DAT 2009 destroyer organic question number 125?

[dallaspage]

I checked answer I understand everything except last part.
Can you explain this part said that " By removal of the double alpha proton, we yield a resonace stabilized Enolate anion as shown"?

And I cannot understand this part said that " Removal of the alpha hydrogen from either C-1 or C-5 will yield an enolate,but with only two resonace forms,not three."

Why does remove the alpha hydrogen from either C-1 or C-5?

Please explain to me..

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[loveoforganic]

I don't have that book.. but I'll do what I can. What contributes to the acidity of alpha protons? The fact that they're adjacent to a carbonyl and the conjugate base can resonate with the carbonyl. If the proton is alpha to two carbonyls instead of one, you have an extra carbonyl the conjugate base can resonate with. Increased delocalization of the anion, increased stability of the conjugate base, increased acidity of that proton.

 

[UCB05]

Assuming the structure you're thinking of is a 3,5 di-keto pentane:

Removal of the alpha C-1 or C-5 gives you two resonance structures: the keto form with the negative charge on the carbon, and the enol form with the negative charge on the oxygen. Removal of the C-3 double alpha hydrogen has three possible resonance structures: one with the negative charge on the carbon, and two with the negative charge on the C-2 or C-4 oxygens on either side of the alpha carbon, for a total of 3 resonance structures. That makes that hydrogen more acidic than either of the two terminal alpha-C hydrogens.