Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: qw098 on December 18, 2011, 11:00:26 AM
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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? :P I'll draw a picture below to clearly explain what I mean :P
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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...
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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!
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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.