Oxidative cleavage of methyl ketones

This two-stage degradation of methyl ketones to carboxylic acids containing one fewer carbon atoms cannot be realized in all cases, but only when the acetyl group is attached to a quaternary carbon as in pinacone and a series of aryl methyl ketones of the acetophenone type. 

In the case of 2,4,6-trichloroacetophenone Fuson, Bertetti, and Ross133 succeeded in isolating the intermediate formulated in the above sense, namely, a, a, a-tribromo- 2,4,6-trichloroacetophenone  

6-Trichloroacetophenone (5g) was prepared in 50% yield from 2,4,6-trichlorobenzoyl chloride and methylmagnesium bromide, and a hypobromite solution by dissolving bromine (50 g) in 10% sodium hydroxide solution (500 ml) at 0°. The ketone (5 g) was stirred with this hypobromite solution for 30 h at about 50° and then for 50 h at 70°, whereupon the tri-bromo ketone (8 g, 80%), m.p. 126-127°, separated. On prolonged boiling with 50% sodium hydroxide solution this product was hydrolysed almost quantitatively to 2,4,6-trichloro-benzoic acid. 

For the haloform reaction to be smooth the group R of the ketone RCOCH3 should not contain hydrogen that is easily replaceable by halogen. However, under the reaction conditions used, methylene groups in R are not so easily substituted, as is shown by the smooth formation of 3-methyl-3-phenylbutyric acid in 84% yield from 4-methyl-4-phenyl-2-pentanone 134 

Levine and Stephens135 showed that ethyl and propyl ketones RCOC2H5 and RCOC3H7 in which R is an aryl or heterocyclic group undergo this reaction smoothly in the same sense as methyl ketones do e.g., benzoic acid is obtained in 64% yield from propiophenone  

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