HCl does not form conventional hydrogen bonds, but in 2 Special Cases It can. Key reasons:
- Cl atom size is larger than N, O, F, resulting in lower electron density, insufficient for strong H-bonds.
- HCl lacks anomalous boiling point seen in H-bonding molecules like HF, H2O, NH3.
- Dipole-dipole forces predominate, not H-bonds.
- Exceptions:
- Weak H-bonding observed between HCl and π-acceptors like vinylacetylene.
- Some evidence of H-bonding in concentrated HCl solutions, likely between H2O and HCl molecules
Reasons Why HCl Does Not Form Conventional Hydrogen Bonds
Size of the Chlorine Atom
- Chlorine has a larger atomic size compared to nitrogen, oxygen, and fluorine.
- The larger size results in a lower electron density around the chlorine atom.
- Insufficient electron density prevents the formation of strong hydrogen bonds.
Lack of Anomalous Boiling Point
- Molecules exhibiting hydrogen bonding (e.g., HF, H2O, NH3) have anomalously high boiling points compared to other hydrides in their group.
- The boiling points of hydrogen halides (HCl, HBr, HI) increase linearly with their molecular mass.
- The linear increase in boiling points indicates the absence of hydrogen bonding in HCl.
Predominance of Dipole-Dipole Forces
- HCl is a polar molecule, but the hydrogen is not bonded to N, O, or F.
- The main intermolecular forces in HCl are dipole-dipole interactions and London dispersion forces, not hydrogen bonds.
| Molecule | Boiling Point (°C) | Hydrogen Bonding |
|---|---|---|
| HF | 19.5 | Yes |
| HCl | -85.1 | No |
| HBr | -66.8 | No |
| HI | -35.4 | No |
Specific Cases of Weak or Unconventional Hydrogen Bonding in HCl
1. Hydrogen Bonding with π-Acceptors
- An investigation found evidence of hydrogen bonding between HCl and vinylacetylene.
- HCl forms a weak hydrogen bond with the π-electrons of the carbon-carbon triple bond.
2. Hydrogen Bonding in Concentrated HCl Solutions
- X-ray diffraction studies have shown some evidence of hydrogen bonding in concentrated hydrochloric acid solutions.
- The hydrogen bonding likely occurs between water molecules and HCl.
Conclusion
In conclusion, HCl does not form conventional hydrogen bonds under most conditions due to the larger size and lower electron density of the chlorine atom compared to nitrogen, oxygen, and fluorine. The predominant intermolecular forces in HCl are dipole-dipole interactions and London dispersion forces. However, in specific cases, such as interactions with π-acceptors or in concentrated HCl solutions, weak or unconventional hydrogen bonding has been observed.
