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Hydrogen bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs between the lone pair of a highly electronegative atom (typically N, O, or F) and the hydrogen atom in a N–H, O–H, or F–H bond. Hydrogen bonds can form between different molecules (intermolecular hydrogen bonding) or between different parts of the same molecule (intramolecular hydrogen bonding). Created by Sal Khan.

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  • blobby green style avatar for user Varad Walde
    Well, I still didn't get the difference between dipole-dipole forces and hydrogen bonds. Could anyone explain?
    (13 votes)
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    • aqualine ultimate style avatar for user famousguy786
      Hydrogen bonds are a special type of dipole-dipole forces. In hydrogen bonding, the dipole-dipole attraction occurs between hydrogen and a highly electronegative atom(O,F,Cl,etc). Hydrogen bonds are much stronger than a general dipole-dipole force.These are the only differences, otherwise everything is same. I hope this helps.
      (29 votes)
  • duskpin ultimate style avatar for user Mohammad Farhan Sadik
    why are there differences between the boiling points of compounds with hydrogen bonds?
    (7 votes)
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  • starky seed style avatar for user Dishita
    Why do we consider only hydrogen bonds? Why not something like Cesium or Francium bonds? Shouldn't attraction between the δ+ and δ- parts be more?

    I mean, why don't we consider a bond between a more electro+ve atom (wrt H) and a highly electro-ve atom? Wouldn't the δ+ and δ- (as well as the net dipole moment)gained be greater as the change in electro-vety increases?

    Is this because H is smaller and can hence form more stable bonds?
    (2 votes)
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    • leaf red style avatar for user Richard
      Well atoms like cesium or francium have very small electronegativity values. And so pairing them with the electronegative atoms commonly associated with hydrogen bonding (nitrogen, oxygen, and fluorine) will have a large enough electronegativity difference that their bonding is no longer considered polar covalent, but rather ionic. So we'll start having ionic interactions instead of dipole-dipole interactions which includes hydrogen bonding.

      The reason we focus on hydrogen specifically is a result of several reasons. First, it's simply a commonplace element in many crucial molecules like water. Second the electronegativity difference between hydrogen and small electronegative atoms is large enough to be considered polar covalent, but not so much that it becomes ionic. Third, since these atoms all quite small, the hydrogen atom on one molecule can approach the small electronegative atom (again N, O, and F) on another molecule very closely. So fundamentally it's the same interaction which occurs in dipole-dipole, but because of the atom's sizes this attraction is enhanced and is stronger than other conventional dipole-dipole interactions.

      Hope that helps.
      (7 votes)
  • leaf blue style avatar for user Jyotiraditya Pradhan
    My textbook shows HCl under examples of dipole-dipole Interaction but not under Hydrogen Bonds. So do HCl molecules not form Hydrogen bonds? If so, why?
    (2 votes)
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    • leaf red style avatar for user Richard
      Well for hydrogen bonding to occur you want the hydrogen atom bonded to an electronegative atom (acting as the hydrogen bond donor) interacting with another electronegative atom (acting as the hydrogen bond acceptor). You also want these electronegative atoms to be small the that donor and acceptor can approach each other more closely adding to the strength of the hydrogen bond. Usually this means atoms like fluorine, oxygen, and nitrogen, but this can also include atoms like carbon, chlorine, and sulfur. Part of the reason why these other atoms aren't commonly recognized as engaging hydrogen bonding is that compared to the traditional atoms, their hydrogen bonding is quite weak. And this is due to the nontraditional atoms either being not as electronegative (in the case of carbon), or not as small (in the case of chlorine and sulfur) compared to the traditional atoms.

      But nowadays it is recognized more elements than just fluorine, oxygen, and nitrogen engage in hydrogen bonding (would would include hydrochloric acid). The strengths of these hydrogen bonds just vary widely.

