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Author: Subject: need help with oxidation conditions
spirocycle
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[*] posted on 21-4-2011 at 08:27
need help with oxidation conditions


I have some polystyrene that I am going to depolymerize today, but before I start working I like to write up my procedure.
After depolymerization, I am going to hydrate and then oxidize.

For hydration, a teacher recommended 10% sulfuric acid.
does 1.5 mL for every gram of styrene sound reasonable? and stir for 15 minutes?

I figure the carbocation intermediate is pretty stable so the reaction will probably progress quickly.

for the oxidation step, I really only have permanganate and 3% peroxide. How concentrated should I make the permanganate to allow the oxidation to the ketone, without cleaving the side chain?
what pH should it be at? heat? time?
I'm actually kind of lost on the specifics of this condition.
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[*] posted on 24-4-2011 at 10:34


anyone gonna help out?
i know I can just experiment, but I dont really have a whole lot of sulfuric acid to play around with.
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[*] posted on 24-4-2011 at 12:44


Have you searched the organics part of the board? Your question maybe better posted there. Specify perhaps also how you plan to depolymerise.

Without experience with that specific hydration I think your styrene/H2SO4 ratio sounds about right... Probably needs refluxing though...

For the permanganate question, stoichiometry of the reaction and solubility of KMnO4 in water should give some clues. Using the stoichiometric amount of permanganate should be a reasonable starting point. Heat very moderately depending on how fast you see the purple permanganate react away. Acid conditions needed but precise pH not very important, at a guess. Say pH 3 - 4, with H2SO4. You should see the reaction proceeding and ending with all the MnO4- reacted away, so 'time' should reveal itself.

Hope this helps...
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spirocycle
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[*] posted on 24-4-2011 at 13:06


the depolymerization went very well with simple distillation of the polymer.
I didn't post in the organics, because i've been told that new topics there require references.
I don't have any specific references for this reaction, just the knowledge that hydration and oxidation generally occur in similar conditions.

The main issue I'm having with the hydration is: how do I know if heat favors the forward or reverse reaction?
Does the large excess of water (10% H2SO4) fully combat that issue?
I guess I could do a bromine test with a small sample every few minutes.
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[*] posted on 24-4-2011 at 13:28


That kind of hydration is often carried out using acetone or methanol as a solvent with quite a low concentration of the target reagent, then reflux at 70 - 85 C. See one example here: http://www.sciencemadness.org/talk/viewthread.php?tid=15171&...

(scroll up and down for info on ratios)

I’m not claiming this is going to work for your specific reaction but it is a hydration of a terpene (double bond).

Testing regularly with Br2 is a possibility for sure.

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spirocycle
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[*] posted on 24-4-2011 at 13:36


wouldn't you get some aldol products using acidified acetone?
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[*] posted on 24-4-2011 at 17:07


Quote: Originally posted by spirocycle  
wouldn't you get some aldol products using acidified acetone?


A small amount of aldol products may be formed but they wil be in equilibrium with acetone and water under aqueous acid conditions.
Self condensation of acetone requires strong acid or base and forcing conditions
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[*] posted on 26-4-2011 at 08:59


an old teacher recomended using household bleach as the oxidant
sound reasonable?
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[*] posted on 26-4-2011 at 12:32


For the oxidation of the sec. alcohol to methyl phenyl ketone? Bleach always contains free Cl2: could halogenate your double bond. K2Cr2O7 and KMnO4 really are the most recommended oxidants here... Neither are hard to get.
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[*] posted on 26-4-2011 at 17:56


if the bond is hydrated, there is no double bond to halogenate.
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[*] posted on 27-4-2011 at 06:52


No, but it depends which reaction get's preference, doesn't it? Would you also consider oxidising in the presence of small amounts of Br2? :P

Commercial bleach (sodium hypochlorite in dilute solution) is a widely used strong oxidiser, especially by hobbyists, but I don’t feel it has a place here… Still, only experiment can tell.

Can you descibe the conditions of depolymerisation a bit more?
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[*] posted on 27-4-2011 at 08:13


I would do the hydration first, and then add bleach.
If the excess styrene were to be halogenated, it wouldnt affect my yield because the alkene wouldnt be oxidized either way.

I dissolve a few foam cups and bowls in minimal acetone, and leave it out to dry for a bit in the sun.
then I put the mostly dry chunks in a 500 ml flask.
It is distilled over a bunsen burner with an air-cooled condenser.
some acetone comes over, but then a hydrocarbon layer comes over.
Hopefully this week I can get an IR spectrum of the product.
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[*] posted on 27-4-2011 at 12:09


Yes, my bad: you carry out the hydration first and separately.

To check if bleach (hypochlorite) has the potential to oxidise the sec. OH group to C=O, compare the reduction potentials of KMnO4, K2Cr2O7 and ClO-.

Did your styrene come over at about 145 C? There’s always the chance of getting dimers and trimers…

Of course you can just test your hypothesis on on oxidising alcohols with hypochlorite: reflux some IPA with bleach and see what happens; you should get acetone if you're correct (personally I doubt it)...

