Purification of P2P's by oxidation?

[Bandil]

Hi!

When using

this

(https://www.thevespiary.org/rhodium/Rhodium/chemistry/p2p.sncl2.html) method for the reduction of nitopropenes to ketones, it seems that the major by-product is the benzaldehyde. It shouldn't be much (app. 5%) when using the ethylacetate method, but with IPA the contamination rises to about 40%. This can be removed by distillation, but it would nice to have a slick way of removing it.

I had thought about suspending the oil (ketone and benzaldehyde after steam distillation) in dilute cold aq. NaOH and titrating with 1M KMnO₄, untill a purple color persists. In a perfect world the benzaldehyde would be oxidized to benzoic acid, and this should dissolve in the basic medium. The ketone could then be isolated directly by the extraction with a proper NP.

The only thing that worries me about this scheme is the possible oxidative cleavage of the benzyl CH₂ group in the ketone. I have not found any conclusive answer to when permanganate will cause oxidative cleavage at the benzyl carbon.

Anyone have some neat suggestions or comments?

Kind regards
Bandil

[Barium]

If you know the ratio benzaldehyde/ketone formed you can simply add enough bisulfite to bound all benzaldehyde as the bisulfite adduct. The benzaldehydes will always form the adduct more easily than a phenylacetone because the aldehyde is less sterically hindered.

[Megatherium]

Strong oxidizing agents (like KMnO₄, CrO₃) are able to oxidize ketones.  The reaction is only of synthetic use for cyclic ketones (to generate terminal aliphatic di-carboxylic acids), since oxidation of aliphatic ketones generates a hopeless mixture of compounds (destructive oxidation).

If one carefully titrates, I don't think there will be any problem, because benzaldehyde is a far better reductant than P2P.  If you want to play it safe, choose a milder oxidizing agent.

Probably the best idea is to form the bisulfite adduct, like Barium said.

I remember from developing a sprayed TLC in a chlorine atmosphere (KMnO₄ / HCl) that it ruined the beaker pretty much ...   , damned MnO₂.


Aztec fanclub member

[hest]

The Bisulfit solution proposed by barium works lika a charm. I think it's the way to doo it. The is a short storie in Vogel abouth it (cant remmeber teh page).

[armageddon]

Hi!

Megatherium: cleaning your glassware from MnO₂ isn't that hard: just use a few ml's of a strong aequ. SnCl₂ solution and a squirt of conc. HCl - it reduces the manganese dioxide, I think. At least the brown remnants dissolve completely in water this way...

Greetz A

[hest]

Post 296793 (missing)

(hest: "The H donor is the alcohol.", Newbee Forum)

[wolff_kishner]

Fehling's solution (a solution of potassium bitartrate and copper sulfate) would oxidize the aldehyde, but leave the ketone unchanged. It is a much milder and more selective agent than permanganate.