(Polar molecules, Non-polar molecules, etc.)
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At the end of Wednesday's lecture this week, the molecule cis-dichloroethene was shown to be a polar molecule because its dipole moments did not cancel out, but I was wondering why the double bond plays a role in this. I thought that the dipole moments would cancel out since the molecule appears to be symmetrical. Could you explain where the dipole moments are and how they are affected by the double bond? Thanks in advance!
I may not be right, but the two Cl atoms are on the same side of the cis-dichloroethene molecule and thus on the same side of the double bond. Since Cl has a higher electronegativity than H, there is a greater shift of electrons towards the Cl atoms. And since they are on the same side of the double bond, they do not cancel out. I hope this helps!
The double bond in this case matters because this way the C atoms can't rotate. Cis means that both of the chlorine atoms are on the same side of the compound. C-Cl bond is polar while C-H bond is non-polar, so the overall dipole moment will point to the chlorine side.
Adding on to what Jiapeng and Brett have already said about this topic, the dichloroethene molecule cannot rotate because of the pi bond in its double bond. In his lecture, Dr. Lavelle discussed the cylindrical symmetry of sigma bonds which allow for atoms on the internuclear axis to rotate without being broken. However, only one sigma bond can form and in a double bond, the second pond is a pi bond. Since pi bonds are located on the side of the internuclear axis, it does not have the same cylindrical symmetry and will be broken if the internuclear axis were to rotate. This is why the double bond locks the carbons and other atoms into a fixed position. If the carbon could rotate and had only a single sigma bond, the dipoles on the chlorine connect to the carbons could rotate to cancel each other out to make dichloroethene a nonpolar molecule. However, this is not the case and if the carbons were to rotate with a double bond, the pi bond in the double bond would be broken.
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