As I have read online, temporary dipoles occur due to the random fluctuations of the electron cloud. The electrons move randomly to one side of the molecule creating a negative partial charge while the nucleus is "left on its own" on the other side - creating a partial positive charge.

My question is : If there is a permanent dipole , won't that stop the electrons from moving wherever they want ? They would be fixed to a certain region, right?

The only solution I see is that the "permanent dipole" isn't really permanent. Could it be that it fluctuates as well? That would allow the electrons to move around and create temporary dipoles.

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    $\begingroup$ Why would anybody care about temporary dipoles when a permanent one is around? But then again, no, a permanent dipole won't stop anything. Electrons in a polar molecule are no less and no more "free" than in any other molecule. $\endgroup$ – Ivan Neretin Nov 20 '18 at 16:10
  • $\begingroup$ I read online that dispersion forces play a greater role in the boiling temperatures of molecules than dipole-dipole forces, so according to that one would care about temporary dipoles... $\endgroup$ – lohey Nov 20 '18 at 16:35
  • $\begingroup$ IT isn't that temporary dipoles don't happen when a molecule has permanent dipoles, it is that they are far weaker and therefore less important. $\endgroup$ – matt_black Nov 21 '18 at 18:38

Both temporary and permanent dipoles will both play a part in what the boiling point of the compound is.

Permanent dipole-dipole forces acting between molecules will mean the majority of molecule will end up aligning themselves to minimise repulsion from partial charges. but this still doesn't stop van der Waal's forces acting between any two molecules. If the idea of electrons continually moving is thought about then it makes sense that just because a bonding pair may spend more time closer to one atom than the other, it doesn't mean they can still move about - so temporary charges can still arise as they move.

So the boiling point will be a result of the permanent and temporary dipole forces acting between molecules. But the contribution from temporary dipoles will be insignificant compared to permanent dipoles.


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