Which is a correct statement?
A. The delocalization ene...

Question

Which is a correct statement?

A. The delocalization energy of an ester is about 18 kcal/mol, and the delocalization energy of an amide is about 10 kcal/mol.

B. The delocalization energy of an ester is about 10 kcal/mol, and the delocalization energy of an amide is about 18 kcal/mol.

Accepted Answer

Hello everyone. Let's do this problem. It says which of the following statements is correct. A the resonance energy of a carboxylic acid is higher than the resonance energy of an amide b. The resonance energy of a carboxylic acid is lower than the resonance energy of an amide. So we're comparing the resonance energy of a carboxylic acid to the resonance energy of an amide. So what is resonance energy also known as de localization energy? And I like to think of it as energy saved from resonance or because a compound has resonance because if I think of resonance energy and a compound having more resonance energy, it can sort of throw you off because I know that having more energy correlates with less stability, right? But having a lot of resonance energy actually correlates with more stability because you're talking about the amount of energy that's being saved due to resonance. So if you're calculating the entropy of an alkene being converted into an Alcaine, for example, the theoretical or the calculated entropy is going to be different than the entropy that you would find experimentally. And that's because of the resonance that's not being considered in the calculation. OK. The difference between those two values is resonance energy. All right. So what makes a compound have higher resonance energy? Well, I mentioned stability right. So you have the first resonance structure for a compound is always the Lewis structure. But then after that the more stable the other resonance contributors are then the more resonance energy a compound has. So like I said, that could be confusing at first because you think, oh how can a compound that's more stable, have higher energy? That's why I think of it as energy saved. OK. So we need to look at stability, stability of these resonance forms. So what are some factors that correlate to stability? Well, stability will decrease. If an atom has incomplete octet, we can look at where the charges are what atoms they are on. And the electron negativity of those atoms because an electron negative atom likes to have a negative charge. So if a more electron negative atom has a positive charge that will make it less stable, we can also look at if there are separated charges that will make it less stable. And by that, I mean, the charges are far away from each other on the compound, we can also look at if we have a carbo Canion, what type of Carbocaine is it primary, secondary tertiary, we know tertiary carbo cad ions are more stable. We also know allylic carbo cads are very stable. So let's look at a carboxylic acid and an aide compare their second or other resonance contributors and compare their stability. So starting with a carboxylic acid, which has the structure c double bond to oxygen and also an alcohol group bonded to that same carbon. And um we just straw and methyl group also bonded to that central carbon. So our oxygen in the carbonyl group has two lone pairs. Our oxygen in the alcohol part of the carboxylic acid has, I guess I should say the O H group also has two lone pairs. So that's the Lewis structure for this carboxylic acid. So what is the other resonance contributor? Well, we can form another resonance contributor by taking a loan pair from this O H group swinging those down in between that carbon and oxygen bond because that carbon is partially positive due to the oxygen atom in the Carboni group. So those electrons will swing up towards that Carboni oxygen giving us the central carbon single bonded to. Now an oxygen with three lone pairs and a negative charge C double bond to the O H group which that oxygen now has one lone pair and a positive charge and the methyl group still stays the same. All right. So let's draw in a mind it's similar, we have a central carbon double bonded to an oxygen with two lone pairs. That carbon is also bonded to single bonded to a nitrogen which has two hydrogen. So, c single bonded to NH 2 and to satisfy the octet of that nitrogen will give it one lone pair in the last group on the carbon can be a methyl group. Drawing a resonance contributor of this same mechanism as the carboxylic acid that lone pair from the nitrogen can swing down towards that central carbon which will break this pi bond in the carbonyl group. So now we have carbon single bonded to oxygen with three lone pairs and a negative charge. The methyl group did not change. And now we have carbon double bonded to nitrogen, That NH 2 group and that nitrogen will now have a positive charge. OK. So let's compare the stability of this second resonance contributor for both the carboxylic acid and the mine all groups have their octet satisfied. They only have a positive one positive and one negative charge and they are the same distance away from each other. There is no carbo cad ion. What about the electron negativity of the atoms with the charge? While the oxygen atom has a negative charge in both the carboxylic acid and the amide. What about the positive charge in the carboxylic acid? The oxygen in the O H group has a positive charge in the amide. The nitrogen in the NH 2 group has a positive charge. Hm. So the positive charge is on oxygen in the carboxylic acid, positive charge on nitrogen in the amide. What is the electron negativity of oxygen and nitrogen, which one is more electron negative oxygen is more electron negative nitrogen is less electron negative. So which one is going to be more stable, the less electron negative atom with the positive charge? Right? Because that electron negative atom wants electrons, it doesn't want a positive charge. So the carboxylic acid will be less stable, which means it'll have less or lower resonance energy. So it's not saving as much energy from resonance and the amide will be more stable and have higher resonance energy. So with that being said, let's look at our statements one more time. The correct statement would be statement B the resonance energy of a carboxylic acid is lower than the resonance energy of an aide. That's it for this problem. I hope this video is helpful and thank you for watching.

Which is a correct statement?
A. The delocalization energy of an ester is about 18 kcal/mol, and the delocalization energy of an amide is about 10 kcal/mol.
B. The delocalization energy of an ester is about 10 kcal/mol, and the delocalization energy of an amide is about 18 kcal/mol.

Key Concepts

Delocalization Energy

Ester and Amide Structures

Comparative Stability

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