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Author: Subject: need help with oxidation conditions
spirocycle
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[*] posted on 10-5-2011 at 12:33


Third and fourth trials were similar failures.
I added 10ml acid, 4ml styrene each time, and tried to gradually add the stoich amount of KMnO4, and despite stirring, cooling, and gradual addition, I'm just left with a thick sludge.
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Nicodem
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[*] posted on 10-5-2011 at 13:10


If I understand you correctly, you want to prepare acetophenone from styrene, no matter how?

If that is the case, then abbandon futile attempts at the hydration of styrene. It is not going to be in no way simple. Have you checked the literature on the conditions and yields?

Why don't you try a chloroalkoxylation followed by elimination? It is much more simple and should give good yields if the methodology is properly developed.
I would first chloromethoxylate styrene with trichloroisocyanuric acid in methanol. This is a high yielding reaction (see DOI: 10.1055/s-2003-36250, the article was already posted in some other thread). Then you just need to find the proper conditions for the elimination step. I would suggest heating the 2-phenyl-2-methoxyethyl chloride in a KOH/glycol solution at 100-120 °C. Use 1.1 equivalents of KOH or NaOH and ethylene or propylene glycol as solvent (or any other suitable solvent you can get). After TLC shows full conversion of the starting material, cool the mixture, add 2 equivalents of conc. HCl and stir at 50 °C for one hour. This should be enough to hydrolyze the alpha-methoxystyrene to acetophenone in a one-pot procedure. After extraction, you will need to fractionate the crude to isolate acetophenone.
I have some experience with a related conversion of a beta-chloroether to the corresponding ketone, though I used ethyl cellosolve as a solvent for the elimination. The yields were relatively good. I don't remember much details and it would not help you if I dig up my lab notes, as that was not done on plain styrene.




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spirocycle
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[*] posted on 10-5-2011 at 14:12


I dont have many of those chemicals (glycol or TCiCA).
Do you think that a hydrobromination, followed by an Sn1 (with AgNO3) in water would give better results?

if the hydrobromination is anywhere near as fast as the bromination, it should be simple enough. I just havent been able to find any resources mentioning it.
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blogfast25
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[*] posted on 11-5-2011 at 05:24


Quote: Originally posted by Nicodem  
If that is the case, then abbandon futile attempts at the hydration of styrene. It is not going to be in no way simple. Have you checked the literature on the conditions and yields?



We found a reference (a google book) to acid catalysed styrene hydration which stated the rate constant (first order reaction in both alkene and hydronium) for styrene was about 200 times higher than for 1-hexene, due to electron pushing by the π ring. No recipe or methodology was provided though.

So what precisely makes simple hydration of styrene so problematic in your view? Tendency to polymerise? Something else?

Trichloroisocyanuric acid is quite OTC, right? I seem to recall someone on SM based an excellent chlorine generator on it…


[Edited on 11-5-2011 by blogfast25]
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[*] posted on 11-5-2011 at 06:39


Quote: Originally posted by blogfast25  
So what precisely makes simple hydration of styrene so problematic in your view? Tendency to polymerise? Something else?

Cationic polymerization can be reduced by performing the reaction in homogeneous media. So it is just a matter of choosing the right cosolvent and ratio with water. Dioxane is probably the best solvent for such things. Diethyl ether is miscible with aqueous H2SO4, so it should be useful as well (depend on how diluted though). Radical polymerization can also be inhibited by using inert atmosphere and radical traps like hydroquinone or 2,6-di-t-butyl-4-methylphenol (less than 1 mol% is usually added). The major obstacle is the reversibility of the reaction, which makes it difficult to develop a preparative reaction and avoiding side reactions. The more acid used, the further left is the equilibrium point. Too little acid and the reaction rate is impractical. Generally this means that you need to screen the conditions (particularly the amount of acid) and follow the reaction at short intervals. It is simply to much work just to obtain a mediocre yield at the end.
The alternative is to do a literature search, as chances are somebody already developed the methodology for a preparative reaction.
Another alternative is to use HgSO4 catalyzed hydration. Hg(II) is a soft acid and thus efficiently catalyzes the addition of water on the double bonds, but the elimination requires a hard acid which can be kept on the minimum required for the Hg(II) catalytic cycle.
Quote:
Trichloroisocyanuric acid is quite OTC, right? I seem to recall someone on SM based an excellent chlorine generator on it…

Yes, trichloroisocyanuric acid can be bought anywhere. Where I live they sell it in any larger supermarket with stuff for house use. Ethylene or propylene glycol can be bought at any gas station (at least in my country). I don't know where spirocycle lives, so that he can not buy such ubiquitous stuff.
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[*] posted on 11-5-2011 at 08:35


