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Dipole–dipole forces

Dipole-dipole forces occur between molecules with permanent dipoles (i.e., polar molecules). For molecules of similar size and mass, the strength of these forces increases with increasing polarity. Polar molecules can also induce dipoles in nonpolar molecules, resulting in dipole–induced dipole forces. Created by Sal Khan.

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  • starky seed style avatar for user Runtian Du
    Is dipole dipole forces the permanent version of London dispersion forces?
    (19 votes)
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    • male robot hal style avatar for user DogzerDogzer777
      Pretty much. Some molecules are arranged in ways where atoms with relatively high electronegativity are on one side while atoms with relatively low electronegativity are on the other. This causes an imbalance of electrons, which makes a permanent dipole as the electrons of the molecule tend to stay closer to the more electronegative atom. I'd actually say that London dispersion forces are just temporary dipole-dipole forces, in fact.
      (17 votes)
  • stelly blue style avatar for user Jordan Roland
    why is it called dipole-dipole
    (5 votes)
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    • male robot hal style avatar for user vinlegend1
      Let's start with an example. Take hydrogen-fluoride for example, we know that fluorine has a high electronegativity, and hydrogen has a low electronegativity relative to fluorine. That means the electrons shared by the covalent bond will "gravitate" or "move" towards the fluorine atom, thus making a dipole. Hydrogen would be partially positive in this case while fluorine is partially negative. An interaction with another "dipoled" molecule would attract the partially positive to the other molecule's partial negative. Thus, the name dipole-dipole.
      (24 votes)
  • leaf red style avatar for user Tejas Singh Sodhi
    Can temporary dipoles induce a permanent dipole? At the end of the video sal says something about inducing dipoles but it is not clear. Could someone tell if temporary dipoles induce permanent ones (or only permanent-permanent/temporary-temporary can be induced)?
    (8 votes)
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  • leafers seed style avatar for user Minkyu Koo
    How can you tell if the intermolecular force is dipole-dipole just by being given the molecular formula?
    (4 votes)
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    • leaf red style avatar for user Richard
      You could if you were really experienced with the formulae. Otherwise you would need the correct Lewis structure to work out if dipole-dipole forces are at play. So you first need to build the Lewis structure if you were only given the chemical formula.

      Hope that helps.
      (6 votes)
  • duskpin ultimate style avatar for user semyonche
    what if we put the substance in an electric field, molecules become more polar, will it cause higher intermolecular forces?
    (4 votes)
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    • leafers ultimate style avatar for user Blake
      It will not become polar, but it will become negatively charged. Those two things are very different from each other because polar molecules have a positive and negative end, or "pole". An electrified atom will keep its polarity the exact same.

      Perhaps the sample will have higher london dispersion forces as the electron cloud will grow substantially, but it will not have any effect on Dipole Dipole
      (4 votes)
  • male robot donald style avatar for user Youssef ElBanna
    Does that mean that Propane is unable to become a dipole? Or is it hard for it to become a dipole because it is a symmetrical molecule?
    (2 votes)
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    • leaf red style avatar for user Richard
      Both molecules have London dispersion forces at play simply because they both have electrons. Any molecule which has London dispersion forces can have a temporary dipole. So in that sense propane has a dipole.

      However pentane has no polar bonds and therefore is not considered a polar molecule. Because of this lack of polarity it cannot form a permanent dipole like acetaldehyde can. And in this sense propane does not have a dipole and cannot engage in dipole-dipole.

      Overall though since every molecule has London dispersion forces and forms temporary dipoles we don't really consider every molecule having a dipole since these aren't permanent dipoles. Only molecules with polar bonds and a dipole moment do we consider having true dipoles since they are permanent. So propane cannot form a dipole because it has no polar bonds to form dipole moments.

      Hope that helps.
      (7 votes)
  • starky tree style avatar for user Corey.Jason.King
    Does anyone here know where to find the Dipole Moments video referenced by Khan in the video? Can't quite find it through the search bar.
    (5 votes)
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  • blobby green style avatar for user victoria omotolani
    What are asymmetric molecules and how can we identify them.
    (4 votes)
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  • eggleston green style avatar for user lennyarms
    I know all molecules have London dispersion forces, but how does it work in polar molecules? There is already a dipole in polar molecules so where are the dipoles in the London dispersion forces?
    (3 votes)
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  • blobby green style avatar for user Maanya
    Why are dipole-induced dipole forces permanent?
    (2 votes)
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    • leaf red style avatar for user Richard
      That sort of interaction depends on the presence of the permanent dipole which as the name suggests is permanently polar due to the electronegativities of the atoms. Electronegativity is constant since it is tied to an element's identity. So if you have a permanently polar molecule then it can create a constant induced dipole in nearby nonpolar molecules.

