Kaplan FL5 P/C Strongest Acid Question

[betterfuture]

Which of the following compounds is expected to be the most acidic

Two answers I was unsure about were CF3COOH and CH3OH2+.

My first thought was to chose CF3COOH since it has 3 electron withdrawing groups as well as the carboxylic acid group. Strong acids means most stable conjugate base. I drew both structures after they deprotonate and found that CF3COOH would seem to be the most stable since it has resonance and the (-) electron on the Oxygen can delocalize between the other Oxygen. But no, the answer is CH3OH2+.(Highlite for answer).

Kaplan explained the answer saying the neutral methanol formed is "eager" to deprotonate making it a strong acid. I thought we were comparing acidity of the acids that I gave above, not the acidity of their conjugate bases. Am I wrong here? I thought we classify molecules as strong acids if they form strong conjugate bases?

Correction: From my last sentence I meant to say strong acids form stable conjugate bases.

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[Ryomagoku]

Which of the following compounds is expected to be the most acidic

Two answers I was unsure about were CF3COOH and CH3OH2+.

My first thought was to chose CF3COOH since it has 3 electron withdrawing groups as well as the carboxylic acid group. Strong acids means most stable conjugate base. I drew both structures after they deprotonate and found that CF3COOH would seem to be the most stable since it has resonance and the (-) electron on the Oxygen can delocalize between the other Oxygen. But no, the answer is CH3OH2+.(Highlite for answer).

Kaplan explained the answer saying the neutral methanol formed is "eager" to deprotonate making it a strong acid. I thought we were comparing acidity of the acids that I gave above, not the acidity of their conjugate bases. Am I wrong here? I thought we classify molecules as strong acids if they form strong conjugate bases?

Yes you are wrong. Strong acid= weak conjugate base
Weak acid=strong conjugate base.

 

[betterfuture]

Sorry. There was some confusion. The last sentence from my last post, I meant to say strong acids form STABLE conjugate bases NOT strong. That was a typo. But could you elaborate on your answer. I understand strong acids form weak conjugate bases and vice versa but how does that apply to the molecules CF3COOH and CH3OH2+ as they are fairly weak acids. And we are not given pKa values so we have to judge on bases of other conditions such as electron withdrawing/donating groups, or resonance etc.

 

[NextStepTutor_3]

Hi there! If you aren't given pKa values, the only reliable way to compare two acids is to compare the stability of their conjugate bases (so you're on the right track). And first of all, although the common wisdom is "strong acid = weak conjugate base," "weak acid = (relatively) strong conjugate base," note that this can also be used to compare two strong or two weak acids. (In other words, even though neither CF3COOH nor CH3OH2+ is one of the strong acids you'd typically memorize, whichever of the two has a weakER / more stable conjugate base must be a strongER acid.)

Now, you're also right that two major factors affecting the stability of a typical conjugate base are resonance and the inductive effect (the presence of electron-withdrawing groups in close proximity). But why do these two factors matter so much, especially inductive effect? Because typically, the conjugate bases that we're discussing are anions. Think of a comparison between CF3COOH and CH3COOH - when we compare their conjugate bases, we're comparing CF3COO- and CH3COO-. The electron-withdrawing nature of fluorine "pulls" some of the electron density away from the anionic oxygen, and resonance allows that anionic nature to be "split" between the two carboxylic acid O atoms.

Back to the question at hand: now, what are the conjugate bases we're actually dealing with? The conjugate base of CF3COOH is CF3COO-, but the conjugate of the other acid is CH3OH (methanol, and NOT an anion). Now, sure, inductive effect and resonance do a pretty good job of delocalizing the negative charge on CF3COO-, but methanol doesn't even have a negative charge, and is significantly less reactive than CF3COO-. (Think of it this way - CF3COOH is still a carboxylic acid, making it weak. So its conjugate is going to want to pick up a proton, just as F- or CN- would. But when's the last time you saw methanol readily pick up a proton in neutral solution?)

In short, this question highlights the importance of thinking about stability on a case-by-case basis, rather than considering inductive effect and resonance alone (though those two factors are very important). Hope this helps!

 

[betterfuture]

Ohh I see. So basically you are saying that the conjugate bases of CF3COOH and CH3OH2+ form CF3COO- and CH3OH respectively. And so we have to decide from these two which one is the stronger acid. So when comparing CF3COO- and CH3OH, we are looking at the tendency of the conjugate bases to react or more specifically ACT as a base? Right? CH3OH doesn't ACT like a typical base because it will likely give off its proton and act more like an "acid" though its the CONJUGATE BASE of CH3COOH. Is that what you mean?

