogen-hydrogen-peroxide-interactions

[curiouscat]

Thanks @opsomath!

It's interesting to see a reaction as simple as H2O2 + HCl seems to still have quite a bit of confusion about it!

[Borek]

Usually it means reaction is far from simple - many possible pathways, which one dominates depends on the details, so there is no single answer. Something like difference between dissolving Mg in HCl (quite simple) and dissolving Cu in HNO₃ (several possibilities, all present at once).

[curiouscat]

Usually it means reaction is far from simple - many possible pathways, which one dominates depends on the details, so there is no single answer.

Right. "seemingly simple" is what I meant.

What confuses me even more is I'm not even sure what species I really need for the reaction!

[curiouscat]

The mechanism from @opsomath's link is this:


H₂O₂ + 2 H⁺ + 2 Cl^-    Cl₂ + 2 H₂O
 
H₂O₂ + Cl₂  O₂ + 2 H⁺ + 2 Cl^-

If this were dominant in my reaction I should see a disparity in the liquid mass balance right? Even if all the nascent Cl reacted and none was lost as Cl₂ gas, the evolved O₂ is almost the entire mass fraction of the peroxide.

But I don't see this mass deficit.

What gives?
 

[camptzak]

for what its worth, your original reaction involving a hypochlorite intermediate seemed to make a lot of sense to me.

how else could it go other than protonation of a peroxide oxygen, and chlorine substitution releasing water?

[opsomath]

It is an interesting scenario, isn't it? The starting material reacts with another compound to form a species which catalyzes the decomposition of the starting material.

I do think that this:

Even if all the nascent Cl reacted and none was lost as Cl2 gas, the evolved O2 is almost the entire mass fraction of the peroxide.

is incorrect. Given the equations from the link, for every mole of chlorine that is produced, one mole of peroxide goes directly to form water. Since the O2 producing reaction consumes a mole of chlorine, the very most you could lose to oxygen is half the peroxide.

Since we get "moderate steady bubbling that last for several hours" I'm assuming that the second reaction is relatively slow (reasonable, since it's the oxidation of something that's already pretty highly oxidized) and that the solution contains a high concentration of latent and dissolved chlorine as Cl2, HOCl, and more complex chlorine oxide species. This is a pretty reasonable conclusion since, after all, this reaction mixture is effective at your halohydrin reaction!

Do you have access to a UV-Vis spectrometer? You could place some of the reaction mixture in a cuvette and monitor the absorption over time. I think molecular Cl2 absorbs at 325-350 nm. You could even get some kinetics for the reaction this way.

I assume the styrene reaction is biphasic. That complicates things considerably, because it is essentially an extraction. Cl2 is extremely polarizable and nonpolar, so it would get schlepped into the styrenic phase (making it unavailable to decompose peroxide further) and virtually ensuring it would react with a molecule of styrene. Then (here's the clever bit) the chloronium ion which formed would be polar enough to re-partition back into the aqueous peroxide phase, where it would most likely react with a water molecule. (Reaction with a peroxide molecule would give you some side products including alpha-chloroacetophenone).

tl;dr Whoever designed your reaction is a smart cookie.

[curiouscat]

It is an interesting scenario, isn't it?

Indeed!

is incorrect. Given the equations from the link, for every mole of chlorine that is produced, one mole of peroxide goes directly to form water. Since the O2 producing reaction consumes a mole of chlorine, the very most you could lose to oxygen is half the peroxide.

Right you are. I'll check my material balance more carefully. Let's see if I can observe this. Though there's a bit of evaporation too that makes it hard to tease out.

Do you have access to a UV-Vis spectrometer? You could place some of the reaction mixture in a cuvette and monitor the absorption over time. I think molecular Cl2 absorbs at 325-350 nm. You could even get some kinetics for the reaction this way.

I'll try! Good ideal. So far quantification was on a GC.

I assume the styrene reaction is biphasic.

Yes. That's where our rate determining step lies I think.

Whoever designed your reaction is a smart cookie.

Is that a bug or a feature though? The bi-phasic part, I mean.

One idea I've been toying with is to get the darn phases to emulsify somehow to see if that will enhance rates. Good idea or bad, you think?

[Archer]

If you add bromine to methanol it forms methylhypobromite and the reaction with an alkene such as yours yeilds the O-methyl bromide analogue of your substance.

This fits with the formation of hypochlorous acid (HOCl) in the reaction between water and chlorine. The hypochlorous acid is what reacts with your alkene.

[opsomath]

Is that a bug or a feature though?  The bi-phasic part, I mean.

One idea I've been toying with is to get the darn phases to emulsify somehow to see if that will enhance rates. Good idea or bad, you think?

I think it's a feature. Essentially your oxidation reaction (Cl- + H2O2) takes place between extremely polar things, but produces an extremely nonpolar byproduct, Cl2. This stuff naturally partitions into your styrene phase and reacts there, producing a polar intermediate which naturally partitions back into the other phase.

Emulsifying might increase the boundary surface area allowing the Cl2 to diffuse into the other phase better. But if it's already mixed well, I wouldn't expect it would matter much.

[magician4]

hypochlorous/chloride solutions are very unstable under acid conditions!  (and those are a given due to the setup), leading to a certain loss of oxygen / chlorine  if the pathway discussed was correct

hence, i would consider standard epoxidation followed by acid-catalysed ringopening with subsequent chloride-addition as another plausible pathway...
... just like we know it from the standardsystem formic acid / H₂O₂  (with alpha-hydroxy formic esters resulting)

regards


Ingo

[curiouscat]

hence, i would consider standard epoxidation followed by acid-catalysed ringopening with subsequent chloride-addition as another plausible pathway...

Interesting. Can you elaborate what you mean by "standard epoxidation"? Do you have a particular route in mind? Since industrially epoxides are very often made via halohydrins I find your counter-pathway intriguing.

[opsomath]

What are the main byproducts of the reaction? That should shed some light on the mechanism.

[magician4]

treatment of alkenes with peracids (from in situ reaction of H₂O₂ with the acid in question) will lead to epoxides in the first place


(from: http://commons.wikimedia.org/wiki/File:Epoixd_Butterfly.png )

depending on the general reaction conditions those epoxides can either be isolated, or will undergo acid-catalyzed ringopening, resulting in alpha-hydroxy-esters as next product (which upon further treatment in aq. systems will undergo hydrolysis, resulting in trans-1,2 dioles)

(from: link)
(though shown as base-catalyzed ester cleavage here: this might well happen with acid catalysts, too )

very reactive olefines, like for example styrene systems, don't require the organic acid as a by-catalyst for epoxidation

(from: link )
(note that the initial epoxide-formation is left out both in the upper as as in the lower row, though it clearly must take place)


regards

Ingo