Hydrogen chloride

The structure of solid DCl, as determined by neutron diffraction of DCl powder at 77 K. DCl was used instead of HCl because the deuterium nucleus is easier to detect than the hydrogen nucleus. The extensible linear structure is indicated by the dashed lines.

Frozen HCl undergoes phase transition at 98.4 K. X-ray powder diffraction of the frozen material shows that the material changes from an orthorhombic structure to a cubic one during this transition. In both structures the chlorine atoms are in a face-centered array. However, the hydrogen atoms could not be located.[9] Analysis of spectroscopic and dielectric data, and determination of the structure of DCl (deuterium chloride) indicates that HCl forms zigzag chains in the solid, as does HF (see figure on right).[10]

Infrared (IR) absorption spectrum

One doublet in the IR spectrum resulting from the isotopic composition of chlorine

The infrared spectrum of gaseous hydrogen chloride, shown on the left, consists of a number of sharp absorption lines grouped around 2886 cm⁻¹ (wavelength ~3.47 µm). At room temperature, almost all molecules are in the ground vibrational state v = 0. Including anharmonicity the vibrational energy can be written as.

${\displaystyle E_{\mathrm {v} }=h\nu _{e}\left(v+{\tfrac {1}{2}}\right)+hx_{e}\nu _{e}\left(v+{\tfrac {1}{2}}\right)^{2}}$

To promote an HCl molecule from the v = 0 to the v = 1 state, we would expect to see an infrared absorption about ν_o = ν_e + 2x_eν_e = 2880 cm⁻¹. However, this absorption corresponding to the Q-branch is not observed due to it being forbidden by symmetry. Instead, two sets of signals (P- and R-branches) are seen owing to a simultaneous change in the rotational state of the molecules. Because of quantum mechanical selection rules, only certain rotational transitions are permitted. The states are characterized by the rotational quantum number J = 0, 1, 2, 3, ... selection rules state that ΔJ is only able to take values of ±1.

${\displaystyle E(J)_{\mathrm {rot} }=h\cdot B\cdot J(J+1)}$

The value of the rotational constant B is much smaller than the vibrational one ν_o, such that a much smaller amount of energy is required to rotate the molecule; for a typical molecule, this lies within the microwave region. However, the vibrational energy of HCl molecule places its absorptions within the infrared region, allowing a spectrum showing the rovibrational transitions of this molecule to be easily collected using an infrared spectrometer with a gas cell. The latter can even be made of quartz as the HCl absorption lies in a window of transparency for this material.

Naturally abundant chlorine consists of two isotopes, ³⁵Cl and ³⁷Cl, in a ratio of approximately 3:1. While the spring constants are nearly identical, the disparate reduced masses of H³⁵Cl and H³⁷Cl cause measurable differences in the rotational energy, thus doublets are observed on close inspection of each absorption line, weighted in the same ratio of 3:1.

In the 17th century, Johann Rudolf Glauber from Karlstadt am Main, Germany used sodium chloride salt and sulfuric acid for the preparation of sodium sulfate in the Mannheim process, releasing hydrogen chloride. Joseph Priestley of Leeds, England prepared pure hydrogen chloride in 1772,[13] and by 1808 Humphry Davy of Penzance, England had proved that the chemical composition included hydrogen and chlorine.[14]

Hydrogen chloride is produced by combining chlorine and hydrogen:

Cl₂ + H₂ → 2 HCl

As the reaction is exothermic, the installation is called an HCl oven or HCl burner. The resulting hydrogen chloride gas is absorbed in deionized water, resulting in chemically pure hydrochloric acid. This reaction can give a very pure product, e.g. for use in the food industry.

The reaction can also be triggered by blue light.[15]

The industrial production of hydrogen chloride is often integrated with the formation of chlorinated and fluorinated organic compounds, e.g., Teflon, Freon, and other CFCs, as well as chloroacetic acid and PVC. Often this production of hydrochloric acid is integrated with captive use of it on-site. In the chemical reactions, hydrogen atoms on the hydrocarbon are replaced by chlorine atoms, whereupon the released hydrogen atom recombines with the spare atom from the chlorine molecule, forming hydrogen chloride. Fluorination is a subsequent chlorine-replacement reaction, producing again hydrogen chloride:

RH + Cl₂ → RCl + HCl

RCl + HF → RF + HCl

The resulting hydrogen chloride is either reused directly or absorbed in water, resulting in hydrochloric acid of technical or industrial grade.

Small amounts of hydrogen chloride for laboratory use can be generated in an HCl generator by dehydrating hydrochloric acid with either sulfuric acid or anhydrous calcium chloride. Alternatively, HCl can be generated by the reaction of sulfuric acid with sodium chloride:[16]

NaCl + H₂SO₄ → NaHSO₄ + HCl

This reaction occurs at room temperature. Provided there is NaCl remaining in the generator and it is heated above 200 °C, the reaction proceeds further:

NaCl + NaHSO₄ → HCl + Na₂SO₄

For such generators to function, the reagents should be dry.

Hydrogen chloride can also be prepared by the hydrolysis of certain reactive chloride compounds such as phosphorus chlorides, thionyl chloride (SOCl₂), and acyl chlorides. For example, cold water can be gradually dripped onto phosphorus pentachloride (PCl₅) to give HCl:

PCl₅ + H₂O → POCl₃ + 2 HCl