[Zalzul]

I'm seeking a little clarification on why N,N-dimethylacetamide is less acidic than an ester.  I am aware that, following deprotonation of the α-carbon, the electrons left behind can be delocalized onto the two atoms more electronegative than carbon.  Whereas in the same scenario for an ester, the electrons on the -OR group compete with those of the α-carbon for delocalization onto the same carbonyl oxygen.  Presumably this is the reasoning, according to my textbook.

However, this seems partly counterintuitive to me.  I would ordinarily assume that N's lone pair in the disubstituted amide would delocalize more so than in an ester onto the carbonyl oxygen, since nitrogen is more electropositive than oxygen.  Is it because the -OR group in an ester has TWO lone pairs?

What if the -OR group has a long alkyl chain?  What if the amide WASN'T disubstituted?

[Benzene]

Which carbonyl carbon has a greater electron density, the carbonyl carbon in an amide? or an ester?

[Zalzul]

Which carbonyl carbon has a greater electron density, the carbonyl carbon in an amide? or an ester?

The amide, right?  Oxygen is more electronegative than nitrogen, thus the -OR group of an ester would be more electron-withdrawing.  So, considering that its carbonyl carbon has greater electron density, why would the deprotonated α-carbon's electrons of an amide delocalize onto a carbon with greater electron density?  Thus, the amide is less acidic than an ester because the α-carbon's electrons cannot delocalize as well.  I think that's what you're getting at, at least.

That makes sense.  I guess I just have a problem with the way the book is phrasing it; that the -OR group's electrons compete for delocalization onto the carbonyl oxygen.

[Benzene]

The proton on the esters alpha carbon is more acidic than the proton on the alpha carbon of amides.
What book are you reading?

[Zalzul]

Latest edition of Organic Chemistry by Paula Bruice.  Yeah, I'm agreeing with you, as does the book.  pKa of N,N-dimethylacetamide = 30 whereas pKa of an ester = 25.  Sorry if I'm being confusing.

I think it makes sense to me now.

[PhDoc]

The problem here is not taking all the variables into consideration.

Nitrogen is less electronegative (3.0) than oxygen (3.5), hence, all things being equal, it more readily participates in resonance than does oxygen. This has the effect of decreasing double bond character of the carbonyl group with consequent lowering of pKa of carbonyl alpha protons.

To really understand this difference in pKa it's important to consider stabilization imparted via hyperconjugation. An alkyl ester has one hyperconjugating group stabilizing the oxonium ion resonance structure whereas a disubstituted amide iminium ion has two hyperconjugating groups. Another way of stating this is that the disubstituted amide has less carbonyl character than the ester. This fact can be corroborated by infrared stretching frequencies, 1662 cm-1 for dimethylacetamide vs. 1746 cm-1 for methyl acetate. The higher stretching frequency is directly proportional to higher double bond character of the carbonyl group.

Compare these results to that of acetyl chloride. The chlorine does not substantially participate in resonance, hence it's primary effect is electron withdrawal via induction. This prediction is empirically observed in the infrared spectrum of acetyl chloride, the pKa of which is 16 and infrared carbonyl stretching frequency of which is 1800 cm-1.

Empirical observation for carboxylic acid derivatives: as infrared stretching frequency increases, acidity of alpha protons increase; as infrared stretching frequency decreases, acidity of alpha protons decrease. This has everything to do with the level of double bond character in the carbonyl group.

Interestingly, this parallels with leaving group conjugate acid pKa. Consider the series Cl-, OMe-, NMe2-; corresponding conjugate acids are HCl (pKa -8, H2O), MeOH (15.5), Me2NH (~36). Notice that, as pKa of leaving group conjugate acid increases, carbonyl stretching frequency decreases (less carbonyl double bond character), and alpha proton acidity of the carboxylic acid derivative decreases.

For better pKa information than published in Bruice, go here: http://evans.harvard.edu/pdf/evans_pka_table.pdf

[Zalzul]

The problem here is not taking all the variables into consideration.

Nitrogen is less electronegative (3.0) than oxygen (3.5), hence, all things being equal, it more readily participates in resonance than does oxygen...

The chlorine does not substantially participate in resonance, hence it's primary effect is electron withdrawal via induction.

Wow, this definitely clarifies it for me.  Up until now I had assumed that electronegativity was directly proportional with resonant ability.  As in, I used resonance as a catch-all term for electron-pushing/movement, and atoms that could better pull electrons were more able to afford a molecule resonance, and thus, stability.

But now after reading these two sentences, it makes sense; that's why acyl halides are so good for making other carboxylic acid derivatives!  The halide's role in such a reaction is twofold.  One, it decreases the double bond character of the carbonyl group, thereby opening up the carbonyl carbon to nucleophilic attack.  Two, it's a great leaving group.

So now I know to remember the distinctions between resonance, electronegativity, and general electron movement.

Thank you both for your help.  I'm confused as to why, in three chapters solely focused on carbonyls, Bruice doesn't talk about double bond character at all.

[Benzene]

So as the carbonyl wavelength increases the acidity of the alpha carbon proton increases?

Why do the electron withdrawing groups increase the wavelength?

is it because the withdrawing group destabilizes the carbonyl bond?

and wavelength is inversely proportional to bond dissociation energy?

Is this rule always true? or does it only apply to esters, amides, and acid chlorides?

Sorry about the question bombardment! any help would be much appreciated on this.

[PhDoc]

In chemistry, exact semantics are the difference between being completely correct and completely incorrect.

"that's why acyl halides are so good for making other carboxylic acid derivatives!  The halide's role in such a reaction is twofold.  One, it decreases the double bond character of the carbonyl group, thereby opening up the carbonyl carbon to nucleophilic attack.  Two, it's a great leaving group."

Chlorine does not appreciably participate in resonance. Period. Because it is an electron withdrawing group (chi = 3.0), and it does not appreciably participate in resonance, then all we can expect it to do is withdraw electron density via induction. Period. Because inductive effects [in this particular example] by far outweigh any potential resonance effects, double bond character of the carbonyl group is a maximum. Period. Finally, since double bond character of the carbonyl carbon is a maximum, in comparison to other carboxylic acid derivatives, the alpha protons are rendered more acidic in acetyl chloride when compared to methyl acetate or dimethylacetamide. Period. Those carboxylic acid derivatives with a maximum of carbonyl double bond character will manifest higher infrared stretching frequency because stretching the C=O bond requires more energy. Period.

As for application of double bond character to compounds other than carboxylic acid derivatives, compare the infrared stretching frequency of acetaldehyde to that of acetone. While you're at it, compare the pKa of acetaldehyde to acetone. What are the differences? What can you conclude?

Please don't memorize any of this stuff. If an explanation is incomplete use the situation as a learning experience to fill in the blanks with your own thought. This is what will permit you to succeed in O-Chem.

[Zalzul]

Because inductive effects [in this particular example] by far outweigh any potential resonance effects, double bond character of the carbonyl group is a maximum. Period. Finally, since double bond character of the carbonyl carbon is a maximum, in comparison to other carboxylic acid derivatives, the alpha protons are rendered more acidic in acetyl chloride when compared to methyl acetate or dimethylacetamide.

I guess this circles me back around to my original question.  There's something here that's not clicking in my head, and I want it to.  The problem question the book is asking me is: "Explain why an N,N-disubstituted amide is less acidic than an ester."  So when I flip back to look for an answer, this is what I get.  Again, I'll quote Bruice:

"The electrons left behind when a proton is removed from the α-carbon of an ester are not as readily delocalized onto the carbonyl oxygen as they would be in an aldehyde or a ketone.  This is because the oxygen of the OR group of the ester also has a lone pair that can be delocalized onto the carbonyl carbon.  Thus, the two pairs of electrons compete for delocalization onto the same oxygen."

Then, on the next page, when taking about amides, she says:

"Nitroalkanes, nitriles, and N,N-disubstituted amides also have a relatively acidic α-hydrogen, because in each case the electrons left behind when the proton is removed can be delocalized onto an atom that is more electronegative than carbon."

I'm having trouble conflating both reasonings into a whole.  Why does Bruice mention that the lone pairs of the ester can delocalize onto the carbonyl carbon, but not the amide's?   If you asked me the same problem that Bruice is on a test, here's what my answer would be: "Nitrogen is more electropositive than oxygen, which should, theoretically, make it better able to accommodate the positive end of a partial dipole, thus stabilizing any intermediate."

Would that be an acceptable answer?

To sum up, I guess here are my two questions to you, Lennox, or to the board, in general:

1) Why can the electrons of the OR of an ester delocalize onto the carbonyl carbon, but not those of the N group of the amide?

2) If the double bond of the carbonyl is at a maximum when a highly electronegative element is attached, then why are such compounds so reactive in nucleophilic addition-elimination reactions?  Normally, I would say because the leaving group (the ^-Cl in an acyl chloride, for example) is a weak base and the carbonyl oxygen can tolerate being an anion for however brief a moment.  But logic is also telling me that if double bond character is at a maximum, then wouldn't that stabilize the original carbonyl molecule?

[Zalzul]

Actually, I think I may have just answered the second of my questions myself.  The fact that the double bond character of the carbonyl group in an acyl chloride is at a maximum means it's at a higher energy level, thus making it more reactive.  The nucleophilic addition-elimination reaction would be exothermic, essentially.

Can someone verify whether I'm correct here?  Thanks.

[Babcock_Hall]

Acid chlorides are reactive because they have a good leaving group (a weak base).  I don't understand what you mean by high energy level and double bond character.

With respect to the question of acidity, you might try comparing esters versus thioesters (compounds deriving from an acid and a thiol, instead of an alcohol).  Which one is more acidic and why?

[orgopete]

I guess this circles me back around to my original question.  There's something here that's not clicking in my head, and I want it to.  The problem question the book is asking me is: "Explain why an N,N-disubstituted amide is less acidic than an ester."  So when I flip back to look for an answer, this is what I get.  Again, I'll quote Bruice:

"The electrons left behind when a proton is removed from the α-carbon of an ester are not as readily delocalized onto the carbonyl oxygen as they would be in an aldehyde or a ketone.  This is because the oxygen of the OR group of the ester also has a lone pair that can be delocalized onto the carbonyl carbon.  Thus, the two pairs of electrons compete for delocalization onto the same oxygen."

Then, on the next page, when taking about amides, she says:

"Nitroalkanes, nitriles, and N,N-disubstituted amides also have a relatively acidic α-hydrogen, because in each case the electrons left behind when the proton is removed can be delocalized onto an atom that is more electronegative than carbon."

As I read this, it appears to me as though Paula Bruice is referring to "groups that enable anion formation". These citations are not meant in opposition to one another, rather besides an ester (or aldehyde and ketones), amides, nitriles, and nitroalkanes can also form anions.

I'm having trouble conflating both reasonings into a whole.  Why does Bruice mention that the lone pairs of the ester can delocalize onto the carbonyl carbon, but not the amide's?   If you asked me the same problem that Bruice is on a test, here's what my answer would be: "Nitrogen is more electropositive than oxygen, which should, theoretically, make it better able to accommodate the positive end of a partial dipole, thus stabilizing any intermediate."

Would that be an acceptable answer?

To sum up, I guess here are my two questions to you, Lennox, or to the board, in general:

1) Why can the electrons of the OR of an ester delocalize onto the carbonyl carbon, but not those of the N group of the amide?

As had been explained, nitrogen does donate its electrons to the carbonyl group of an amide. Bruice wasn't excluding this, simply referring to other groups that may show anion formation. You may note the sum of the explanations given here are much greater than given by Paula Bruice. I don't know one can arrive at a universally proper explanation. Too much information can be unnecessarily confusing.

If one were to examine ester and amide acidities, carbonyl stretching frequencies, and carbonyl reactivity then you might conclude the nitrogen is reducing the electron withdrawing properties of a carbonyl. One might use these data to compare substituents ability to donate or withdraw electrons. Granted this data may be messy. Nitrogen can be both more electron withdrawing than carbon and also able to donate its non-bonded electrons.