      Hope that helps.
      (3 votes)

Video transcript

- [Instructor] So I have these two molecules here, propane on the left and acetaldehyde here on the right. And we've already calculated their molar masses for you, and you see that they have very close molar masses. And so based on what you see in front of you, which of these, you think, would have a higher boiling point, a sample of pure propane or a sample of pure acetaldehyde? Pause this video, and think about that. All right, well, in previous videos, when we talked about boiling points and why they might be different, we talked about intermolecular forces. Because you could imagine, if you have a bunch of molecules, let's say, in a liquid state, the boiling point is going to be dependent on how much energy you need to put into the system in order for the intermolecular forces between the molecules to be overcome so that molecules could break free and enter into a gaseous state. And so when we're thinking about which might have a higher boiling point, we really just need to think about which one would have higher intermolecular forces. Now, in a previous video, we talked about London dispersion forces, which you can view as random dipoles forming in one molecule, and then that can induce dipoles in a neighboring molecule. And then the positive end, even temporarily positive end, of one could be attracted to the temporarily negative end of another and vice versa, and that whole phenomenon can domino. And we said that you're going to have more of those London dispersion forces the more polarizable your molecule is, which is related to how large of an electron cloud it has, which is related to its molar mass. And when we look at these two molecules, they have near identical molar masses. So you might expect them to have near identical boiling points, but it turns out that that is not the case. The boiling point of propane is negative 42.1 degrees Celsius, while the boiling point of acetaldehyde is 20.1 degrees Celsius. So what makes the difference? Why does acetaldehyde have such a higher boiling point? Why does it take more energy for the molecules in liquid acetaldehyde to be able to break free of each other to overcome their intermolecular forces? Well, the answer, you might imagine, is other things are at play on top of the London dispersion forces. And what we're going to talk about in this video is dipole-dipole forces. So you might already imagine where this is going. In the video on London dispersion forces, we talked about a temporary dipole inducing a dipole in a neighboring molecule and then them being attracted to each other. Now we're going to talk about permanent dipoles. So when you look at both of these molecules, which one would you think has a stronger permanent dipole? Or another way of thinking about it is which one has a larger dipole moment? Remember, molecular dipole moments are just the vector sum of all of the dipole moments of the individual bonds, and the dipole moments are all proportional to the differences in electronegativity. When we look at propane here on the left, carbon is a little bit more electronegative than hydrogen but not a lot more electronegative. So you will have these dipole moments on each of the bonds that might look something like this. So you would have these things that look like that. If that is looking unfamiliar to you, I encourage you to review the videos on dipole moments. But as you can see, there's a symmetry to propane as well. So if you were to take all of these arrows that I'm drawing, if you were to take all of these arrows that I'm drawing and net them together, you're not going to get much of a molecular dipole moment. You will get a little bit of one, but they, for the most part, cancel out. Now what about acetaldehyde? Well, acetaldehyde, there's a few giveaways here. One is it's an asymmetric molecule. So asymmetric molecules are good suspects for having a higher dipole moment. Another good indicator is you have some character here that's quite electronegative. In this case, oxygen is quite electronegative. And even more important, it's a good bit more electronegative than carbon. So right over here, this carbon-oxygen double bond, you're going to have a pretty significant dipole moment just on this double bond. It might look like that. And all of the other dipole moments for all of the other bonds aren't going to cancel this large one out. In fact, they might add to it a little bit because of the molecule's asymmetry. And so net-net, your whole molecule is going to have a pretty significant dipole moment. It'll look something like this, and I'm just going to approximate it. But we're going to point towards the more negative end, so it might look something like this, pointing towards the more negative end. And I'll put this little cross here at the more positive end. And so you would expect a partial negative charge at that end and a partial positive charge at this end. And so what's going to happen if it's next to another acetaldehyde? Well, the partially negative end of one acetaldehyde is going to be attracted to the partially positive end of another acetaldehyde. And so this is what people are talking about when they say dipole-dipole forces. We are talking about a permanent dipole being attracted to another permanent dipole. And so acetaldehyde is experiencing that on top of the London dispersion forces, which is why it has a higher boiling point. Now some of you might be wondering, hey, can a permanent dipole induce a dipole in a neighboring molecule and then those get attracted to each other? And the simple answer is yes, it makes a lot of sense. You can absolutely have a dipole and then induced dipole interaction. And we might cover that in a few examples in the future, but this can also occur. You can have a temporary dipole inducing a dipole in the neighbor, and then they get attracted to each other. And you could have a bit of a domino effect. You can have a permanent dipole interacting with another permanent dipole. They get attracted to each other. And you could have a permanent dipole inducing a dipole in a neighboring molecule.