Glutamine and Aspargine Side Chains

[SyrianHero]

Why doesn't the nitrogen that is part of the amide group in Glutamine and Aspargine gain a proton and become NH3 when PH changes?

---

Members don't see this ad.

 

[Hospitalized]

Because the amide group has resonance stabilization. Amides are very unreactive because of this resonance, hence they will not lose or gain protons easily.

 

[betterfuture]

My question kind of relates. I was wondering when amide bonds are broken, like in peptide hydrolysis, I was wondering what the acidic conditions do? It states that the carbonyl oxygen gets protonated making it more susceptible to nucleophilic attack by water molecule aka the nucleophile. But how does protonation help it? It also states it can work under basic conditions. How does that work?

 

[aldol16]

My question kind of relates. I was wondering when amide bonds are broken, like in peptide hydrolysis, I was wondering what the acidic conditions do? It states that the carbonyl oxygen gets protonated making it more susceptible to nucleophilic attack by water molecule aka the nucleophile. But how does protonation help it? It also states it can work under basic conditions. How does that work?

It's similar to nucleophilic attack at any carbonyl. In acidic conditions, the carbonyl is protonated, making it more electrophilic. You need this because water isn't a particularly good nucleophile. In base catalyzed hydrolysis, the base will pluck a proton off water easily, making a hydroxyl group, which is a good nucleophile.

 

[betterfuture]

I mean I understand the part where protonation helps the C of the carbonyl group become more electrophilic. My question was what does protonation do to contribute to the electrophilicity of the C?

 

[aldol16]

I mean I understand the part where protonation helps the C of the carbonyl group become more electrophilic. My question was what does protonation do to contribute to the electrophilicity of the C?

I don't get how the second sentence isn't answered by the first. Do you want the specifics? The carbonyl O has a lone pair that can donate via a Lewis base interaction with a proton. This results in a net positive charge on the oxygen. So oxygen is very electronegative and doesn't want to be electropositive, so it will withdraw even more electron density from the carbon atom of the carbonyl to stabilize the positive charge. So the resonance contributor where there is a carbocation gets a bigger coefficient = higher electrophilicity on the carbon.

 

[betterfuture]

Pardon me. I am kind of slow. I have to reread things more than a few times to understand them.

You explained the answer I was looking for with your second response. I just did not understand before how protonation would help C become more electrophilic. It's because of the nature of the Oxygen atom wanting to become more negative hence it's reason for pulling electron density even more.