DAT Destroyer 2014 Ochem Question #9

[urbanthread]

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So I have a hard time understanding this problem conceptually..
The question is asking "Which compound has the most acidic proton?"
and the answer is the H coming off carbonyl oxygen of a carboxylic acid
could someone explain the answer?
I'm trying to apply what Chad talked about on the acidity of alpha-carbons.. is that related?

I took a picture of the problem but I can't upload it because the file is too big 🙁 any way around this? (I'm new to SDN..)

 

[SmileDesignersDDS]

Bump

 

[Potential22]

that is because oxygen is a highly electronegative atom and it really does not want a positive charge on it, hence the name electroNEGATIVE as it has a high affinity for electrons

 

[DogeDDS]

Chad's rules for acid (or base) strength ranking is CARDIO:
1. Charge: the more + charged species = more acidic
2. Atom: the more electronegative = more acidic
3. Resonance: the more resonance = more acidic
4. Dipole Induction: the more electron-withdrawing groups that are near the atom w/ proton= more acidic
5. Orbitals: the more sp character = more acidic
(flip these around for base strength)

The carboxylic acid is the most acidic because (1) the atom with the proton (oxygen) is electronegative, (2) it has very good resonance (electrons are shared between 2 oxygen molecules which are very electronegative= good quality of resonance!). Now, theres 2 carboxylic acid looking molecules but one has an extra proton on the C=O, giving the oxygen a + charge. No good! This molecule would much rather donate that proton so that the electronegative oxygen isn't positively charged and the overall molecule is neutral.

The acidity of alpha carbons is irrelevant because it's not as acidic as the proton on the carboxyl group. Alpha-carbon acidity is more with ketones/aldehydes.

Just a tip, you should try to memorize the rough pKa's of some of the common molecules (it's easy if you think about them in increments of 5)
Carboxylic acids ~5
Phenols ~10
Alcohols ~15
Ketones/aldehydes ~20
Esters ~25
And in proteins, carboxyl group ~ 2.5, amino group ~ 9.5