acidity of carboxylic acid derivatives

[twenty7]

I was reading the answer #606 for EK's 1001 Questions in O-Chem which states that acid chlorides have more acidic hydrogens than ketones, aldehydes or esters.

From what I understand, acid chlorides would have hydrogens that are LESS acidic than the other carboxylic acid derivatives as the chlorine is electron withdrawing, thus further destabilizing the alpha carbon for example on the acid chloride.

So why do acid chlorides have more acidic hydrogens?

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[wish2bMD]

I was reading the answer #606 for EK's 1001 Questions in O-Chem which states that acid chlorides have more acidic hydrogens than ketones, aldehydes or esters.

From what I understand, acid chlorides would have hydrogens that are LESS acidic than the other carboxylic acid derivatives as the chlorine is electron withdrawing, thus further destabilizing the alpha carbon for example on the acid chloride.

So why do acid chlorides have more acidic hydrogens?

Acid Chlorides have more acidic hydrogens because of the reason you just stated. Chlorine is electron withdrawing (think about it as pulling the electrons toward itself) and so losing a proton is easier for an Acid Chloride than for aldehydes, ketones, or esters that do not have the extra electron withdrawing component. It is true that the alpha carbon is more destabilized (more positive), which further pulls the electrons from the beta carbon towards itself facilitating the loss of a hydrogen (proton).
Remember that when the Acid Chloride loses a proton it keeps the electrons that it had in the bond to the hydrogen.

 

[Erythropoietin]

I was reading the answer #606 for EK's 1001 Questions in O-Chem which states that acid chlorides have more acidic hydrogens than ketones, aldehydes or esters.

From what I understand, acid chlorides would have hydrogens that are LESS acidic than the other carboxylic acid derivatives as the chlorine is electron withdrawing, thus further destabilizing the alpha carbon for example on the acid chloride.

So why do acid chlorides have more acidic hydrogens?

You explained it correctly, except that the withdrawing by Cl would make the hydrogen MORE acidic, not less.

 

[twenty7]

Thanks for the explanation! So I guess the further destabilizing the alpha carbon makes it a stronger acid if we define the acid as a Lewis acid. A Lewis acid means that an acid is an electron acceptor; therefore, the more positive the charge is on the alpha carbon, the more willing it is to accept electrons as the molecule is so electron-poor.

 

[MedPR]

Thanks for the explanation! So I guess the further destabilizing the alpha carbon makes it a stronger acid if we define the acid as a Lewis acid. A Lewis acid means that an acid is an electron acceptor; therefore, the more positive the charge is on the alpha carbon, the more willing it is to accept electrons as the molecule is so electron-poor.

Same is true for a Bronsted acid (proton donor). The electron withdrawal by the Cl will make it easier to lose (donate) the acidic hydrogens.

 

[Hexon]

You explained it correctly, except that the withdrawing by Cl would make the hydrogen MORE acidic, not less.

EPO,
nice points, just need a clarification;
doesn't a molecule's acidity relate to how 'stable' its resonance structure is? wouldn't that mean the chloride's electorn withdrawing ability cause relatively less resonance and there cause the entire molecule to be less acidic than the other derivatives?

i always thought of acidity as being 'more resonance stabilised = more acidic'

 

[Ssina]

I think it's resonance and electron withdrawing ability together... so just as resonance is able to spread out the extra electron density as the hydrogen leaves, any electron withdrawing groups also pulls some of the density toward itself!
just as you mentioned: more resonance more acidic.... and with more EWG around more acidic also... now put them together and it becomes even better

 

[twenty7]

EPO,
nice points, just need a clarification;
doesn't a molecule's acidity relate to how 'stable' its resonance structure is? wouldn't that mean the chloride's electorn withdrawing ability cause relatively less resonance and there cause the entire molecule to be less acidic than the other derivatives?

i always thought of acidity as being 'more resonance stabilised = more acidic'

I was sort of confused with this as well. I may be digressing here, but take the concept of carbocation rearrangement. From what I understand, the cation wants to be placed on a more substituted carbon because the surrounding electron donating alkyl groups are able to stabilize the positive charge. Therefore, a cation that is surrounded by electron DONATING groups is typically favoured because of stability.