      Hope that helps.
      (6 votes)
  • leafers sapling style avatar for user sophiaw1445
    How electronegative does an atom have to be to form a hydrogen bond? Is it only N, O, & F that can form these bonds?
    (1 vote)
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    • leaf red style avatar for user Richard
      There’s no single electronegativity number, but generally the nonmetals of the upper right of the periodic table engage in hydrogen bonding. The criteria we want for hydrogen bonding is small, electronegative atoms. The elements which best fit those requirements are fluorine, oxygen, and nitrogen. And indeed those are the most common, but they are not the only instances of hydrogen bonding. We’ve also observed hydrogen bonding with elements such as chlorine, sulfur, and certain carbon molecules.

      Hope that helps.
      (4 votes)
  • blobby purple style avatar for user Ella :)
    Are there hydrogen bonds between water and HCl (if not, why, since oxycen has lone pair electrons and hydrogen has delta + ?)? Or do hydrogen bonds only form between same molecules (like HCl and HCl)?
    (2 votes)
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    • male robot hal style avatar for user Johnny
      Hydrogen bonds can form between different molecules, as long as one molecule has H and the other has N, O, or F. So yes, we can have hydrogen bonding between one H2O molecule and one HCl molecule, in which case the O molecule in H2O forms a hydrogen bond with the H from HCl.
      (2 votes)
  • blobby green style avatar for user renashoby
    what about dipole induced dipole forces......can u please explain deeply about that
    (1 vote)
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  • blobby green style avatar for user Steven Chelney
    Isn't Chlorine more electronegative than Nitrogen? Why is nitrogen usually seen in a hydrogen bond, but chlorine isn't?

    Also, what makes hydrogen special? Wouldn't francium or another element be better because of its low electronegativity? I'm having trouble understanding why hydrogen bonds are significant and why they are so strong.
    (2 votes)
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    • blobby green style avatar for user philip duvall
      I believe Nitrogen and Chlorine bear the same value for electronegativity, and what makes Nitrogen a better atom for Hydrogen to H-Bond with is the size of its highest occupied molecular orbital. Nitrogen has a smaller highest occupied molecular orbit than Chlorine. A greater difference between the lowest occupied molecular orbital (which belongs to hydrogen in both cases) and highest occupied energy orbital will lead to less mixing between orbitals. Since Nitrogen carries a smaller unit area than Chlorine, it has a greater charge density which then leads to a greater attraction.
      (2 votes)
  • eggleston blue style avatar for user Harsh
    At Sal says "In fact all the molecules have similar molar mass" but the molar mass of all the molecules are not same
    For instance, NH3 has a molar mass of 17g/mol while HF has a molar mass of 20g/mol

    What is the thing thatI am missing?
    (1 vote)
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  • aqualine tree style avatar for user Ink
    Why does pentanone not have hydrogen bonds even though it contains oxygen and hydrogen?
    (1 vote)
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    • leaf red style avatar for user Richard
      For hydrogen bonding to occur, there not only needs to be a small electronegative atom like oxygen, but also have that small electronegative atom must be directly bonded to hydrogen.

      Pentanone (both the 2-pentanone and 3-pentanone isomers) are known as ketones in organic chemistry. This means there is a carbonyl group where a carbon is double bonded to an oxygen and that carbon has two additional carbon groups bonded to it. So the sole oxygen in a ketone is bonded to a carbon and only a carbon. So since the oxygen is not directly bonded to a hydrogen, it doesn’t display hydrogen bonding.

      Hope that helps.
      (2 votes)

