Ranking Acidic Hydrogens

[clarinethero214]

Hi there. I'm having SO much trouble trying to figure out the reasoning to this answer.

I'm given this molecule (with some hydrogens I labeled A, B, C, D, and E):

 

I need to rank the hydrogens in order of increasing acidity. I know the answer is DACEB, but I'm a little confused as to why. My textbook tells me that the most important contributing factor is what atom the hydrogen is bonded to (with the most acidic lying to the right of a period and down a group). This would, undoubtedly, make hydrogen E the most acidic since it's bonded to oxygen while the others are bonded to carbon, but this isn't the case. It must be due to the resonance stabilization that the double bonded oxygens supply to hydrogen B, but this seems to go straight against my textbook in which order of significance I look at things at! I understand that B and C are affected by resonance stabilization, but my main concern is why B is the MOST acidic if I'm supposed to look at what atom the hydrogen is bonded to first and foremost (even this website says so: http://masterorganic...luence-acidity/

 

Please, any help will be MUCH appreciated.

Edited June 2, 2012 by clarinethero214

[khatereh]

The withdrawing groups can affect on acidity as well. Carbonyl group is almost withdrawing; therefore the CH which is placed between two carbonyl groups (proton B) is enough acidic. Also when proton B is removed the remaining carboanion can be stabilized by two carbonyl groups through resonance. the acidic nature of this proton is famous and it has been used in "base catalyzed Aldol reaction".

 

The proton E is related to the alcoholic group. Alcohols are not considered acidic. but it is more acidic than proton A,C and D.because the proton is attached to the electronegative atom (oxygen). after removing the proton the oxygen anion is kinda stable. but not as stable as the carboanione which can have resonance with two carbonyl.

[hypervalent_iodine]

The withdrawing groups can affect on acidity as well. Carbonyl group is almost withdrawing; therefore the CH which is placed between two carbonyl groups (proton B) is enough acidic. Also when proton B is removed the remaining carboanion can be stabilized by two carbonyl groups through resonance. the acidic nature of this proton is famous and it has been used in "base catalyzed Aldol reaction".

 

The proton E is related to the alcoholic group. Alcohols are not considered acidic. but it is more acidic than proton A,C and D.because the proton is attached to the electronegative atom (oxygen). after removing the proton the oxygen anion is kinda stable. but not as stable as the carboanione which can have resonance with two carbonyl.

 

 

It's less to do with how electron withdrawing the carbonyls are and more to do with resonance stabilisation of the resulting anion. The more stable the product (in this case the anion), the lower the energy barrier to its formation and therefore the more labile / acidic the proton.

 

And FYI, alcohols can act as acids.

 

 

I need to rank the hydrogens in order of increasing acidity. I know the answer is DACEB, but I'm a little confused as to why. My textbook tells me that the most important contributing factor is what atom the hydrogen is bonded to (with the most acidic lying to the right of a period and down a group). This would, undoubtedly, make hydrogen E the most acidic since it's bonded to oxygen while the others are bonded to carbon, but this isn't the case. It must be due to the resonance stabilization that the double bonded oxygens supply to hydrogen B, but this seems to go straight against my textbook in which order of significance I look at things at! I understand that B and C are affected by resonance stabilization, but my main concern is why B is the MOST acidic if I'm supposed to look at what atom the hydrogen is bonded to first and foremost (even this website says so: http://masterorganic...luence-acidity/

 

Please, any help will be MUCH appreciated.

 

With B, you have multiple resonance contributors (compare it to the resonance structures for C), which makes the anion of B more stable than the anion of C and therefore, the proton on B more acidic. The 'rules' you linked aren't so much rules as they are guidelines, which is something of a common theme in most all the 'rules' you'll encounter.

[chemlover_098]

Check out how Orgo Made Simple explains it :http://orgomadesimple.com/organic-chemistry-acids-and-bases/. I found it really helpful, and it's easy to understand.