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The Reactivity of Aldehydes and Ketones

Hi,
I was wondering if anyone could help explain to me the difference in reactivity between an aldehyde and a ketone when heated under reflux with acidified potassium dichromate (K₂Cr₂O₇/H⁺).

I understand the basic principles of the reaction:

Where by the aldehyde is oxidised by the potassium dichromate which is reduced, to produce the carboxylic acid (propanoic acid) turning the orange solution to green.

And that the Ketone can not be further oxidised and therefore remains orange.

But I don't understand how the mechanism actually takes place?
What actually happens to the aldehyde to become oxidised? is it through the loss of the hydrogen from the carbonyl group (C=O)?.
And if so, where does it go?

I've looked at several contradicting sources and the best answer I can find is that the Ketone can not be further oxidised due to the 'lack of oxygen' and that the aldehyde can be due to the contrary?

But that isn't enough for me to understand the reaction.

So if anyone can help me, I'd greatly appreciate it

Reply 1

marta99

Aldehydes are attached to a carbon with a hydrogen group (-CHO).
Ketones are attached to a carbon that is attached to two carbons (-CCOC).

This absence of a hydrogen makes ketones very difficult to oxidise, only the use of strong oxidising catalysts as well as carbon-carbon breaking methods allows you to oxidise a ketone.
Where as, in aldehydes, the hydrogen attatched to the aldehyde group can be easily oxidised into an alcohol group, making a carboxylic acid (-COOH).

Because of the resistance to oxidation of ketones, you can use specific tests to see the difference between them.

E.g. use of Cr2O7-2 and Tollen's reagent where aldehydes will be oxidised, but ketones will not react.

Hope this helped :smile: