TBR Orgo I Ch.1

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ajumobim

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#85 Which of the following types of compounds is the MOST basic?
A) Primary alcohols B) Esters C) Secondary amines D) Tertiary thiols

#26 asked, "The most stable hydrogen-bond between amides extends from the.." and the answer was "the carbonyl oxygen to the H on N." I chose B when working on #85, primarily because I remembered reading in the explanation for #26 that the carbonyl oxygen of an amide forms the strongest hydrogen bond because the N of the amide donates electrons through resonance pushing out electrons on the O, making O a better electron donor. Why doesn't the same hold true for #85 in regards to esters? I was also confused by the fact that the explanation for #85 says that esters have no lone pairs to readily donate to a proton.

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The simplest reason is that esters cannot favorably accept a proton! That's what they're trying to say with the statement about lone pairs. Amines, on the other hand, often accept protons as their conjugate acids are fairly stable (just think of NH4+, for example). In the image, I showed what the conjugate acids of these two species would look like (bear with me, I'm terrible at drawing haha).

I'm not sure what you mean regarding hydrogen bonding - what you say about #26 makes perfect sense, but an ester can't hydrogen bond in the way described because its O is not bound to a proton. I think you might be confusing the fact that the carbonyl oxygen is a good electron donor with basicity - the carbonyl still won't be prone to acting like a base. Does this make sense or is it still confusing, or were you thinking of something else with regard to #26?

Good luck! I always appreciate your questions, they're good ones.
 

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Thank you, Clara! Well, I thought #26 and #85 were similar because in #26 it asks which choice would form the strongest hydrogen-bond and that would be the most basic site attacking the most acidic site. At first, I thought the amide nitrogen forms the strongest hydrogen bond since it is less electronegative, but then the explanation said the most basic site is the O on the amide because N donates electrons and pushes electrons out to O. So, I thought it would be same for #85. The O of the ester donates electrons through resonance and pushes electrons out to the carbonyl oxygen making it the most basic. Isn't it the case that the basicity has a direct relationship to ability to donate electrons?
 
@NextStepTutor_3 Okay, I had an epiphany! You can completely ignore the post above. I was tackling the question in a completely incorrect way. In #26, it was essentially asking which site on the compound is the most basic. Whereas, this question is asking which compound, in itself, is the most basic. So, I was thinking simply because there is resonance donation and greater electron density on the carbonyl oxygen of the ester that it would be the most basic compound, which is not the case. I'm still not understanding why it is not favorable for esters to accept a proton. Is it because O is highly electronegative?
 
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Which one is most basic?

As a functional group esters do not participate in H+ exchange. They do affect other functional groups and carbon chains of course and can indirectly change pkas etc - but the functional group of esters do not participate in H+ ion exchange. This is because the carbonyl group is not involved in acid base reactions and the ether portion does not either. As far as I can understand without knowing the context I don't see why hydrogen bonding would be involved in determining the strength of a base expect for comparing protic and aprotic solvents. The ester has a dipole, but I wouldn't say it hydrogen bonds.

Thiols are similar to alcohol groups. Intuitively, I think the primary alcohol would be a stronger base than the tertiary thiol. Someone can correct me if I'm wrong.

Amines are the strongest base. Just remember that amines are better nucleophiles than alcohols. Alcohols make great nucleophiles but amines are better. (This is assuming protic - aqueous consitions)
 
If you're wondering why esters aren't basic, why not consider amides? After all, amines are fairly good weak bases, but amides are nearly neutral. @ajumobim , I think you mentioned the carbonyl's electron density? The key is that electron-withdrawing groups make ACIDS stronger (inductive effect), NOT bases. Since the nitrogen of an amide is adjacent to a carbonyl group, some of the electron density of its reactive lone pair - the very thing that makes an amine basic - is being pulled away. This makes the amide more stable and consequently less basic. Back to esters - in general, esters are pretty unreactive already, and fairly stable. Gaining a proton would seriously destabilize the molecule.

And it's true that hydrogen bonding in itself doesn't relate to acidity or basicity, like other intermolecular interactions. What actually does determine the strength of an acid is the stability of its conjugate base, and the strength of a base is determined by the stability of its conjugate acid. Anything that contributes to those qualities (resonance, electron withdrawing effects for an acid, electron donating effects for a base, etc) is what you should look at.

Did that clear it up at all?
 
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