Oxidation of mandelic acid to benzaldehyde etc. , Hive Methods Discourse

moo
(Hive Bee)
09-18-03 09:55
No 459724

Oxidation of mandelic acid to benzaldehyde etc.
(Rated as: excellent)

Acta Chem. Scand. B38(4), 343-344 (1984)

Oxidation of alpha-Hydroxy Carboxylic Acids with Sodium Hypochlorite

General Procedure. A solution containing 10 mmol of the alpha-hydroxy carboxylic acid in 25 ml of diethyl ether was cooled in an ice-bath. Then 40 ml of a commercial bleach solution (containing 4.2% sodium hypochlorite and 0.5% sodium hydroxide), 22 mmol, was added over a 3 min period. The reaction mixture was then allowed to warm up to room temperature for 2-3 h. The organic layer was separated, washed with water, dried over anhydrous magnesium sulfate and the solvert evaporated. The crude product was purified by molecular distillation or recrystallization.

Entry	Substrate	Reaction time, h	Product	Yield, %
1	PhCHOHCOOH	2	PhCHO	95
2	Ph₂CHOHCOOH	1	(Ph)₂CO	91
3	PhC(CH₃)OHCOOH	1	PhCOCH₃	93
4	C₆H₁₁CHOHCOOH	2	C₆H₁₁CHO	85
5	(CH₃)₂CHCHOHCOOH	2	(CH₃)₂CHCHO	47
6	PhCHOHCOONa	2	PhCHO	93
7	5-androsten-3beta,17alpha-diol-17-carboxylic acid	3	5-androsten-3beta-ol-17-one	85

A modification comes to mind. If the benzaldehyde to be prepared is a water-insoluble solid (such as piperonal), and the sodium salt of the corresponding mandelic acid is water-soluble enough, it might be possible to omit the organic phase from the reaction and filter the precipitated benzaldehyde from the reaction mass.

The procedure posted could come in handy for those wishing to prepare benzaldehydes from mandelic acids, which can be prepared from the corresponding benzenes. The route is a close cousin of the chloral route. Related posts/threads:
Post 226201 (Bwiti: "3,4-methylenedioxymandelic acid", Chemistry Discourse)
Post 328397 (demorol: "Piperonal synthesis", Novel Discourse)

Especially the following ref is a must-read for those interested in the preparation of piperonal from benzodioxole via 3,4-methylenedioxymandelic acid, or any other benzaldehyde.
Org. Proc. Res. Dev. 4, 534-543 (2000) (../rhodium/pdf /md-mandelic2piperonal.pdf)

It is a pity glyoxylic acid is a bitch to purify.

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