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Sigma and Pi Bonds

Posted: Sun Nov 25, 2018 10:58 pm
by Nell Mitchell 1E
I understand the rules regarding sigma and pi bonds - that sigma can rotate because the bond is on the axis but pi bonds can't because they're not on the axis - but I can't visualize how the pi bonds would break from rotation. Can someone explain this?

Re: Sigma and Pi Bonds

Posted: Sun Nov 25, 2018 11:03 pm
by Andre_Galenchik_2L
Just try to remember the demonstration that Dr. Lavelle did during class. He put two sticks between his two index fingers and tried to turn his index fingers. From rotating his fingers, the sticks fell out of their formation.

Re: Sigma and Pi Bonds

Posted: Sun Nov 25, 2018 11:29 pm
by Luc Lorain 1L
It can help to use the example of a figure skater to imagine this concept. When the figure skater is balanced on one foot, they can rotate in only one direction, resulting in smooth rotation-- this would be analogous to the range of motion of a sigma bond. When the figure skater places both feet on the ground they must both be moving in the same direction (rotating the entire figure skater rather than one point on the ground). If the two legs had free range of rotation, they could rotate in opposite directions-- which would cost the figure skater a broken pelvis at the very least if they were to attempt it. That is the same way a pi bond operates-- the whole molecule must rotate in a unified direction along the pi bond.