[Strike]

Hey, I just have a quick question.

When acetone is used and reacted with an acid, it breaks the double bond and forms the alcohol group as well as attaching the nucleophile.

But when acetone reacts with labeled water it gives CH3C(=18O)CH3 with acid catalyst. Why?

Acetone + H2(18-O) --> CH3C(=O)CH3.

Why/Mechanism?

[Dan]

When acetone is used and reacted with an acid, it breaks the double bond and forms the alcohol group as well as attaching the nucleophile.

^This is why.

Labelled water is the nucleophile, and will add reversibly across the C=O double bond as usual. What can happen next?

[Strike]

So the original oxygen gets ripped off completely and is replaced by 18O?

I'm having some trouble understanding this. I have this in my head so far:

The double bond in C=O is broken, leaving C-O. Normally a nucleophile comes in and we end up with Nu-C-OH. But instead of that, it's completely replaced by the 18O and double bond is formed.

From your post, I'm getting:

C=O is broken, we get C-O. Labeled water attacks and we get 18O-C-O (What happens now? Oxygen leaves and forms a double bond with the 18O?)

[sjb]

So the original oxygen gets ripped off completely and is replaced by 18O?

I'm having some trouble understanding this. I have this in my head so far:

The double bond in C=O is broken, leaving C-O. Normally a nucleophile comes in and we end up with Nu-C-OH. But instead of that, it's completely replaced by the 18O and double bond is formed.

From your post, I'm getting:

C=O is broken, we get C-O. Labeled water attacks and we get 18O-C-O (What happens now? Oxygen leaves and forms a double bond with the 18O?)

The C=O is converted to C-O by addition of the nucleophile, ¹⁸OH₂, which can then undergo proton transfer between the oxygens to give..?

[Strike]

The C=O is converted to C-O by addition of the nucleophile, ¹⁸OH₂, which can then undergo proton transfer between the oxygens to give..?

Er.. I don't think we went over proton transfers. No idea that protons could transfer.. wouldn't that form a whole new element? Or did you mean neutron transfer? (Never went through neutron transfer either)

[sjb]

The C=O is converted to C-O by addition of the nucleophile, ¹⁸OH₂, which can then undergo proton transfer between the oxygens to give..?

Er.. I don't think we went over proton transfers. No idea that protons could transfer.. wouldn't that form a whole new element? Or did you mean neutron transfer? (Never went through neutron transfer either)

Sorry, perhaps slightly loose terminology here. Here's what I meant. Can you complete this mechanism?

[Strike]

Yeah I found the full mechanm in my textbook. It only talks about using labeled water to test if the hydrate reaction actually occured.

What I can't explain is why the original oxygen is replaced by the isotope 18O. And why it happens only for labeled water and not normal water.

[sjb]

Yeah I found the full mechanm in my textbook. It only talks about using labeled water to test if the hydrate reaction actually occured.

What I can't explain is why the original oxygen is replaced by the isotope 18O. And why it happens only for labeled water and not normal water.

For point 1, what is the concentration of labelled water compared to the concentration of acetone going to be like?
For point 2, what would be the product if you used normal water?

[Strike]

Oh I'm assuming enough and equal part concentration for the reaction to complete.

If it where just normal water, it would form two alcohol groups on the carbon instead of having 18O kick off the original oxygen and forming the double bond.