6
$\begingroup$

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?

$\endgroup$
6
  • 2
    $\begingroup$ 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
    Commented Feb 22, 2015 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
    Commented Feb 22, 2015 at 2:21
  • $\begingroup$ @ron what's the "Good Book"? $\endgroup$
    – Dissenter
    Commented Feb 22, 2015 at 2:34
  • 1
    $\begingroup$ What book is this? $\endgroup$
    – jerepierre
    Commented Feb 22, 2015 at 18:58
  • $\begingroup$ @jerepierre - Organic chemistry by Brown. $\endgroup$
    – Dissenter
    Commented Feb 22, 2015 at 18:59

3 Answers 3

8
$\begingroup$

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.

$\endgroup$
3
$\begingroup$

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.

$\endgroup$
2
  • 1
    $\begingroup$ 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
    Commented Feb 22, 2015 at 3:10
  • 1
    $\begingroup$ 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
    Commented Feb 22, 2015 at 4:23
0
$\begingroup$

That Cr[6] compounds oxidize benzylic hydrogens does not mean that they do not oxidize ketones. The presence of a H seems sufficient but not necessary. It depends on the time and conditions. I did my Senior research on chromic acid oxidation of methyl-2-propylphenylcarbinols and the alkenes and alkanes of the same carbon arrangement. The conditions were glacial acetic acid, 25 degrees Celsius, a relatively mild condition. The products were good yields of acetophenone and iso-butyrophenone. [This is from memory I do not have the thesis any more.] Compounds with no benzylic H especially the carbinol were rapidly oxidized to the ketones. Our conclusion was that oxidation of a side chain yielded ketones with carbinols possible intermediates, carbon-carbon bonds were easily broken, and that ketones were more difficult to oxidize.

There are many references about acidic dichromate oxidizing side chains to acids. It is the reagent of choice for chemical oxidation demand [COD] measurements because it seems to be a more thorough oxidizer than permanganate. However, that could be also because it agrees more closely with biological oxygen demand [BOD]. The general opinion is that quaternary benzyl carbons are not oxidized altho a quick search gave no specific data. Tertiary alcohols are oxidized and ketones seem to be less reactive but still oxidizable. This seems to be a subject that could use some lab research; it is a subject for many specific catalysis studies.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.