Differences in Rotation between σ-Bonds and π-Bonds

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Chris_Butler_1A
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Differences in Rotation between σ-Bonds and π-Bonds

Postby Chris_Butler_1A » Sun Dec 06, 2020 10:24 pm

I was wondering if someone could help clarify why pi-bonds restrict rotation while sigma-bonds allow for it. From what I can tell, the orientation of the electron density in relation to the intermolecular axis seems to play a role. Is it that the p-orbitals seen in pi-bonds are perpendicular to the intermolecular axis and thus if they were to rotate they would break? The diagrams that I found seem to have conflicting answers, but overall they seem to depict the pi-bonds seen in double and triple bonds as somewhat "around" the sigma-bonds. I appreciate any help as I figure this will also tie heavily into molecular shape.

Hasan Mirza 3F
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Re: Differences in Rotation between σ-Bonds and π-Bonds

Postby Hasan Mirza 3F » Sun Dec 06, 2020 10:30 pm

From what I've read, you are right in thinking that rotation would cause the pi bonds to break. This is because the pi bonds depend on the parallel orientation of the p orbitals and any rotation would result in the orbitals no longer being parallel.

Brennan McGurrr 3C
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Re: Differences in Rotation between σ-Bonds and π-Bonds

Postby Brennan McGurrr 3C » Sun Dec 06, 2020 10:31 pm

A pi bond restricts rotation due to having two regions of electron density overlap. The orbitals involved in the pi bond overlap side to side, so there are two overlapping regions in addition to the end to end overlap of the sigma bond. The bond cannot rotate without the pi bond breaking. It's like [censored] two things together with two screws and trying to twist it. With only one screw, the things could rotate.

Charlie Russell 2L
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Re: Differences in Rotation between σ-Bonds and π-Bonds

Postby Charlie Russell 2L » Sun Dec 06, 2020 10:34 pm

Pi-bonds have 2 areas of contact (the 2 p orbitals line up side to side!). Think of a ladder. If you try to move 1 ring but not the other, the ladder (bond) will break.

For sigma bonds, they have 1 point of contact (end to end), think about wringing out a towel (lol), you can twist either way without the towel (bond) breaking.

Chris_Butler_1A
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Re: Differences in Rotation between σ-Bonds and π-Bonds

Postby Chris_Butler_1A » Sat Dec 12, 2020 4:43 pm

Charlie Russell 3F wrote:Pi-bonds have 2 areas of contact (the 2 p orbitals line up side to side!). Think of a ladder. If you try to move 1 ring but not the other, the ladder (bond) will break.

For sigma bonds, they have 1 point of contact (end to end), think about wringing out a towel (lol), you can twist either way without the towel (bond) breaking.

That makes much more sense, thank you! Does this same logic also apply to Triple Bonds? From what I understand, Triple Bonds include one sigma bond and two pi bonds. So I would imagine that this would increase the areas of contact from 3 in a double bond to 5. Would I be correct in assuming this? Plus, would the adding of one pi bond to a triple bond make it even harder to bend than a double bond? For example, in a C double bond to O have more of a potential to bend as compared to a C with a triple bond to O?

Charlie Russell 2L
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Re: Differences in Rotation between σ-Bonds and π-Bonds

Postby Charlie Russell 2L » Thu Jan 14, 2021 11:02 pm

Sorry for the late response but I figured I would answer, regardless:

Yes, you are correct the the same logic does apply and the triple will have, in total 5 "points of contact."


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