TBR OChem Ch. 5 passage IV question 19

[altogether]

formic acid has a pka of 3.78. what b value is expected for methylethanoate?

I believe this is a transesterification reaction so the leaving group should be ethanol or ethoxide. However the question implies that formate is the l.g.

Can anyone tell my why formate is the leaving group as opposed to ethanol?

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[Isoprop]

I don't have the question/problem so I can't be sure, but I can answer why formate is a better leaving group than ethoxide.

In general, the more thermodynamically stable molecule will be the better leaving group. Formate can stabilize the negative charge through resonance between each oxygen. Ethoxide cannot do this.

This is why formic acid has a lower pKa ~5 (strong acid), it's conjugate base is thermodynamically stable.

 

[altogether]

I don't have the question/problem so I can't be sure, but I can answer why formate is a better leaving group than ethoxide.

In general, the more thermodynamically stable molecule will be the better leaving group. Formate can stabilize the negative charge through resonance between each oxygen. Ethoxide cannot do this.

This is why formic acid has a lower pKa ~5 (strong acid), it's conjugate base is thermodynamically stable.

I understand this but I just don't understand how you can have formate as a leaving group from methylethanoate. Maybe I have the wrong structure in my head... This methylethanoate is an ester according to wiki. So the leaving group is always the alkoxy. This question is implying that formate will leave and ethyl remains?

 

[chemtopper]

methylethanoate is formed by esterification reaction between ethanoic acid and methanol
On hydrolysis it gives ethanoic acid and methanol
I dont think that we have formic acid anywhere in this ester formation or hydrolysis.

 

[altogether]

methylethanoate is formed by esterification reaction between ethanoic acid and methanol
On hydrolysis it gives ethanoic acid and methanol
I dont think that we have formic acid anywhere in this ester formation or hydrolysis.

so... bad question, perhaps?

 

[inasensegone]

Sorry to bump an old thread, but I figured it was more appropriate than starting a new one...

I was having the same issue with this answer explanation. It seems like the leaving group would actually be methoxide (CH3O-) and not Formate/Methanoate (HCOO-) as the book explains. Anyway, I'm just posting this here in case others googled the same thing in the future.

 

[StephWright]

This is super helpful. Thank you all!

 

[RiPSTONE]

I think it's just a bad question haha. I was quite confused myself just now.

 

[BerkReviewTeach]

Dang, that is a bad nomenclature typo. The question is supposed to be comparing H3CCO-O-COCH3 (an anhydride) with H3CCO-O-COH (an anhydride).

 

[Lunasly]

Dang, that is a bad nomenclature typo. The question is supposed to be comparing H3CCO-O-COCH3 (an anhydride) with H3CCO-O-COH (an anhydride).

I found the same issue in the 2014 review book, as well. It's the first question of the 1st review passage.

I don't even understand what the question is asking. Even if we were comparing the two structures below, would they still have similar beta values? It's the anhydride with the methyl group a better leaving group? I guess since it's not that much better than the beta value will only be slightly higher (1.8 vs 1.7)?

 

[BerkReviewTeach]

I found the same issue in the 2014 review book, as well. It's the first question of the 1st review passage.

I don't even understand what the question is asking. Even if we were comparing the two structures below, would they still have similar beta values? It's the anhydride with the methyl group a better leaving group? I guess since it's not that much better than the beta value will only be slightly higher (1.8 vs 1.7)?

That's disheartening that it's still there.

The gist of the question is to compare the reactivity of two comparable acid anhydrides. We are taught in organic chem that the strength of a leaving group can be compared to the strength of the conjugate acid of the leaving group. It makes sense when you think that the ability of a group to leave from H+ correlates with its ability to leave from O=CR+. Because formic acid is only slightly more acidic that acetic acid, a formate leaving group is only slightly better than an acetate leaving group. Does that perspective help at all?

 

[Lunasly]

That's disheartening that it's still there.

The gist of the question is to compare the reactivity of two comparable acid anhydrides. We are taught in organic chem that the strength of a leaving group can be compared to the strength of the conjugate acid of the leaving group. It makes sense when you think that the ability of a group to leave from H+ correlates with its ability to leave from O=CR+. Because formic acid is only slightly more acidic that acetic acid, a formate leaving group is only slightly better than an acetate leaving group. Does that perspective help at all?

Yes, thank you that makes a lot more sense. Just another side question: I really like the TBR material, but should we contact you if we notice a mistake or what we think is a typo? I found a few so far in the readings and one in particular in the physics passages I have done.