Most acidic proton: first semester organic chemistry

[cookiemnstr510510]

Hello All,
I have attached my questions with pictures in attached photos. Look at screen shot 46 first then screen shot 47.
I am having trouble ranking acidic protons.
In my Organic book it has the order of:
1)Charge
2)Atom Type
3)Hybridization
4)Resonance
5)Induction

Things I know:
S is able to accommodate the negative charge after deprotonation better than O due to size.
I also see that if O is deprotonated that the negative charge is able to go through resonance which would stabilize the conjugate base better.
If the list shows Atom type being more important than resonance, why is the H on the O the most acidic proton?

The only thing I can think of is that O and S are not too different in terms of being able to hold charge (We already know that S can accommodate the negative charge better), but since O has another property from the list that S doesn't then O wins because the effects are additive?
Very confused.

I know if you look at the PKa's that OH has a lower PKa than SH. I want to understand this on a more fundamental level rather than just PKa.

Thanks,
WIll

 

[TeethWhitener]

Yes, this is a bit confusing. I admit, I'd never before seen an explicit list like the one from your book. It's best probably just to remember that carboxylic acids are more acidic (by a lot) than sulfides.

However, if you want to try to shoehorn this into the list you gave, you can take another look at the resonance structures of the carboxylate. Notice that the two oxygens are indistinguishable by resonance, so you can say (roughly) that each oxygen really only feels a charge of 1/2. So "charge" will win out over "atom type" in your list. I'm not sure I can do any better than that if you want to stick to the list in your orgo book.

 

[cookiemnstr510510]

Thank you! I appreciate your response, it was the best explanation I have received so far.

 

[epenguin]

Two preliminary observations. A first semester question like this should go into the homework section not here. Secondly when you say you want to understand at a more fundamental level than pK's, I'd say that pK's are not an understanding at all, they are just the thermodynamic measure of the readiness of molecules to donate a proton to water in aqueous solution. They are what needs to be explained, not an explanation. However I get that you are asking for a more fundamental explanation than what you have.

You can of course just remember. Just the name carboxylic acid gives away that carboxyl groups are acid (these compounds used to be called more often just 'organic acids ').The acid dissociation of RSH comes up in biochemistry w where reaction of ( nucleophilic) RS^- with molecules is part of enzymatic mechanisms, or better still there is a thematic in old-style school qualitative chemical analysis, in which the solubility of certain sulphides depends on the concentration of HS^- which is less when you make the solution acid and convert it into H₂S whereupon some sulphides redissolve. But if that is not familiar we can come back to it later. Anyway none of this tells you what the pK's are, just that RSH does have some effective acidity. Anyway I just remember that most sulphydyls have pK_a's about neutral or slightly alkaline (7-8 or so).

I doubt anyone can show you a calculation just from the structure of what the pK of either of these substancesis : if it can be done it is no doubt heavily computational.But what you do in order to understand systematically general chemistry is, as you will already realize if you think about it, understanding via systematic comparisons. For example you may have met and you will need to know comparisons of the acidity of hydrogen halides.That is, vary just one thing, the general nature of the anion is the same for all.

You've got in your examples just two substances, but two big differences between each so you cannot yet begin to make a systematic comparison.

To start if we compare the pK_a of Ethanoic acid (≈4.8) and ethanol (15.9) that's a very big difference.Seems to require an extraordinary explanation, and maybe the resonance explanation could be considered that! ( One could have wished for a better comparison, I would have preferred hydroxyethanal CH₂OHCHO but I don't know its pK_a or whether anyone does).

A good comparison could be between O compounds in the corresponding S compounds. I will give you some pK_a's, maybe you would like to see if you could come to some conclusions using them and to tell us here, in that case I will come back too.

H₂O. 15.7
H₂S. 8.7
C₂H₅SH. 10.6
Thioacetic acid CH₃C(=O)SH 3.4. (Note that this is in tautomeric equilibrium with CH₃C(=S)OH but the first is the predominant form.
For the dithio acids, -C(=S)SH, Wikipedia gives that they are in general about 0.5 below the corresponding thioacids.I can't find any actual pK_a's for compounds corresponding to the above ones, but I found the benzoyl ones:
Benzoic acid C₆H₅C(O)OH 4.2
Thiobenzoic acid C₆H₅C(O)SH. 2.5
Dithiobenzoic acid C₆H₅C(S)SH. 1.92

Nine pK_a's plus any relevant anyone else can throw in. Care to try some rationalisation?