Amide -> Keto enol tautomers?

[regeneration]

Can an amide (the "keto") form an enol tautomer with a double bond to the nitrogen and a hydroxy group instead of the ketone? What is this called?

I'm familiar with the imine <-> enamine tautomerization, but this is with N alpha to a carboxyl group.

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[Trayshawn]

your question is really confusing. maybe rephrase and be a bit clearer?

 

[regeneration]

Ok: Can an amide undergo tautomerization such that there is a double bond between the carboxyl carbon and the N, and an alcohol instead of the carboxyl group?

 

[ouroboros39]

You mean the carbonyl group - just a carbon oxygen double bond with an "R" group instead of OH (and the amide+its substituents)? I imagine so, the lone pair on the nitrogen will likely be in resonance as is - depending on structure.

But this would not be very favored since carbonyls are more stable, so it would be in very present in very small quantities. For the purposes of a reaction you might be able to treat it as an alcohol as long as the reaction is irreversible and funnels to the products.

 

[Trayshawn]

^yup. Its much less favored of the two tautomers

 

[regeneration]

Hmm, I'm not sure if we're thinking about the same thing. To rephrase again: Can an amide tautomerize to an imine

that has an OH instead of R1.

 

[ChemTutor]

it is called iminol. for standard amides it does not really form. this creature is clearly beyond MCAT.

here is one example: http://www.wikipremed.com/image.php?img=040103_68zzzz254500_Uracil-Resonance_68.jpg&image_id=254500

 

[BerkReviewTeach]

As stated above, it's not a stable compound and should be thought of as merely a resonance contributor. That said, it's a big enough contributor that when amides form H-bonds, they donate the electrons from O rather than N (despite O being more electronegative than N). This is why the H-bonding in a beta-pleated sheet is between an H on N and the amide O, rather than the amide N.

 

[regeneration]

Thank you, that's it! And I see it has an important role in proteins.

despite O being more electronegative than N

Wouldn't we anyway expect a more electronegative atom to accept the H bond, since the electrons on the more electronegative species will "pull" the H harder? I would think that a F-H hydrogen bond is stronger than a N-H?

Or is it that less electronegative atoms can hold onto partial positive charges better? Is this why amines (RNH2) are better bases than carboxyl groups -- because they are less electronegative and can hold the +1 charge better?

Thanks!