Most Acidic Hydrogen on OChem problem??

[ephesus32]

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Hi,

Can you explain why a is the most acidic hydrogen in this problem?? I thought it was b since its between a carboxy acid and alcohol. Thanks.

 

[Theillestill]

because the hydrogen in A is the hydrogen involved in Carboxylic acid, and it's conjugate base is resonance stabilized, making it very very easily deprotonated.

 

[Sublimation]

because the hydrogen in A is the hydrogen involved in Carboxylic acid, and it's conjugate base is resonance stabilized, making it very very easily deprotonated.

Very tru. Another way to think of this is due to the electronegative nature of the oxygen causing them to posses a stronger hold over the electrons, think of them as "hogging" the electrons that they share with the hydrogen. therefore deprotonation will follow with ease.

 

[doc3232]

This type of question is actually hard IMO, until you learn all the concepts of O-chem. You need to know how acidic each one is also.
You are thinking H-NMR...

 

[dominican drums]

The fact that the hydrogens in B are in between two functional group should alert you that this could not possilbly be the answer. Functional groups "function" ie they react easily, so those hydrogens will probably leave first.

Also, the carboxilic acid is a very reactive functioal group, and since its an acid, it will lose the hydrogen real easy.

 

[ephesus32]

Thanks guys, I guess it's just something I have to memorize. But if you had a molecule such as :

CH3(C=O)CH2(C=O)CH3

, the middle hydrogens would be the most acidic right??

 

[doc3232]

Thanks guys, I guess it's just something I have to memorize. But if you had a molecule such as :

CH3(C=O)CH2(C=O)CH3

, the middle hydrogens would be the most acidic right??

Yes, but only due to the two carbonyls. These pull electrons very easily.
Probably pKa of about 16ish which could get deprotonated by OH-.

 

[joonkimdds]

Very tru. Another way to think of this is due to the electronegative nature of the oxygen causing them to posses a stronger hold over the electrons, think of them as "hogging" the electrons that they share with the hydrogen. therefore deprotonation will follow with ease.

shouldn't E be the answer too if oxygen was the main factor?

I have been thinking that the alpha hydrogen is most acidic.

Now I am thinking that alpha hydrogen can be the hydrogen in alpha carbon or alpha oxygen?

 

[joonkimdds]

...

 

[WOAHHI]

I think this is because E is not the BEST answer. It is part of an OH group with the highly electronegative oxygen willing to give up the hydrogen easily, but the carboxylic acid OH is also resonance stabilized by the carbonyl group so this hydrogen (A) is given up even easier than hydrogen E.