Amide Resonance

From WikidChem

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Slide 9

Resonance is defined as mixing of a HOMO and a LUMO within a molecule, as in, for example, an amide.

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Slide 10

This slide discusses the reasons why the amide functional group cannot be considered as an amine and a ketone put together.

If we think of the amide as an amine and a ketone, we would neglect the properties of resonance that an amide actually has. Thus, our predictions for an amide composed of an amine and a ketone versus the experimental observations for an actual amide would be different.

Putting an amine and a ketone together would predict certain things about the resulting molecule, and we can see by experiment that those things are not the case.

Predictions:

-Stable

-Long N-C bond (single bond length)

-Short C=O bond (double bond length)

-Pyramidal N (most stable configuration)

-Easy rotation around the N-C bond

-Both basic and acidic because the molecule contains both a HOMO and a LUMO

-Polarity mostly cancels out because the dipole directions oppose each other (at least vertically)

Experiment:

-Even more stable than predicted

-N-C bond is shorter than a single bond

-C=O bond is longer than a double bond

-Atoms are planar around N

-No rotation around the N-C bond, because it would break the overlap between the 2p orbitals of C and N

-Relatively unreactive (does not contain a HOMO and LUMO)

-Strongly dipolar (in the direction from N to O)

Note that the last four comments are crucial for structural biology.