Wrecking Bereserker
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Rotation of atoms around single/double/triple bonds
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!
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DraconicAcid
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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.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Metacelsus
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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.
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Wrecking Bereserker
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| 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?
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Metacelsus
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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]
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DraconicAcid
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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.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Endo
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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
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DraconicAcid
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Yeah, but that's breaking the pi bond, so you can rotate around the single bond that's left.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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qt314
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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 
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happyfooddance
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These may be helpful
Attachment: CNMR_Dynamics.pdf (73kB)
This file has been downloaded 387 times
Attachment: carbonbondrotation.pdf (242kB)
This file has been downloaded 384 times
Attachment: toluenerotation.pdf (496kB)
This file has been downloaded 373 times
Attachment: ethanerotation.pdf (296kB)
This file has been downloaded 374 times
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]
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Wrecking Bereserker
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| 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...
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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]
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Metacelsus
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| 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
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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.
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qt314
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Many times molecules are referred as cis/trans in newmann projections. This may contribute to confusion.
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