Pi bonds

Deborah Cheng 1F
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Joined: Thu Jul 13, 2017 3:00 am

Pi bonds

Why can't atoms bound by pi bonds rotate like atoms bound by sigma bonds?

Nancy Le - 1F
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Joined: Fri Sep 29, 2017 7:07 am

Re: Pi bonds

Since the electrons overlap side-by-side in pi bonds, rotating one atom would break the parallel orientation of the orbitals.

Mika Sonnleitner 1A
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Joined: Fri Sep 29, 2017 7:04 am
Been upvoted: 2 times

Re: Pi bonds

If we look at C2H4 as an example, there is a pi bond between the two carbons. However, assuming you are looking at the Lewis structure of C2H2, you must think of the pi bonds as having one lobe pointing out of the page towards you, and the second lobe pointing back behind the page.

This picture might help with the visualization: http://assets.openstudy.com/updates/att ... ibonds.gif

Essentially, the pi bonds are at a 90 degree angle with the sigma bonds. The pi bonds do not allow free rotation around the axis, because the pi bond is a weak bond (since it is the most exposed). Therefore, rotating the pi bond would break the bond.

Posts: 52
Joined: Fri Sep 29, 2017 7:04 am

Re: Pi bonds

To have a pi bond present means that there is already a sigma bond present, thus making the bond strength much stronger. Additionally, pi bonds will bond side by side, so if the atoms rotate, then the pi bonds will be broken.

Sonja Kobayashi 1H
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Joined: Fri Sep 29, 2017 7:04 am

Re: Pi bonds

So does that mean that if there is a sigma bond, there will always be a pi bond as well?

Erik Khong 2E
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Joined: Fri Sep 29, 2017 7:07 am

Re: Pi bonds

Sonja Kobayashi 1H wrote:So does that mean that if there is a sigma bond, there will always be a pi bond as well?

No this is not the case. In fact, it's backwards. If there is a pi bond, there is always a sigma bond. This is because sigma bonds are ALWAYS the first bond to be made between two atoms. Any others will result in pi bonds.

Sophie 1I
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Joined: Fri Sep 29, 2017 7:04 am

Re: Pi bonds

Pi bonds can only overlap side by side with electron densities on each side of the internuclear axis so if it rotated the bond would be broken.