Quantcast

Acidic H Question, HELP.

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

m617911622

Full Member
5+ Year Member
Joined
Aug 2, 2015
Messages
38
Reaction score
7

Members don't see this ad.
How do you determine the most acidic hydrogen molecule? I am so confused with this topic.

There is the CAIRO rule, that the atom will higher S percent, and it will result more acidic.
There is the electronegativity rule where H hydrogen connects to higher electronegativity molecule, and it will result higher acidic H.
And I saw this one from one of the DESTORYER problem, it said the easier H to remove, the more acidic it is.

But it kinda contradicts each other.

Anyone can help me with this, thanks.

This is a problem from bootcamp, I thought the answer can be B, D, and E ?

Screen Shot 2017-08-05 at 7.18.10 PM.png
 

princecoup

Full Member
5+ Year Member
Joined
Jul 18, 2015
Messages
39
Reaction score
53
for these types of questions, i usually go with the electronegativity rule because in the above question, you can automatically eliminate B, C, and D since those hydrogen's are attached to a carbon.

now you're left with A and E because those hydrogen's are attached to the more electronegative oxygen, but E is the more acidic hydrogen because it is attached to an aromatic benzene ring, which has more delocalized electrons and thus allows for more resonance --> makes the compound more stable. thus, if you were to deprotonate the hydrogen at E, you would get a more stable (lower energy) conjugate base and a weak conjugate base is a strong acid!

hope that helps
 
  • Like
Reactions: 2 users

DAT Destroyer

Dr. Romano and Nancy
Lifetime Donor
Sponsor
Vendor
15+ Year Member
Joined
Mar 12, 2005
Messages
4,650
Reaction score
3,414
How do you determine the most acidic hydrogen molecule? I am so confused with this topic.

There is the CAIRO rule, that the atom will higher S percent, and it will result more acidic.
There is the electronegativity rule where H hydrogen connects to higher electronegativity molecule, and it will result higher acidic H.
And I saw this one from one of the DESTORYER problem, it said the easier H to remove, the more acidic it is.

But it kinda contradicts each other.

Anyone can help me with this, thanks.

This is a problem from bootcamp, I thought the answer can be B, D, and E ?

View attachment 222210
The best way to approach acidity is by resonance. Almost always a resonance stabilized intermediate will show that hydrogen most easy to come off. For example, in butanal, the most acidic hydrogen is the alpha proton on C2 because it yields a resonance stabilized enolate anion. I think it is best for an exam like the DAT, that you know a few basic pKa values. The smaller the pKa value, the more acidic the proton. For example, carboxy acids are at 5, double alpha protons are at 9...phenols and thiols are at 10. alcohols 16......aldehydes 17, ketones 18,...alkynes 25...alkenes 45, and alkanes in the 50s.

I hope this helps.

Dr. Romano
 
  • Like
Reactions: 1 users

Dr.CinnamonRoll

Full Member
5+ Year Member
Joined
May 2, 2017
Messages
115
Reaction score
80
What helped me to not get confused on acidity is is to relate acidity with being more likely to drop off a proton. An electronegative element is a "selfish" molecule that wants to keep electrons, and O is more electronegative than C, so O is more "selfish" and willing to steal hydrogen's electrons, which would mean H gets dropped off.
 
  • Like
Reactions: 1 user

m617911622

Full Member
5+ Year Member
Joined
Aug 2, 2015
Messages
38
Reaction score
7
Thank you, all.

So, I will look the resonance stabilized intermediate ( the atom that is one away from the double bond) and the electronegativity of the atom to determine the most acidic H.
 

DAT Destroyer

Dr. Romano and Nancy
Lifetime Donor
Sponsor
Vendor
15+ Year Member
Joined
Mar 12, 2005
Messages
4,650
Reaction score
3,414
What helped me to not get confused on acidity is is to relate acidity with being more likely to drop off a proton. An electronegative element is a "selfish" molecule that wants to keep electrons, and O is more electronegative than C, so O is more "selfish" and willing to steal hydrogen's electrons, which would mean H gets dropped off.
Yes,,,,,you have the idea. A hydrogen bonded to an O would generally come off easier than one bonded to either an sp, sp2, or sp3 carbon. Keep up the great work.
 
Top