[qw098]

Hey guys,

I had a pretty cool question on my Final Exam for Org III, and I was wondering if I answered the question correctly .

We had an alpha,beta unsaturated ketone and we reacted that compound with D2O under basic conditions. We were asked to draw all the hydrogens that would be turned into deuterium.

I said this: I said the gamma carbon from the carbonyl would have a deuterium hydrogen attached to it and that's it and also the alpha carbon to the right of the carbonyl. I said the alpha hydrogen to the left would NOT be removed and replaced by deuterium because that would create too much ring strain!

Was my "rationale" thinking correct? I'll draw a picture below to clearly explain what I mean

[fledarmus]

I'm not sure what ring strain has to do with it, but I agree with your selection of exchangeable protons.

You might want to look at the mechanism of keto-enol tautomerization...

[qw098]

Well if I take that hydrogen off that I said I can't take off because of ring strain... look at my corresponding enol drawn below:

I have an sp hybridized carbon in a six-membered ring!! Extreme ring strain!

[Dan]

I think that position will exchange, but not via the strained intermediate you show. I can think of two mechanisms: alpha protonation of an extended enolate, followed by tautomerisation, ie via a beta-gamma unsaturated ketone; and a Baylis-Hillman-like mechanism (Hydration followed by dehydration). I prefer the latter mechanism, the intermediates look lower in energy.