Aldehyde Reactivity in Aqueous Base: Mechanism and Species Identification

[Sourabh N]

Homework Statement

In aqueous base, 97% of 2,2-diphenylethanal is present as a species different from the aldehyde, enol or aldol addition or condensation products. What is this species? Please also try to explain the mechanism.

Homework Equations

The Attempt at a Solution

I'm confused in the first step. Would the hydroxyl ion attack carbonyl carbon or the diphenyl carbon; or it will attack on the enol form. please help.

 

[Sourabh N]

Please someone try the question. I have struggled with it for a long time.

 

[Sourabh N]

Please someone try the question. I have struggled with it for a long time.

 

[chemisttree]

The hydroxide is negatively charged and there are no alpha protons to deprotonate. Try adding the hydroxide anion to the carbonyl carbon since it is the most electropositive species. What do you generate? Is it a stronger base than hydroxide?

Remember that it is in water... lots of protons available. Also, the base is a catalyst and reactant in this reaction, meaning that at the end of the process there is still the original amount of hydroxide concentration around.

 

[Sourabh N]

The hydroxide is negatively charged and there are no alpha protons to deprotonate.

The structure is (C6H5)2CHCH=O. There is a Hydrogen(attached to 3 deg carbon havin phenyl substituents) that can be captured by hydroxide.

Actually this is my confusion : which H will be removed : carbonyl C's or 3 deg C's?

neway thanks for reply.

 

[chaoseverlasting]

If you attack the carbonyl carbon, you get a negative charge on the oxygen, and since there are protons in the medium, you end up getting two hydroxyl groups on the same carbon which again gives you your original product.

I don't think that's what they're asking you to do. If, however, you do take the 3 deg c, then the negative charge you do get is delocalised through resonance among the two phenyl groups and the double bond of the aldehyde oxygen. I think here that's what will happen cause it seems like the resonance is sufficient to stabalise the negative charge enough so that the species exists in that state.

 

[Sourabh N]

ur reasoning is correct; does nething happen after that?

 

[chemisttree]

The hydroxide is negatively charged and there are no alpha protons to deprotonate.

Sorry about that. I should have been more careful about the structure.

You will notice that the alpha proton is available for deprotonation and can easily form the enol. From the enol, you might consider the aldol addition product but the question specifically states that this is not reflected in the product distribution. The enol reaction is reversible and even though it can form another reaction occurs that removes the diphenylethanal from the equilibrium.

What would you get if you added hydroxide to the carbonyl carbon?

 

[chemisttree]

If you attack the carbonyl carbon, you get a negative charge on the oxygen, and since there are protons in the medium, you end up getting two hydroxyl groups on the same carbon which again gives you your original product.

This is not the original product (an aldehyde) but is a geminal diol...

 

[chaoseverlasting]

This is not the original product (an aldehyde) but is a geminal diol...

But two hydroxide groups can't exist on the same carbon atom. They release a water molecule don't they? Unless here some kind of carbocation is formed which causes the molecule to undergo rearrangement. Unless the positive charge is on the carbonyl carbon, it won't even undergo rearrangement, as the other carbon has two phenyl groups on it.

 

[Sourabh N]

What would you get if you added hydroxide to the carbonyl carbon?

Conjugate base of a geminal diol!

 

[chemisttree]

Conjugate base of a geminal diol!

What would that do in an aqueous environment?