Organic Chem Q

etymon

Just wondering if someone can help me out - can't figure out the reasoning to this question.

Why does the Tollens' test react with aldehydes and not with ketones?

1. Ketones are more sterically hindered than aldehydes (i know this is not the answer)
2. Aldehydes can be oxidised to a carboxylic acid but not ketones
3. Ketones do not have an acidic proton while aldehydes do
4. Aldehydes are more sterically hindered than ketones (again know not the answer)

Now I marked 3 as correct which came up in similar questions and was correct. However, the answer booklet gives number 2 as the answer and further says that 3 is false! I know 2. is true but is it a better answer than 3. and is 3 actually false? So confused. Any input appreciated.

Thanks!

Diabhal_Glas

If you cant get an answer here, Yahoo Answers is amazing for this sort of thing!

Marvinthefish

With regards to point number 3, I don't know what this "acidic proton" is about. Seems like the questioner is trying to trick you into thinking that the CHO proton is acidic. Any H's BESIDE the carbonyl group will be more acidic than the CHO proton. So forget "acidic"; think in terms of reduction and oxidation, because this is what Tollens' test is all about.

I think the main thing you need to grasp is that a C-H bond is breaking when you do the Tollen's test with the aldehyde (i.e. C-HO bond), whereas you need to break a C-C bond (harder) when you do the test with a ketone (there's no H bonded to the carbonyl carbon.

You can oxidise ketones (vigorous acidic conditions with nitric or chromic acids) but they won't oxidise under the same conditions as aldehydes will.

Marvinthefish

I hope that made sense. I wrote it very late last night and I was tired. If you really want to oxidise a ketone you could try the Baeyer-Villiger oxidation. Won't give you a carboxylic acid though.

etymon

cheers clever cloggses!