 

[NextStepTutor_3]

Basically, yes! (I think you meant to say that CH3OH is the conjugate base of CH3OH2+, not CH3COOH, but other than that you're exactly correct. On the other hand, if you're subconsciously confusing CH3OH2+ with CH3COOH, that would explain the confusion.) When comparing the strength of two acids, we need to look at their conjugate bases (here, CF3COO- and CH3OH). We can look at conjugate bases in two ways, both of which are really exactly the same:

- Which of these conjugate bases is more stable? The more stable conjugate base always corresponds to the stronger acid. Here, we can conclude that CH3OH is more stable than CF3COO-, as it's uncharged and a generally unreactive species. This means that the conjugate acid of CH3OH (which is CH3OH2+) is a stronger acid than the conjugate acid of CF3COO- (which is CF3COOH).

- Which of these conjugate bases tends to react more as a base? This is just another way of saying, "Which of these conjugate bases is less stable?" but somehow tends to be easier to think about. Here, CF3COO- certainly acts more as a base than CH3OH, as (like you said) methanol really acts more like a weak acid than like a base at all. Since CF3COO- tends to react more, it is less stable and corresponds to the weaker conjugate acid (CF3COOH). Since CH3OH tends to react less, it is more stable and corresponds to the stronger conjugate acid (CH3OH2+).

Let me know if anything is still confusing - this is a great topic to master!

 

[betterfuture]

You are awesome! This makes soo much sense now.

 

[flyingeagle]

Yes you are wrong. Strong acid= weak conjugate base
Weak acid=strong conjugate base.

This is not something you should rely on (from what I have learned in TBR, most people think like this despite it being wrong).

An acid could have a pKa of 6 which is a weak acid but this means its conjugate base has a pKb of 8, which is a weak base (pKa + pKb = 14). So, the rule you are using is a RELATIVE rule, not an absolute rule. That is, if you are comparing two acid/base pairs, the pair with a stronger acid will have a weaker conjugate base in comparison to the other conjugate base. E.g. if Acid HX has pKa of 3, Acid HY has pKa of 5, then base X- will have pKb of 11 and base Y- will have pKb of 9 (thus Y- is a stronger base than X-). Hope that helps!

 

[NextStepTutor_3]

Just wanted to add to @flyingeagle 's good point! Like I mentioned above, a safer but still easy way to think of this conventional wisdom is "strong acid = weak conjugate base," "weak(er) acid = (relatively) strong(er) conjugate base." The issue is always with the term "strong," which should only be used very carefully. If we thought that all weak bases had "strong" conjugate acids, we'd wrongly think that hundreds of common acids (HF, HCN, acetic acid, etc.) would completely dissociate in aqueous solution, which is not the case. Instead, we can say that since HCN is a weakER acid than HF, HF will have a weakER conjugate base. Neither conjugate base is necessarily "strong" as that term is typically defined.

On the other hand, it's technically accurate to say "strong acid = weak conjugate base," as all of the strong acids you need to know for the MCAT have very weak (to the point of being virtually neutral) conjugate bases. That follows from the definition of a strong acid - since HI dissociates completely in solution, I- ions do not tend to pick up protons. If you think of this entire concept as a balance between the forward and reverse reaction, it becomes much simpler.

 

[swimedkid]

Basically, yes! (I think you meant to say that CH3OH is the conjugate base of CH3OH2+, not CH3COOH, but other than that you're exactly correct. On the other hand, if you're subconsciously confusing CH3OH2+ with CH3COOH, that would explain the confusion.) When comparing the strength of two acids, we need to look at their conjugate bases (here, CF3COO- and CH3OH). We can look at conjugate bases in two ways, both of which are really exactly the same:

- Which of these conjugate bases is more stable? The more stable conjugate base always corresponds to the stronger acid. Here, we can conclude that CH3OH is more stable than CF3COO-, as it's uncharged and a generally unreactive species. This means that the conjugate acid of CH3OH (which is CH3OH2+) is a stronger acid than the conjugate acid of CF3COO- (which is CF3COOH).

- Which of these conjugate bases tends to react more as a base? This is just another way of saying, "Which of these conjugate bases is less stable?" but somehow tends to be easier to think about. Here, CF3COO- certainly acts more as a base than CH3OH, as (like you said) methanol really acts more like a weak acid than like a base at all. Since CF3COO- tends to react more, it is less stable and corresponds to the weaker conjugate acid (CF3COOH). Since CH3OH tends to react less, it is more stable and corresponds to the stronger conjugate acid (CH3OH2+).

Let me know if anything is still confusing - this is a great topic to master!

Just a quick question, going off of looking at a case by case basis. If the question is just: The conjugate base of acetic acid is a: (answers: weak acid, weak base, strong acid, strong base), why is the answer a weak base???