trans- & cis- Molecules

[Jonathan Batac - 2D]

I think Lavelle said something about how sigma and pi bonds have an effect on how a molecule like dichloroethene can be either in a fixed shape (cis-dichloroethene = polar) or loose shape (trans-dichloroethene = nonpolar). Are there any other kind of molecules that can help further emphasize and visualize this concept?

[Jessie Hsu 1C]

I believe that both cis-dichloroethene and trans-dichloroethene are fixed and cannot rotate because of the double bond between the two carbons. I think only single bonds are "loose" and can rotate because it is the only bond that doesn't have pi bonds.

[Pranav Daggubati 3C]

Ethenes are fixed molecules and cannot rotate. However, visualizing cis versus trans molecules by drawing them out and then identifying the dipoles is a way to make this easier.

[Chinyere Okeke 2J]

I believe that in the case of cis-dichloroethene and trans-dichloroethene what makes them non-polar and polar is the dipole moments and how they either cancel out or don't cancel out. They can't rotate because of the double bond between the two C's which fixes them in place, but I would look at the dipole moments to determine polarity, at least in this case.

[Jack_Pearce_2H]

honestly I usually just check out the dipole moments to help visualize if the molecule will be polar or non polar. If all dipole moments offset one another=nonpolar. If they are leaning or pushing to one side, they are polar. an example would be cis- and trans-dimethylcyclohexane

[Ryan Hoang 1D]

I believe that both cis-dichloroethene and trans-dichloroethene are fixed and cannot rotate because of the double bond between the two carbons. I think only single bonds are "loose" and can rotate because it is the only bond that doesn't have pi bonds.

Yeah I have to agree with Jessie here, I think you might be misinterpreting the difference between the two. The two molecules both have fixed carbon atoms, and won't be able to change from being cis (chlorines on the same side) vs trans (chlorine on opposite sides). However, it is the dipole moments that result from these different distributions that cause polarity in molecules, yes.

[Rob Tsai 2F]

I believe that both cis-dichloroethene and trans-dichloroethene are fixed and cannot rotate because of the double bond between the two carbons. I think only single bonds are "loose" and can rotate because it is the only bond that doesn't have pi bonds.

Yeah I have to agree with Jessie here, I think you might be misinterpreting the difference between the two. The two molecules both have fixed carbon atoms, and won't be able to change from being cis (chlorines on the same side) vs trans (chlorine on opposite sides). However, it is the dipole moments that result from these different distributions that cause polarity in molecules, yes.

The use of these molecules as examples was to show how a molecule can be polar or nonpolar depending on whether or not the dipole moments end up canceling each other out. To Jonathan's original question, though, I think generally that if a molecule has a double bond, the whole molecule has to move on the same axis, while if a molecule is purely connected by single bonds, it can rotate on the axis of the bond itself since it wouldn't be breaking any bonds by doing so. Correct me if I'm wrong, though.