It's not that I can't find it, its that I can't afford it.
I am actually pretty sick right now, so I can't really do any more experiments, but when I recover I will try to hydrobrominate the styrene, and from there, hydroxylating should be easy enough with AgNO3 around, right?
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[*] posted on 11-5-2011 at 10:20


Thanks Nicodem, very interesting...
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[*] posted on 13-5-2011 at 09:22


Quote: Originally posted by spirocycle  
but when I recover I will try to hydrobrominate the styrene, and from there, hydroxylating should be easy enough with AgNO3 around, right?

I don't know about the hydrobromination of styrene, but I'm almost sure I saw a post at Hyperlab where the hydrochlorination of styrene is described. Try searching for "styrene" at the forum search engine. Also, I guess you can find examples of hydrobromination in the literature, if you only try. It's nothing exotic, so there should be some preparative reaction described in the journals.
As for the reaction with AgNO3, I can't know. Have you done a literature search already? It might just give styrene back.
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[*] posted on 13-5-2011 at 09:59


I'll do a literature search to confirm, but it doesn't seem likely to give styrene back.

Assuming the hydrobomination worked, and I mix rapidly with silver nitrate, the benzyllic carbocation will likely form, and water should act at the nucleophile. the only way for styrene to reform would be if there was a hydride source.
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[*] posted on 14-5-2011 at 13:54


Quote: Originally posted by spirocycle  
the benzyllic carbocation will likely form, and water should act at the nucleophile. the only way for styrene to reform would be if there was a hydride source.

That does not make much sense to me. If the carbocation forms it can react with water, but in two different ways, as water can act as nucleophile or as a base. Therefore, given the ease of proton elimination from 1-phenylethyl carbocation, you can easily get styrene back. I don't know what you get, because I have not done a literature search (not interested enough).

Hydration of styrene via Co(II) catalysed oxidation-reduction:
Bull. Chem. Soc. Jpn., 63 (1990) 179-186.
Inorganica Chimica Acta, 157 (1989) 171-174.
J. Am. Chem. Soc., 107 (1985) 3534-3540.

Photochemical hydration of styrene:
J. Org. Chem., 48 (1983) 869-876.

I have not read the whole thread, so I do not know what's the purpose of all this. But obviously it is not about obtaining acetophenone, as this is as cheap as it can get. In any case, I do hope you are aware that the Wacker oxidation of styrene gives acetophenone in a single step?




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[*] posted on 14-5-2011 at 19:38


I am aware, and I see how the purpose may seem obscure but this is the deal. I'm pretty inexperienced in the lab and don't really have a whole lot of cash to spend. I want to make acetophenone (just a couple grams) cheaply. Yields don't need to be great because styrene is cheap. I can't afford the catalyst for the wacker oxidation, and the only (kind of) polar aprotic solvent I have at hand is acetone.
Since the hydration seems kind of tricky based on what you said and my several attempts, I'll try the hydrobromin(chorin)ation and then try the silver nitrate thing.
I see how I could end up with some styrene again, but I feel that given the reactivity of the double bond in styrene (at least in hydration), it would likely give mostly the alcohol.
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[*] posted on 15-5-2011 at 04:43


Quote: Originally posted by Nicodem  
That does not make much sense to me. If the carbocation forms it can react with water, but in two different ways, as water can act as nucleophile or as a base. Therefore, given the ease of proton elimination from 1-phenylethyl carbocation, you can easily get styrene back. I don't know what you get, because I have not done a literature search (not interested enough).



I don’t quite understand how that squares with the high rates of styrene hydration compared to the textbook example of 1-hexene, the former being reported having a rate constant about 200 times higher than the latter. That points to a very stable carbocation, no? Although I can see how that can lead to polymerisation…
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[*] posted on 15-5-2011 at 13:09


Direct H+ catalysed hydration of styrene to 1-phenylethanol seems to be simple to do. However I could not find any preparative procedure for this .....
The books I have say that "hydration of alkenes has no larger preparative meaning".
In case of simple alkenes, reaction with H2SO4 (40-80%) gives alkyl mono- and di- sulfates as main primary produsts. In the next step these sulfates are converted (more or less easily) to alcohol.
For styrene it is not so simple because it is prone to self condensations giving various "dimers" or just oligomers. It is also valid for benzyl alcohols - they are not stable in strong acidic media, especially H2SO4.
Example article (via DOI) : 10.1021/ja01167a019
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