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Cl is an electronegative element and hydrogen is electropositive. Why are intermolecular interactions involving H and Cl not considered as H bonding? I read it in a book but there was no reason given for it.

Mithoron
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Srijan
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  • H bonds have their bond energies in the order of ~10-20 kJ per mol. The bond energy of H-Cl is ~427 kJ per mol, which is too strong to be a H-bond – Aniruddha Deb Oct 26 '20 at 04:17
  • Why does H bonding depend on bond energy? – Srijan Oct 26 '20 at 04:21
  • I am asking this because there are the molecules needed to form H bond.Then why does the bond energy I.e the energy required to break their bond matters here? – Srijan Oct 26 '20 at 04:22
  • Also Cl has less charge density.How is the bond so strong? – Srijan Oct 26 '20 at 04:23
  • Please see the definition of Hydrogen Bond here (the first paragraph). – Aniruddha Deb Oct 26 '20 at 04:24
  • I did read it . – Srijan Oct 26 '20 at 04:37
  • Cl is more electronegative than N and less than O. – Srijan Oct 26 '20 at 04:38
  • You can't have hydrogen bonds as the basis for forming the molecule itself, there has to be at least one covalent or ionic bond inside the molecule. The hydrogen bond then features as an electrostatic interaction between a bonded hydrogen and another sufficiently electronegative group, which will act as the H-bond acceptor – Yusuf Hasan Oct 26 '20 at 04:42
  • What do you mean by inside the molecule? – Srijan Oct 26 '20 at 04:46
  • Cl and H are forming polar covalent bond. – Srijan Oct 26 '20 at 04:47
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    A hydrogen bond by definition needs three centers to be formed: the donor atom(Dn), the hydrogen itself (H) and an acceptor atom (Ac) to form a setup like Dn–H···Ac .A bond between H and Cl has only two centres: the hydrogen(H) and chlorine(Cl). Without a third center to act as the donor or acceptor (depending on the convention you have followed) you can't have a "hydrogen bond" per se, it will just be a regular covalent or ionic bond – Yusuf Hasan Oct 26 '20 at 04:48
  • H20 has bonding because there is one Ac(O) and one Dn(H) with H.Does it mean there have to be 3 atoms for H bonding to form. – Srijan Oct 26 '20 at 04:53
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    Yes, in each hydrogen bond of H2O, you can trace out three centres : O-H···O, where the O from one water molecule acts as the donor and also the first center, the hydrogen attached with donor oxygen forms the second center, while the oxygen from another water molecule as an acceptor and the third center – Yusuf Hasan Oct 26 '20 at 05:05
  • Ok thank you very much.Just one question , there are no 2 shared pair of electrons in Cl to share with HCL whereas in H20 O could bond with 2 H atoms.IS THIS RIGHT TO SAY? – Srijan Oct 26 '20 at 05:08
  • @YusufHasan, you basically have an entire answer there. Can I suggest converting it to an answer? – orthocresol Oct 26 '20 at 14:10
  • @Mithoron, surprised you didn't choose one of the better questions linked on that one. :-) – orthocresol Oct 26 '20 at 14:21
  • @orthocresol I am afraid that the meaning of the question is slightly different than what I originally assumed after looking at the edit. I believed the OP was asking why is the intramolecular covalent bond b/w H and Cl not called an H-bond, but after the edit it seems that the question was about the intermolecular interactions between multiple HCl molecules. Hence I feel my answer above was not what the OP wanted – Yusuf Hasan Oct 27 '20 at 00:48
  • @YusufHasan, hmm... that isn't OP's edit so arguably it doesn't tell us anything about what OP intended. In any case, I see what you mean. – orthocresol Oct 27 '20 at 00:54
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    @orthocresol Maybe closing as unclear would be better idea... – Mithoron Oct 27 '20 at 13:59

1 Answers1

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As per my teacher, the reason we say that Chlorine does not form hydrogen bonds, even though its electronegativity is almost same as N (both have electronegativity values around 3.0 as per the Pauling Scale) is because :

Chlorine being a third period element, has a relatively larger size as compared with Nitrogen and Oxygen, so it has a quite large and more diffused electron cloud. Therefore the dipole-dipole interactions in HCl are not as strong as the ones encountered in HF and H₂O, and so it wouldnt be quite accurate for them to be called as Hydrogen Bonds, as the ones in HF and H₂O are much stronger.

Hope this answered your question.

SpaceSkiier
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    This is not the inherent reason why the bonding between H and Cl is not considered hydrogen bonding, as on the Pauling scale , the electronegativity comes quite close to N, which is often involved in H bonds. The reason in this case has more to do with the definition of what we call a hydrogen bond, check my comments above for more details on the same. I believe that you have answered something else, while the OP has a misconception on a different aspect – Yusuf Hasan Oct 26 '20 at 08:32
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    As to why HCl might not show H-bonding in an extended network at low concentrations,yes, your reasoning could work as explained here in a related question. Although do note that clusters of HCl in a concentrated form may be obtained as shown here.As I said, I believe you have answered something else, and the OP was confused on a different point.Please modify accordingly – Yusuf Hasan Oct 26 '20 at 08:36
  • I do believe this was the actual question which the OP intended to ask...! I myself have had this doubt before. – SpaceSkiier Oct 26 '20 at 11:33
  • I think the question is ambiguous enough to admit both interpretations. I wouldn't downvote just based on that. But also, this question has been addressed elsewhere on SE. – orthocresol Oct 26 '20 at 14:19