Ochem question alpha hydrogen acidity

[dannybht]

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I have a question on the BR ochem chapter #13 problem for passage 2 of chapter 6 practice problems.

I know why the most acidic hydrogen is on the reactant of step 4 but why is the alpha hydrogen on the sp2 carbon for the reactant of step 4 the least acid when there is an alpha sp3 hydrogen on the reactant for step 1?

 

[dannybht]

I mean chapter #5* sorry

 

[monkeyvokes]

Could you post the problem?

 

[dannybht]

 

[monkeyvokes]

View attachment 24718View attachment 24719

reactant for step for resembles a betaketo acid and has an extra cidic alpha hydrogen between the two carbonyls. If you used the Smith textbook in college you may remember these such as acetoacetic acid.

 

[dannybht]

Yeah I know why that one has the most acidic alpha hydrogen but I don't know why the 2nd reactant has the least acidic alpha hydrogen.

 

[MedSprinter]

Yeah I know why that one has the most acidic alpha hydrogen but I don't know why the 2nd reactant has the least acidic alpha hydrogen.

The second reactant is stabilized by conjugation, breaking that by giving up a proton would make it less stable and unfavorable. The difference in energy levels between reactant 1 and its deprotonated form is less than the difference in energy levels between reactant 2 and its deprotonated form. I hope that helps.

 

[dannybht]

If the proton is removed, then the negative charge on the alpha carbon can't be moved around. Is that why its so unstable? I thought sp2 hydrogens would be more easier to remove via heterolytic cleavage since there is more s character.