[thedy]

Hi,can molecule with double bond rotate?Because in school,they teach us,no.But here: http://en.wikipedia.org/wiki/Peptide_bond is picture of X-Pro peptide bond and it looks like bond between C-N in peptide bond rotates.How is it possible?Is it some kind of energy,what makes molucule to rotate?Or what?I m confused....Peptide bond is planar and cannot rotate,isn it?
Thanks

[sjb]

Hi,can molecule with double bond rotate?Because in school,they teach us,no.But here: http://en.wikipedia.org/wiki/Peptide_bond is picture of X-Pro peptide bond and it looks like bond between C-N in peptide bond rotates.How is it possible?Is it some kind of energy,what makes molucule to rotate?Or what?I m confused....Peptide bond is planar and cannot rotate,isn it?
Thanks

There is a difference between planarity and rotation barriers. In the case of the C-N peptide bond there is some double bond character, look into tautomerism or similar.

[thedy]

There is a difference between planarity and rotation barriers. In the case of the C-N peptide bond there is some double bond character, look into tautomerism or similar.

Exactly,in C-N,there is a double bond due to resonance,that means peptide bond cannot rotate,or not?

[fledarmus]

In amides, there is a barrier to rotation due to the partial double bond character of the bond, but it is not as high as the barrier to rotation of a true double bond. In most cases, it is still high enough that it doesn't occur readily at room temperature.

There may also be a difference in stability between the cis and trans isomers, especially on secondary amides, which would prevent rotation.

Depending on substituents, solvents, and other effects, it is possible to lower the double bond character still further and rotation may be rapid at room temperature.

For true double bonds, you have to provide enough energy to completely break the pi bond in order to get rotation to occur - during the rotation, there is a point where the p orbitals on the two atoms in the bond are at 90 degrees to each other and cannot overlap at all.

[juanrga]

Hi,can molecule with double bond rotate?Because in school,they teach us,no.But here: http://en.wikipedia.org/wiki/Peptide_bond is picture of X-Pro peptide bond and it looks like bond between C-N in peptide bond rotates.How is it possible?Is it some kind of energy,what makes molucule to rotate?Or what?I m confused....Peptide bond is planar and cannot rotate,isn it?
Thanks

What is rotating is not a double bond C=N but a single C-N bond. The structure ^-O-C(···)=N··· cannot rotate around the double bond C=N but has a resonant structure



that can rotate around the single bond C-N