 

[MedPR]

You explained it correctly, except that the withdrawing by Cl would make the hydrogen MORE acidic, not less.

Secondary effects depend on the effect of the molecule on the atom bonded to the acidic proton. The more electron-poor that atom, the more acidic the proton. The more electron-rich that atom, the less acidic the proton." - Page 31 in TBR OCHEM 1.

Isn't Chlorine electron rich? So, according to the above, it would decrease the acidity...

 

[Ssina]

Secondary effects depend on the effect of the molecule on the atom bonded to the acidic proton. The more electron-poor that atom, the more acidic the proton. The more electron-rich that atom, the less acidic the proton." - Page 31 in TBR OCHEM 1.

Isn't Chlorine electron rich? So, according to the above, it would decrease the acidity...

Erythropoietin is not talking about the Cl bonded directly to the leaving H+... so as you correctly stated, if the the atom bonded to the H is electron poor, it will be more acidic because it is holding on to the hydrogen less tightly and after the H+ leaves, it will have less electron density to deal with as well..... vs. if it was an electron-rich atom which after H+ leaves now has more electron density on it.

Now going back to the original OP, the Cl is not directly bounded to the hydrogen(in acid chloride) so it is acting as an EWG.... which will pull away the electron density from the atom directly connected to the H.... which will make that atom electron poor (and as you mentioned this will make it more acidic).

Looking at their structures, the acidic hydrogen is connected to the R group, and the Cl is more EWG, making acid chloride more acidic and willing to lose the H+ vs. a let say a Keton

a better illustration that makes this point, I think, is comparing the acidity of a propanoic acid vs. chloropropanoic acid:

*sorry couldn't find better pictures! lol

 

[MedPR]

Erythropoietin is not talking about the Cl bonded directly to the leaving H+... so as you correctly stated, if the the atom bonded to the H is electron poor, it will be more acidic because it is holding on to the hydrogen less tightly and after the H+ leaves, it will have less electron density to deal with as well..... vs. if it was an electron-rich atom which after H+ leaves now has more electron density on it.

Now going back to the original OP, the Cl is not directly bounded to the hydrogen(in acid chloride) so it is acting as an EWG.... which will pull away the electron density from the atom directly connected to the H.... which will make that atom electron poor (and as you mentioned this will make it more acidic).

Looking at their structures, the acidic hydrogen is connected to the R group, and the Cl is more EWG, making acid chloride more acidic and willing to lose the H+ vs. a let say a Keton


*sorry couldn't find better pictures! lol

Neither was I...

"...effect of the molecule on the atom bonded to the acidic proton." The molecule (Cl) on the atom (C) to the acidic proton. I didn't state that the atom bonded to the H is electron poor, I stated that the atom on the atom bonded to the H is electron poor.

As far as I know, Cl- is not electron poor, since it has 7 valence and is electronegative. I understand that an the Cl in an acid chloride is an electron withdrawing group, thus it increases the acidity of the hydrogen. The only thing I can think of is that an "electron withdrawing group" and an "electron rich molecule" are not the same thing.

TBR says electron withdrawing group = increase acidity
TBR also says electron rich = decrease acidity

 

[Ssina]

aaaa myy bad.. I think maybe they are just referring to electron donating groups as electron rich and electron withdrawing groups as electron poor?! which in that case Cl would be an EWG/electron-poor because of it's electronegativity vs. a carbon

 

[MedPR]

aaaa myy bad.. I think maybe they are just referring to electron donating groups as electron rich and electron withdrawing groups as electron poor?! which in that case Cl would be an EWG/electron-poor because of it's electronegativity vs. a carbon

Yea, the only way both statements could be true is if electron-rich = electron donating.

I just can't imagine how you would call something like Cl- electron-poor since it has 7 valence.. Does electron-rich mean full octet + no lone pairs? Like a Methyl group? Because then it makes sense that electron-rich = electron donor...


Edit: Nevermind, I just read about this in TBR. The reason why electron-poor groups increase acidity is because they are more likely to accept a pair of electrons, which is the definition of a Lewis Acid. Electron-rich and electron-poor have nothing to do with acidity of a hydrogen, but the basicity/acidity of the overall compound.