[curiouscat]

There's a bunch of patents describing the typical epoxidation process to make Propylene Oxide from Propylene.

The part that confuses me is this:

"..reacting a chlorinating species, such as hypochlorous acid, with.... propylene or butylene, at a pH of greater than 6.0. The chlorinating species is formed in a first step by reacting a source of chlorine, such as Cl2 gas, with an aqueous pH adjusting source, such as aqueous NaOH.."

Does this mean NaOH is a reactant that gets consumed stoichiometrically? Or is it some sort of catalyst / buffer? i.e. Not consumed stoichiometrically.

https://www.google.com/patents/WO1996000709A1?cl=en

What's the operative reaction here?

Cl2 + NaOH    HOCl + NaCl ?


PS. I'm not worried about the mechanism. More what the overall material balance will look like.

[discodermolide]

So I guess the NaOH is there to maintain the pH above 6. But what pH the maximum is I'm not sure. So you will need a pH electrode in your reactor. If you are making propylene oxide then the NaOH should also do this, so a pH of 9 may be enough to mop up HCl and form the epoxide.
This is just a guess.

[curiouscat]

So I guess the NaOH is there to maintain the pH above 6.

Thanks @disco.

But can it just maintain the pH or will it constantly get consumed by the Cl2 coming in? i.e. Does it have to be replenished in essentially stoichiometric amounts?

[Dan]

But can it just maintain the pH or will it constantly get consumed by the Cl2 coming in? i.e. Does it have to be replenished in essentially stoichiometric amounts?

Well, the reactions is this, right?

NaOH + Cl₂ + alkene NaCl + chlorohydrin

If you run out of base, the stoichiometric side product would surely be HCl...

[curiouscat]

If you run out of base, the stoichiometric side product would surely be HCl...

That's the part that is confusing. If you bubble Cl2 through H2O without any base you do get this, right?

Cl2 + H2O  HOCl + HCl

So with or without the base you ought to get HOCl. Which is what we need to react with the Alkene.

Does the base push the reaction forward in some way?

Or is having HCl in there bad for epoxidation (say by-products) but having NaCl is not bad?

[discodermolide]

I imagine that HCl will prevent epoxidation, or will open any epoxide that is formed. The base should also drive any equlibrium in the HOCl formation towards completion.
So the NaOH is there to mop up HCl and also form the epoxide. Therefore NaOH should be present throughout the entire process. So pH control here will be essential.
I have no idea at what pH epoxide formation begins or, indeed, if it goes to high epoxide opening may start to happen. This is why I choose a pH of around 9, an educated guess, but something you might need to check in the lab.

[clarkstill]

The pKa of HOCl is 7.5:

http://www.wileyshortcourse.com/cellbiology/textboxes/textbox_2_4.pdf

So presumably the HOCl is immediately and irreversibly deprotonated by NaOH? I guess the anion is more oxidizing, so perhaps ClO- is the active species.

[curiouscat]

I imagine that HCl will prevent epoxidation, or will open any epoxide that is formed. The base should also drive any equlibrium in the HOCl formation towards completion.

Maybe. But looking at the patents this part seems only concerned with making the chlorohydrin. They have one tower where they bubble Cl2 through alkaline water and presumably get a solution with HOCl in it. (and NaCl I suppose)

This then gets put into a second tower where they bubble propylene through it. That forms the chlorohydrin which then indeed gets contacted with NaOH or Lime in a seperate section to make the epoxide.

Ergo, the little epoxide that gets formed in stage 1 is incidental. The goal was to make the Chlorohydrin

So then the question is: Would the HCl also inhibit chlorohydrin formation from the alkene? Why?

e.g. The composition of the organic material in the CSTR output contained 91.3 molar-% propylene chlorohydrin (PCH), 4.1 molar-% propylene oxide (PO), and 4.6 molar-% byproducts.

[discodermolide]

HCl will add to the propylene, competing with the HOCl, therefore reducing the yield of chlorohydrin.

[curiouscat]

HCl will add to the propylene, competing with the HOCl, therefore reducing the yield of chlorohydrin.

Understood. Thanks.

Is HOCl stable in the vapor phase? Do you know? I wonder if there's a way to avoid the alkali entirely by evaporating off the HOCl / HCl mixture and getting an HOCl enhanced stream by partial condensation of the HCl.

[discodermolide]

Don't really know. But these hypohalous acids are usually not so thermally stable.

[orgopete]

In order to get an epoxide from and alkene and chlorine, you need two equivalents of hydroxide. Should the second equivalent be added simultaneously with the chlorine? Hydroxide might open the epoxide, so I'm guessing the reaction is actually done by adding chlorine to the alkene in water and then adding the hydroxide until the chlorohydrin disappears (to form epoxide).

[curiouscat]

Hydroxide might open the epoxide, so I'm guessing the reaction is actually done by adding chlorine to the alkene in water and then adding the hydroxide until the chlorohydrin disappears (to form epoxide).

Indeed. That is how it seems to be done.

Three stages: (a) Add Cl2 to water & NaOH to make HOCl (b) Add HOCl solution to alkene to make chlorohydrin & (c) Add caustic or lime to chlorohydrin solution to make epoxide.