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We usually get some questions about how many hydrogen bonds these compounds can create, we use it to differentiate between the boiling point of each one of them so it would be great if you could help me.

So what I know that $\ce{H2O}$ has 4 hydrogen bonds those are 2 from hydrogen atoms and 2 from other molecules that has 2 hydrogen atoms connected to the 2 lone pair of electron that the oxygen has.

In $\ce{HF}$, It has 2 hydrogen bonds. The same explanation implies here. In $\ce{NH3}$, 2 hydrogen bonds because $\ce{N}$ doesn't have the required amount of lone pairs to make more hydrogen bonds.

But what I really want to know is about alcohols. In the way that I am explaining it, it should have 2-3 hydrogen bonds but when I check some books and articles they say that it only creates 1.

So if you can explain how many each of them creates and what it depends on that would be awesome.

Freddy
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Biker
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    Interesting question. I suspect the answer is more complicated than you might expect. – bon Dec 31 '15 at 18:37
  • Like I am pretty sure there are some other factors that play here. That why I am asking about the alcohols. But the way of me thinking seems to work with everything except alcohols. – Biker Dec 31 '15 at 18:55
  • Just summing up the number of hydrogen bonds will not do, accepting and donating are two different things. – Martin - マーチン Jan 01 '16 at 09:33
  • Various papers suggest that the average number of hydrogen bonds per molecule in water is less than 4 and possibly as low as 2 and that it varies depending on temperature. I suspect a similar effect will be seen in alcohols but I can't find any specific references to it. – bon Jan 01 '16 at 11:03

2 Answers2

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Hydrogen bonds require a $\ce{\delta{+}}$ hydrogen atom capable of forming a hydrogen-bond, so $\ce{H2O}$ makes 2 H-bonds, and $\ce{NH3}$ has 3 H$\ce{\delta{+}}$ atoms capable of forming these bonds.

However hydrogen bonds also require an "active" lone pair capable of accepting a hydrogen-bond, where $\ce{H2O}$ can do 2 of these again, but $\ce{NH3}$ can only accept 1.

So, does water make 4 bonds or 2? Does ammonia make 1 bond or 2? The answer is both, but it depends on how you count it. If you look at an individual molecule in a cluster of water molecules it may appear to be surrounded and held in place by four H-bonds; it makes two with the hydrogen and accepts two with the lone pairs. That molecule is involved in 4 hydrogen bonds.

But if you take 100 water molecules and count how many hydrogen bonds there are between them, the answer will be about 200 because each molecules makes 2 bonds. If you consider each molecule making 4 bonds then you are double counting each bond being made and accepted. The same is true for ammonia, each molecule makes one hydrogen-bond, it also accepts one. You could say that an individual molecule is involved in two bonds, but if you count them all up each molecule only makes one each.

Because of these two different viewpoints, some texts will say water makes 4 and alcohols make 2 bonds, and some will say water makes 2 and alcohols make 1 bond. They are both right, but right in different ways. Whichever way you count them, you must be consistent. If this is related to an exam syllabus, the correct way is defined by that.

Spontification
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  • Well, It doesnt make sense. Ammonia will create 1 h bonds through it's hydrogen atom but it will also have another hydrogen atom that another molecule provides a H atom and the Nitrogen in the first molecules provides its pair of lone electrons. So as in total it has 2 of hydrogen bonds."

    Same applies to water.

     – Biker Dec 31 '15 at 19:41
  • @Biker that other hydrogen bond in created by another ammonia molecule. I'll try an edit to clarify. – Spontification Jan 01 '16 at 00:45
  • never thought about these different perspectives, Great! @Spontification – KingBoomie Sep 25 '16 at 08:22
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This a good question for discussion to clear concept on H-bonding in alcohol.

Hydrogen bonding occurs between molecules where you have a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen.

In the case of alcohols, there are hydrogen bonds set up between the slightly positive hydrogen atoms and lone pairs on oxygens in other molecules.

The hydrogen atoms are slightly positive because the bonding electrons are pulled away from them towards the very electronegative oxygen atoms.

enter image description here

Note: If you want to be fussy, the diagram is slightly misleading in that it suggests that all of the lone pairs on the oxygen atoms are forming hydrogen bonds. In an alcohol that can't happen. Taking the alcohol as a whole, there are only half as many slightly positive hydrogen atoms as there are lone pairs. At any one time, half of the lone pairs in the total liquid alcohol won't have hydrogen bonds from them because there aren't enough slightly positive hydrogens to go around. In the diagram, to show the 3-dimensional arrangement, the wedge-shaped lines show bonds coming out of the screen or paper towards you. The dotted bonds (other than the hydrogen bonds) show bonds going back into the screen or paper away from you.

solanki...
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  • So as a result from what the picture says, That it can create up to 3 bonds but if that happen then another molecule have to create 1 bond hydrogen bond. So In average alcohol can create 2 bonds for each molecule right? – Biker Dec 31 '15 at 19:10
  • yeah. 2 H-bonds per molecule of alcohol – solanki... Dec 31 '15 at 19:11
  • There are also such things as bifurcated hydrogen bonds and there is the rest of the molecule to consider and the steric effects that it has. These complicate matters and I would be interested to see a higher level answer than this. – bon Dec 31 '15 at 20:51