Sciencemadness Discussion Board

Rotation of atoms around single/double/triple bonds

Wrecking Bereserker - 23-4-2019 at 04:19

Can someone explain how rotation of atoms work around single,double,triple bonds?
I'm kind of confused around saying something is either the same molecule or cis/trans!

DraconicAcid - 23-4-2019 at 07:48

There is generally free rotation around single bonds.

There is no free rotation around a double bond.

There may not be any free rotation around a triple bond, but it doesn't matter, because each sp hybridized carbon has only one other bond, and there is free rotation around that one.

Metacelsus - 23-4-2019 at 08:38

Quote: Originally posted by DraconicAcid  

There may not be any free rotation around a triple bond, but it doesn't matter, because each sp hybridized carbon has only one other bond, and there is free rotation around that one.


It doesn't matter for carbon, but some transition metal complexes (for example, hexakis(dimethylamido)dimolybdenum) have triple bonds where the rotation matters. And yes, it can freely rotate. This came up on one of my inorganic chemistry exams back in undergrad.

Wrecking Bereserker - 23-4-2019 at 11:15

Quote: Originally posted by Metacelsus  
Quote: Originally posted by DraconicAcid  

There may not be any free rotation around a triple bond, but it doesn't matter, because each sp hybridized carbon has only one other bond, and there is free rotation around that one.


It doesn't matter for carbon, but some transition metal complexes (for example, hexakis(dimethylamido)dimolybdenum) have triple bonds where the rotation matters. And yes, it can freely rotate. This came up on one of my inorganic chemistry exams back in undergrad.


Can you explain how is this rotation possible?

Metacelsus - 23-4-2019 at 11:16

The molecular orbital has cylindrical symmetry. For an experiment showing the rotation, see here: https://pubs.acs.org/doi/abs/10.1021/ja00538a063

[Edited on 2019-4-23 by Metacelsus]

DraconicAcid - 23-4-2019 at 11:23

That kinda makes sense. If you rotate a carbon-carbon double bond 90 degrees, you completely break the pi bond. If you rotate a triple bond 90 degrees, the p orbitals can start overlapping with the other set of p orbitals as soon as you're halfway there.

Endo - 23-4-2019 at 11:55

Also with double bonds if you have a source of UV energy that can excite or temporarily disrupt the Pi bond it can rotate. For example with the rearrangement of Cis-Stilbene.

https://en.wikipedia.org/wiki/(E)-Stilbene

DraconicAcid - 23-4-2019 at 12:00

Quote: Originally posted by Endo  
Also with double bonds if you have a source of UV energy that can excite or temporarily disrupt the Pi bond it can rotate. For example with the rearrangement of Cis-Stilbene.

https://en.wikipedia.org/wiki/(E)-Stilbene

Yeah, but that's breaking the pi bond, so you can rotate around the single bond that's left.

qt314 - 25-4-2019 at 03:35

Its also very straightforward to visualize if you draw the actual atomics orbitals and see with logic how the molecular orbitals (arising from the linear combination of atomic orbitals) can behave... Try it :)

happyfooddance - 25-4-2019 at 14:02

These may be helpful :)

Attachment: CNMR_Dynamics.pdf (73kB)
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Attachment: carbonbondrotation.pdf (242kB)
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Attachment: toluenerotation.pdf (496kB)
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Attachment: ethanerotation.pdf (296kB)
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I also want to point out that rotation in a molecule is completely unrelated to isomerism and stereochemistry (in any example I can think of at least), OP seems to be confusing or conflating the two by referring to cis/trans as if they were somehow related to bond rotation. Unless there are cis/trans-designated conformations, which I wouldn't be terribly surprised to find out...

[Edited on 4-26-2019 by happyfooddance]

Wrecking Bereserker - 25-4-2019 at 22:13

Quote: Originally posted by happyfooddance  
I also want to point out that rotation in a molecule is completely unrelated to isomerism and stereochemistry (in any example I can think of at least), OP seems to be confusing or conflating the two by referring to cis/trans as if they were somehow related to bond rotation. Unless there are cis/trans-designated conformations, which I wouldn't be terribly surprised to find out...

Thank you for the reply;)

There is no cis/trans configuration for organic compounds involving solely single bonds right because they can rotate all they want and no matter how we change the bond orientation it will always the same compound
But when it comes to compounds with double bond there is cis/trans isomers for it since it can't rotate to be those isomers and the only way is to break the bonds so they are different compounds ie-isomers that's why im connecting rotation with isomerism


[Edited on 26-4-2019 by Wrecking Bereserker]

Metacelsus - 25-4-2019 at 22:40

Quote: Originally posted by Wrecking Bereserker  

There is no cis/trans configuration for organic compounds involving solely single bonds right because they can rotate all they want and no matter how we change the bond orientation it will always the same compound


Usually. But sometimes bonds are hindered from rotating, and this results in atropisomers. However, they still aren't called cis/trans; instead they're P/M, or alternatively Δ/Λ.

qt314 - 25-4-2019 at 23:36

Many times molecules are referred as cis/trans in newmann projections. This may contribute to confusion.