Why are carboxylic acids of amino acids stronger?

[chiddler]

Stronger than regular carboxylic acids, anyway. Is it due to the nitrogen producing an inductive effect?

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[Ssina]

Oxygen on the OH group is more electronegative than the Nitrogen, so it'll donate less of it's electron density vs. Nitrogen (when looking at the resonance delocalized bonds) e.g. in a peptide bond... so that might contribute as well!

 

[chiddler]

Oxygen on the OH group is more electronegative than the Nitrogen, so it'll donate less of it's electron density vs. Nitrogen (when looking at the resonance delocalized bonds) e.g. in a peptide bond... so that might contribute as well!

hmm can you please rephrase that? i don't understand what you mean.

 

[MedPR]

hmm can you please rephrase that? i don't understand what you mean.

I think he misunderstood your question.

I don't have an answer, but I guess you're reading TBR because I noticed that when they were talking about pKa too.

 

[Ssina]

I think he misunderstood your question.

I don't have an answer, but I guess you're reading TBR because I noticed that when they were talking about pKa too.

WOOOW waaay off! lol sorry
yeah no idea, but inductive sounds better than nothing!

 

[milski]

WOOOW waaay off! lol sorry
yeah no idea, but inductive sounds better than nothing!

Yes, I'd go with inductive too. N is fairly electro negative, compared to just plain C. So there will be some inductive effect from the N group.

 

[IH8ColdWeath3r]

it could be due to inductive effects of N, but I would also say because peptide bonds have partial double bond character. Remember that the N has a lone pair and sits adjacent to the Carbonyl carbon, so its electrons can migrate and form a double bond between N and C if the electrons in the pi bond on the carbonyl migrate upward.

http://en.wikipedia.org/wiki/File:Mesomeric_peptide_bond.svg

I would say this is what really accounts for the added stability.

 

[mcloaf]

it could be due to inductive effects of N, but I would also say because peptide bonds have partial double bond character. Remember that the N has a lone pair and sits adjacent to the Carbonyl carbon, so its electrons can migrate and form a double bond between N and C if the electrons in the pi bond on the carbonyl migrate upward.

http://en.wikipedia.org/wiki/File:Mesomeric_peptide_bond.svg

I would say this is what really accounts for the added stability.

He's asking about the acidity of the carboxylic acid group of an amino acid compared to a regular carboxylic acid, not bond strength--though you're right on about the stronger bond with double bond character. In an individual amino acid, however, the nitrogen is not attached to the carbonyl carbon, so this resonance is precluded. Additionally, having the nitrogen donate electron density to carboxyl group would decrease, rather than increase acidity.

Induction is what accounts for the pKa differences as the N withdraws electron density.