Video transcript

- [Instructor] Let's talk about hydrogen bonds. Depicted here, I have three different types of molecules. On the left, I have ammonia. Each ammonia molecule has one nitrogen bonded to three hydrogens. In the middle, I have something you're probably very familiar with, in fact, you're made up of it, which is water. Each oxygen is bonded to two hydrogens. And then here on the right, I have hydrogen fluoride. Each fluorine is bonded to one hydrogen. Now, why are these types of molecules interesting? And what does that have to do with hydrogen bonds? And the simple answer is, in each of these cases, you have hydrogen bonded to a much more electronegative atom. Even though these are covalent bonds, they're going to be polar covalent bonds. You are going to have a bond dipole moment that goes from the hydrogen to the more electronegative atom, from the hydrogen to the more electronegative atom, from the hydrogen to the more electronegative atom. The more electronegative atom is going to hog the electrons. The electrons are gonna spend more time around that. So that end of the molecule is going to have a partial negative charge. And then the ends with the hydrogens, those are gonna have partial positive charges. Another way to think about it is, if you added these dipole moments, you would have a net dipole for the entire molecule that would look something like that. So we are dealing with polar molecules. And the polarity comes from both the asymmetry, and you have a very electronegative atom bonded to hydrogen, oxygen, very electronegative atom, bonded to hydrogen. So this end of the molecule is partially negative. This end of the molecule or these ends of the molecule are partially positive. For hydrogen fluoride, this end is partially positive. This end is partially negative. And so what do you think could happen when these molecules interact with each other? The nitrogen end right over here, of this ammonia, could be attracted to one of these hydrogens that has a partially positive charge right over there. Or this hydrogen, the partial positive charge, might be attracted to that nitrogen that has a partial negative charge. And this attraction between the partial positive hydrogen end and the partially negative end of another molecule, those are hydrogen bonds. And they are an intermolecular force that will be additive to the total intermolecular force from, say, things like London dispersion forces, which makes you have a higher boiling point than you would have if you just thought about London dispersion forces. And to make that clear, you can look at this chart. You can see all of these molecules are formed between period two elements and hydrogen. In fact, all of these molecules have similar molar masses, methane, ammonia, hydrogen fluoride, and water. If we were just thinking about London dispersion forces, London dispersion forces are proportional to the polarizability of a molecule, which is proportional to the electron cloud size, which is proportional to the molar mass. And generally speaking, as you go from molecules formed with period two elements to period three elements to period four elements to period five elements, you do see that as the molar mass of those molecules increase, there is that general upward trend of the boiling point, and that's due to the London dispersion forces. But for any given period, you do see the separation. And in particular, you see a lot of separation for the molecules formed with oxygen, fluorine, and nitrogen. These molecules, despite having similar molar masses, have very different boiling points. So there must be some other type of intermolecular forces at play above and beyond London dispersion forces. And the simple answer is yes. What you have at play are the hydrogen bonds. Now, some of you might be wondering, well, look at these molecules formed with period three elements and hydrogen or period four elements and hydrogen, they also don't have the same boiling point, even though you would expect similar London dispersion forces because they have similar molar masses. And the separation that you see here in boiling points, this, too, would be due to other things, other than London dispersion forces. In particular, dipole-dipole forces would be at play. But what you can see is the spread is much higher for these molecules formed with nitrogen and hydrogen, fluorine and hydrogen, and oxygen and hydrogen. And that's because hydrogen bonds can be viewed as the strongest form of dipole-dipole forces. Hydrogen bonds are a special case of dipole-dipole forces. When we're talking about hydrogen bonds, we're usually talking about a specific bond dipole, the bond between hydrogen and a more electronegative atom like nitrogen, oxygen, and fluorine. And so we're specifically talking about that part of the molecule, that hydrogen part that has a partially positive charge being attracted to the partially negative end of another molecule. So it's really about a bond dipole with hydrogen bonds versus a total molecular dipole when we talk about dipole-dipole interactions in general. And so you could imagine, it doesn't even just have to be hydrogen bonds between a like molecule. You could have hydrogen bonds between an ammonia molecule and a water molecule or between a water molecule and a hydrogen fluoride molecule. And I mentioned that these are really important in biology. This right over here is a closeup of DNA. You can see that the base pairs in DNA, you can imagine the rungs of the ladder, those are formed by hydrogen bonds between base pairs. So those hydrogen bonds are strong enough to keep that double helix together, but then they're not so strong that they can't be pulled apart when it's time to replicate or transcribe the DNA. Hydrogen bonds are also a big deal in proteins. You learn in biology class that proteins are made up of chains of amino acids, and the function is heavily influenced by the shape of that protein. And that shape is influenced by hydrogen bonds that might form between the amino acids that make up the protein. So hydrogen bonds are everywhere. There are many hydrogen bonds in your body right now mainly, not just because of the DNA, mainly because you're mostly water. So life, as we know it, would not exist without hydrogen bonds.