Question

what happens when alpha hydroxy ketone reacts with Fehling's solution

Answer 1

Okay I will explain this here.

First of all we know that group i.e. aldehyde gives positive test with Fehling’s solution whereas ketones don't give positive test with it. But on the other side alpha hydroxy ketones give positive test with Fehling’s solution. Why? Because of ‘ Tautomerism’.

Alpha hydroxy ketone gets converted into enol by tautomerism and then eventually turns into aldehyde group because of tautomerism. And that aldehyde group gives positive test with Fehling’s solution.

Now how this whole process takes place?

First let me tell you what tautomerism is.

Tautomerism is a special type of functional isomerism in which migration of H atom from one polyvalent atom to another polyvalent atom takes place.

The two isomers are in dynamic equilibrium with each other.

Now have a look at this picture to understand the above two points.

Now conditions for tautomerism are:-

Compound should have electronegative atom( in the above case it's oxygen) and there should be multiple bonds like in the above example we have double bond.

Atleast one alpha hydrogen atom should be there.

Now coming to your question about alpha hydroxy ketone.

Alpha hydroxy ketone when treated with Fehlings solution undergoes tautomerism.

Let's have a look at this whole process.

Let's us take CH3COCH2OH as an example of alpha hydroxy ketone.

Now have a look at this picture to understand the whole process of tautomerism of this compound.

So this is the whole process of tautomerism in which alpha hydroxy ketone is converted into alpha hydroxy aldehyde. And then it eventually gives positive test with Fehling’s solution.

The mechanism I have provided is the basic mechanism. The actual mechanism of tautomerism in case if the compound is reacting with Fehling’s solution involves cu complex and in case if it's reacting with Tollen’s reagent then it involves Ag complex and mechanism for that is very complex. But eventually alpha hydroxy ketone gets converted into aldehyde and thus gives positive test for both the reagents.

I hope I have answered your question.