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According to the solutions manual, the oxidation of acetophenone to benzoic acid can be achieved through addition of chromic acid and application of heat.

The solutions manual refers me to a section in the book which explains that when a benzylic hydrogen is present, a strong oxidizing agent combined with heat can perform oxidative cleavage.

Problem is that I don't see any benzylic hydrogens on acetophenone. Neither does its enol tautomer have any benzylic hydrogens ...

So, how is the oxidation of acetphenone to benzoic acid possible given what the book and solutions manual has told me? Self-consistency aside is this even a possible reaction?

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  • $\begingroup$ The mechanism of benzylic oxidation is not well understood. For acetophenone, consider what would happen if the enol tautomer were to undergo oxidative cleavage of its $\ce{C=C}$ bond, say using $\ce{KMnO4}$? $\endgroup$ – Ben Norris Feb 22 '15 at 2:12
  • $\begingroup$ @BenNorris is that possible? It wasn't introduced in the text so I'm confused about a) why the text would introduce a reaction it doesn't even cover itself, unless I'm missing something and b) what exactly do we know strong oxidizers such as KMno4 and H2CrO4 to do? $\endgroup$ – Dissenter Feb 22 '15 at 2:21
  • $\begingroup$ @ron what's the "Good Book"? $\endgroup$ – Dissenter Feb 22 '15 at 2:34
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    $\begingroup$ What book is this? $\endgroup$ – jerepierre Feb 22 '15 at 18:58
  • $\begingroup$ @jerepierre - Organic chemistry by Brown. $\endgroup$ – Dissenter Feb 22 '15 at 18:59
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From a SciFinder search, there are ~10 examples of oxidations of aryl methyl ketones resulting in a carboxylic acid using a chromium based reagent (usually dichromate but also chromium trioxide). This suggests the reaction is at least possible, although I think including it in a textbook is unwarranted.

One reaction slipped through my search that is informative (below). Notice that in the presence of a benzylic position that does contain a hydrogen, the methyl ketone is unchanged, while the other group is oxidized.

enter image description here

Taken together, it seems that chromium oxidation of an acetophenone to a carboxylic acid is slower than alkyl benzylic oxidation.

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I'd like to clear up one issue that has not been addressed in comments, but provides a weak answer.

There are NO benzylic hydrogens in acetophenone. So it is not possible based on what the textbook tells you.

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    $\begingroup$ you've nailed one of the points I was trying to make ... why does my text fail to be self-consistent? Well, I guess I already know the answer ... $\endgroup$ – Dissenter Feb 22 '15 at 3:10
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    $\begingroup$ This may be Bayer-Villager variant, but I've been away from teaching for awhile, and never used this reaction in my lab work, so it is not familiar to me. My intuition tells me chromic acid is not strong enough, but I'd never bet my intuition against a good literature search. $\endgroup$ – Lighthart Feb 22 '15 at 4:23

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