[Edited on 28-4-2011 by blogfast25]
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[*] posted on 28-4-2011 at 13:21


turns out that my "styrene" was mostly acetone. . .
So I'm going to reattempt and only collect the portion that is immiscible with water.
I'll get an IR spectrum to see what I have.
I'm fairly competent at IR analysis, but honestly, I am stumped at how I could tell the dimer from the monomer.

usually alkene c=c stretch is 1680-1640, and aromatic c-c stretch is 1600-1585 and 1500-1400.
But i'm not sure if a peak 50 wavenumbers away will be that distinct
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[*] posted on 29-4-2011 at 07:36


I did think you were a bit cavalier about the depolymerisation of polystyrene, which is a decidedly stable polymer (no conjugated double bonds for instance).

Perhaps try this: make a fairly concentrated solution of styrofoam in acetome in a clear bottle (UV transparent material!) and allow it as much sunshine as you can muster for about a week or so. Then distill.

Telling the monomer from the dimer (trimer etc.) may be hard with IR but of course the BP of the oligomers will be that much higher that they should be separable quite easily from the lower boiling styrene. Perhaps distill the obtained styrene one more time, collecting only the 145 C fraction?
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[*] posted on 29-4-2011 at 07:45


I'll do that.
But while i'm waiting for the UV depolymerization, I'll try another pyrolysis, with more prolonged heating.
If I'm lucky, i'll get to use the rotovap.
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[*] posted on 29-4-2011 at 09:29


I wonder if a bit of peroxide, either H2O2 or slower acting organic peroxides (caution! Dangerous stuff although many are quite tame...) might help cleave the backbone. In polymers like polyolefins, radicals are the enemy of the methylene (-CH2-) backbone. A form of accelerated ageing or weathering if you prefer...
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[*] posted on 29-4-2011 at 14:25


Quote: Originally posted by spirocycle  
the depolymerization went very well with simple distillation of the polymer.

It is very probable that you obtained mixture, with low styrene content. The rest is benzene and toluene.
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[*] posted on 1-5-2011 at 05:58


Hmm... possibly the but the main cleavage point will be the methylene backbone, at a guess. There are some patents out there that speak of high styrene yields for the depol. of polystyrene, as always for what they're worth of course.
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[*] posted on 3-5-2011 at 11:12


Well I reran it, this time heating more and for longer.


Rather than collecting in a regular flask, I collected in a sep funnel charged with a few ml tap water.
This way, the phases will separate.
After about an hour of distilling, I obtained two distinct phases, the top, organic phase was a pale yellow, and the bottom, aqueous phase was murky white.

The organic phase was collected, and washed with 20 mL of water. The phase was then dried with 1.01g MgSO4
This product passed the bromine test.
I'll get around to writing it up more formally, but my notebook got wet and its drying now
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[*] posted on 3-5-2011 at 13:13


Oh, the 'wet notebook defense', eh? :)

Sounds promising though... Details will be appreciated.
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[*] posted on 3-5-2011 at 14:10


32.87 g PS (CD cases) was dissolved in acetone and dried to a final weight of 40.37 g.
This was distilled for 65 minutes. The plastic became frothy and discharged a thick white smoke that condensed to form the distillate. The distillate was collected in a separatory funnel charged with around 5 mL tap water to form two distinct layer. The aqeous layer (bottom) was cloudy, and the organic layer was pale yellow. The Aqueous layer was drained, and the organic layer was washed with 20 mL fresh DI water. Separation yielded 18.31g of product, presumably styrene among other things, which is 55.7% yield.
It should be noted that the boiling flask was left with a mass of thick black, tarry substance that has proven marginally soluble in kerosene.
An IR spectrum will be taken tomorrow.
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[*] posted on 4-5-2011 at 04:46


So no UV or peroxide in this case. Still, 18 g of (we assume) hydrocarbon distillate is very respectable. Any idea of the temperature at which the distillate came over?

As kmno4 suggested, when taking the IR, look out for signatures of benzene, toluene, ethylbenzene… apart from styrene itself of course.

What’s your bromination procedure? Modified slightly it could be used quantitatively…

Cleaning up your distillation flask may be a job and a half: boling up with very strong alkali should dissolve the carbon.
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[*] posted on 4-5-2011 at 07:49


I have no thermometer attachment for my 19/22 set so I really have no idea on the boiling point.
My bromination procedure is pretty simple,
dilute about 1 mL liquid bromine with about 10 mL DI water.
Add a couple drops to a test tube.
Add a couple drops of the hydrocarbon to the tube, and shake
the color disappeared quite quickly
I dont really have a way of accurately measuring small volumes, so quantitative analysis may be tricky
ethyl benzene and toluene will be easy to discriminate against (methyl rock and methylene scissor), but benzene may be a bit trickier. I'll upload it later this afternoon once I get it.
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[*] posted on 4-5-2011 at 12:15


No thermometer is a drawback: it would make a redistillation of the styrene suspected liquid also fairly meaningless.

A semi-quantative procedure with bromine could be as follows. Either use a styrene standard or another compound of well known unsaturation content (and fairly reactive too - any low MW alkene would do). Add the same (small) amounts of both liquids to two identical test tubes.

Now add bromine solution to each tube, drop by drop, allowing plenty time between additions (the reaction will be slow when full bromination is near). Comparing the number of drops needed to brominate the standard and the sample would give you a pretty good idea of how the two compare in terms of unsaturation content. A ‘comparative titration in a test tube’, if you will…
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