fully buffered peracetic: failure.

[hypo]

hello, bees!

first of all thanks to chromic for working out a viable peracetic procedure:

Post 250788

(Chromic: "Peracetic in DCM - dreams of success", Methods Discourse).

the goal was to make the epoxide of anethole as opposed to the glycol / acetyl ester.
the procedure tried was chromic's (100 mmol anethole) with an 0.5 molar amount
of Na2CO3 (1/2 mol Na2CO3 per mol CH3COOH) instead of NaAc and TCE instead of DCM. some
mishaps occured. yield was very very low and it isn't granted that the epoxide was formed.

to make a long story short: about 2/3rd of the Na2CO3 was added to the peracetic acid
(which had to be slightly diluted with water), the other part was dissolved in some water
and directly added to the TCE in which the reaction was performed. in the beginning
the reaction was cooled on ice bath, but not noticing any exothermic reaction, it was
stirred for 8h at rt. pH was 7 throughout the reaction. after distillation, ended up
with 9.5g anethole and 2.0g of a dark yellow, viscous residue smelling like PMP2P which was
not further distilled.

i see many reasons why this reaction may not work. the most convincing ones:
* the chemist is unable.  
* with a pH around seven, most of the peracetic is dissociated and stays in the aqueous layer.

ideas:
* heat the reaction
* let it run longer
* don't add water
* use MeOH or EtOH like in oxone procedure

suggestions very welcome!

(usually this would not be post worthy, but i'll be away for some weeks, so i thought
post it, so other bees trying this can learn from it.)
couch terrorist

[Rhodium]

The pKa of peracetic acid is 8.2, so if you had a pH of 7, only ~5% of your acid was dissociated.

[GC_MS]

You can't make the epoxide using a peracid oxidation, since the formed epoxide is more like an 'intermediate'. Once the epoxide is formed, it immediately reacts with the acid form of your peracid (HOAc, HCOOH, ...) to yield glycol esters. The isolation of epoxides is very difficult, and trying to achieve this with 'simple' equipment is near to impossible.
About suggesting heating: never go higher then 40°C, unless you like low(ered) yields.
About the runtime: SWiM doesn't really look at the time, but at the colours of the reaction mixture. In case of anethole: anethole/DCM is nearly colourless, changes to yellow when adding peracid, but SWiM stops his glycol formation reaction only when his DCM phase is deep red. He is sure that the reaction is 'finished' then.
WOMAN.ZIP: Great Shareware, but be careful of viruses...

[Rhodium]

GC_MS: Read the references in the document hypo is talking about first:

https://www.thevespiary.org/rhodium/Rhodium/chemistry/peracetic.chromic.html

Peracetic acid most certainly can be used to form epoxides, as long as the reaction medium is buffered to keep the concentration of free acetic acid to a minimum, but not as far as to deprotonate the peracetic acid. Peracetic acid will not react with the epoxide to form a glycol, as it is almost neutral itself.

[Chromic]

For 100mmol scale fully buffered, this is what I'd use:

Into the flask: 14.8g anethole, 130ml TCE (or DCM), 26.6g Na2CO3 (or 42.2g NaHCO3 -- as you can see, these high values of buffering to alkene are why I chose not to go for the epoxide...)

Then make your peracid (let sit 5-7 days at room temp -- NECESSARY!): 30.2g 35% H2O2, 29.0g GAA, 0.6g H2SO4

Put the flask into an ice-water bath, then drip in the peracid with stirring as fast as you can (but don't dump it in wholesale, a lot of CO2 release occurs). Run until the rxn is over, ice-cubes have melted, etc. ~5hrs.

Add 130ml water to the flask, keep the TCE. Wash once with 87ml of 5% NaOH (4.3g NaOH). Distill off the TCE to claim the epoxide.

Sure enough, you should get the epoxide. Epoxides are frequently made from peracids! Don't give up.

In Patent EP 1140788 trans-cinnamyl alcohol is epoxidized using a similar procedure.

My bet is that you didn't wait long enough for the peracetic to form or you didn't use sulfuric acid to catalyze the production of the peracid. Or that by neutralizing the peracetic before the addition, you broke it down to sodium acetate, carbon dioxide and oxygen.

[hypo]

oh! i didn't realise that peracids were such weak acids!
but it makes sense, the -O3^- anion doesn't look stable.

could it be that the pH measuring was wrong? those universal pH
papers where used, but they work only in aqueous medium. they
definitely showed 7.

so another suggestion would be:
* less Na2CO3?
couch terrorist

[hypo]

> 14.8g anethole, 130ml TCE (or DCM), 26.6g Na2CO3

34g Na2CO3 were used to neutralise 38.3g CH3COOH.
(too much?)

> My bet is that you didn't wait long enough for the peracetic to form

14 days  

> or you didn't use sulfuric acid to catalyze the production of the peracid.

sulfuric acid was used.

> Or that by neutralizing the peracetic before the addition,
> you broke it down to sodium acetate, carbon dioxide and oxygen.

that is a possibility... the next time, the Na2CO3 will be added
to the TCE. what about dissolving it in the minimum amount of water?

thanks, bees! the experiments will be continued!


couch terrorist

[Chromic]

>> 14.8g anethole, 130ml TCE (or DCM), 26.6g Na2CO3
>34g Na2CO3 were used to neutralise 38.3g CH3COOH. (too much?)

The amount of peracetic acid that you used is a 56% molar excess (156mmol). (assuming you used 39.9g 35% H2O2, and 0.8g H2SO4) The proper amount of Na2CO3 to buffer, would be ~35.2g (if you follow the same assumptions below). The amount of sodium carbonate you used, in relation to the peracetic that you used, is correct.

EXCEPT... I'm confused on the amount of peracetic you used, because my writeup would recommend:

30.2g 35% H2O2, 29.0g GAA, 0.6g H2SO4 giving 0.48mol of (per)acetic acid to be buffered (this actually forms 118mmol of peracetic acid at equilibrium, the 18% excess that is recommended in the EP patent... perhaps you had read an earlier post in the thread by me where I was using more peracetic than necessary), and about 0.01mol of sulfuric acid to be neutralized. If you want to neutralize 100% of the peracetic/acetic, and 150% of the sulfuric (numbers pulled from hat), you'd use 25.6+1.0g of Na2CO3 or 40.6+1.5g of NaHCO3.

Try repeating your experiment, with the suggestion of keeping the buffer with the alkene in chloronated solvent (and perhaps modifying the amount of peracetic to use to fit inline with the patent and my updated procedure...) This must be the trouble, if it still doesn't work something is really wrong.

You might also try using baking soda, as it's a weaker base (?).

[Chromic]

Anethole epoxide really does smell like the ketone and the glycol (very much unlike the alkene), so part of your experiment sounded right... but I bet you already know that as you've sampled PMA before.  

[hypo]

>  perhaps you had read an earlier post in the thread by me where I
> was using more peracetic than necessary

exactly, the first one where you said 36.9g. the H2O2/AcOH ratio was somewhat
different, because over here we have a different 35% H2O2 (w/v vs. v/v!)

NaHCO3 will be used the next time. then most of the variables should be eliminated.


couch terrorist