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Author: Subject: Phthalic acids from xylene using Oxone® as an oxidising agent
Keras
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[*] posted on 14-2-2022 at 08:40
Phthalic acids from xylene using Oxone® as an oxidising agent


Guys,

this was done with just a 25-ml beaker on the back of an envelope, so don’t be too harsh on me, and don’t expect anything but a crude report.

To 5 mL of distilled water are add about 3 mL of xylene (mix of isomers). The resulting biphasic mixture is warmed on a hot plate until the water is nearly boiling (that happens pretty fast, given it is capped with the xylene layer).

As the temperature reaches 100 °C, a few grams (1 or 2, give or take) of potassium peroxymonosulphate (Oxone®) is added almost crystal by crystal, each crystal dropped into the mixture resulting in water boiling and bubbling, which also somewhat stirs the pot.

After all the Oxone has been added, the mixture is left to boil for a further 5 min, with a small metal rod that serves as boiling chip. The xylene layer turns slightly yellow, at which point the beaker is taken off the hotplate.

The ‘smell’ of xylene is definitely gone, replaced by a smell somewhat redolent of benzaldehyde, but definitely different (sourer, more acrid, or more ‘roasted’). The water layer turns cloudy, but I see no evidence of precipitate. I suspect phthalic acids, if any, are trapped in the organic layer.

Now that’s about it. I don’t have any equipment here, no sep, no cooler, nothing that could help identifying what's inside the organic layer (or even that could help separate it cleanly)

So the question is: can I reasonably assume that Oxone, like potassium permanganate, can oxidise xylenes to phthalic acids, to something else (4-carboxybenzaldehyde, which would account for the smell) or am I just daydreaming?

[Edited on 14-2-2022 by Keras]
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[*] posted on 14-2-2022 at 10:56


Based on the findings of Kennedy and Stock in J. Org. Chem. 25, 11, 1901-1906, it's likely that at least one of the methyl groups has been converted to an acid - when Oxone was reacted with toluene, the result was benzoic acid, in a yield of around 50%. However, the reaction time was much longer: 22 hours under reflux.

Where it gets a little more complicated is that when Oxone wasa reacted with salicaldehyde (2-hydroxybenzaldehyde), the result was catechol - the molecule lost the aldehyde group and -OH was added directly to the ring. Similarly, when reacted with phenol, the result was hydroquinone (1,4-dihydroxybenzene). Both of these reactions were conducted at room temperature or below.

Given that you started with a mixture of isomers, my view is that you have a mixture of products with phenolic and acidic functionalities. As for the smell, that could be the result of cross-reaction products between different components of the mixture - but in any case, it wouldn't be possible to directly produce an aldehyde from phthalic acid under oxidising conditions.




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[*] posted on 14-2-2022 at 11:17


Separate the organic phase and shake it with an excess of a solution of sodium bicarbonate.
The organic acids will dissolve in the aqueous layer. Bicarbonate isn't a strong enough base to deprotonate (most) phenols.

Then take the aqueous extract and add excess acid.
That will convert the phthalic acid (and its isomers) back to the free acids which are not very soluble in water.
They will produce crystal or, at least some cloudiness.

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[*] posted on 14-2-2022 at 11:57


Quote: Originally posted by Lionel Spanner  
Based on the findings of Kennedy and Stock in J. Org. Chem. 25, 11, 1901-1906, it's likely that at least one of the methyl groups has been converted to an acid - when Oxone was reacted with toluene, the result was benzoic acid, in a yield of around 50%. However, the reaction time was much longer: 22 hours under reflux.

Where it gets a little more complicated is that when Oxone was a reacted with salicaldehyde (2-hydroxybenzaldehyde), the result was catechol - the molecule lost the aldehyde group and -OH was added directly to the ring. Similarly, when reacted with phenol, the result was hydroquinone (1,4-dihydroxybenzene). Both of these reactions were conducted at room temperature or below.

Given that you started with a mixture of isomers, my view is that you have a mixture of products with phenolic and acidic functionalities. As for the smell, that could be the result of cross-reaction products between different components of the mixture - but in any case, it wouldn't be possible to directly produce an aldehyde from phthalic acid under oxidising conditions.


Thanks for your input, quite interesting.

My idea was that:

1. Given that potassium permanganate can oxidise xylene to phthalic acid (there’s even a video of that somewhere on the webspace), and since Oxone's redox couple is higher than KMnO₄ which, by the way, make Oxone one of the strongest oxidising agents known;

2. But given that the industrial process by which xylenes are transformed into phthalic acid produces also carboxybenzaldehyde

The oxidation could stop at some middle point. I also read that while the first methyl group in xylene will oxidise to carboxyl quite easily, the second seems to be more reluctant. Wherefore my idea that the second oxidation could stop at the aldehyde phase, giving as a result carboxybenzaldehyde in various isomers.

Quote: Originally posted by unionised  
Separate the organic phase and shake it with an excess of a solution of sodium bicarbonate.
The organic acids will dissolve in the aqueous layer. Bicarbonate isn't a strong enough base to deprotonate (most) phenols.


I was thinking about that, but using sodium hydroxide. However, you’re right, sodium bicarbonate is way smarter to use. Thanks for the suggestion!

---

By the way, something I never really figured out: is the redox potential of a given red/ox couple related to its ability to oxidise organic molecules? In other way, can we assume that the lower the potential, the milder the oxidiser?

When I think about the mechanisms behind the oxidation of organic molecules, with all the orbitals, σ, σ*, π and π* stuff, HOMO, LUMO, etc. I don’t clearly see how the potential relates to those mechanisms.

[Edited on 14-2-2022 by Keras]
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