Definitions

# Hydrogen chloride

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The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry. The formula HCl is often used to refer, somewhat misleadingly, to hydrochloric acid, an aqueous solution derived from hydrogen chloride.

## Chemistry

Hydrogen chloride is composed of diatomic molecules, each consisting of a hydrogen atom H and a chlorine atom Cl connected by a covalent single bond. Since the chlorine atom is much more electronegative than the hydrogen atom, the covalent bond between the two atoms is quite polar. Consequently, the molecule has a large dipole moment with a negative partial charge δ- at the chlorine atom and a positive partial charge δ+ at the hydrogen atom. In part due to its high polarity, HCl is very soluble in water (and in other polar solvents).

Upon contact, H2O and HCl combine to form hydronium cations H3O+ and chloride anions Cl- through a reversible chemical reaction:

HCl + H2O → H3O+ + Cl

The resulting solution is called hydrochloric acid and is a strong acid. The acid dissociation or ionization constant, Ka, is large, which means HCl dissociates or ionizes practically completely in water. Even in the absence of water, hydrogen chloride can still act as an acid. For example, hydrogen chloride can dissolve in certain other solvents such as methanol, protonate molecules or ions, and serve as an acid-catalyst for chemical reactions where anhydrous (water-free) conditions are desired.

HCl + CH3OH → CH3O+H2 + Cl

HCl protonating a methanol (CH3OH) molecule

Because of its acidic nature, hydrogen chloride is a corrosive gas, particularly in the presence of any moisture.

## Health effects

Hydrogen chloride forms corrosive hydrochloric acid on contact with water found in body tissue. Inhalation of the fumes can cause coughing, choking, inflammation of the nose, throat, and upper respiratory tract, and in severe cases, pulmonary edema, circulatory system failure, and death. Skin contact can cause redness, pain, and severe skin burns. Hydrogen chloride may cause severe burns to the eye and permanent eye damage.

## History

Alchemists of the Middle Ages recognized that hydrochloric acid (then known as spirit of salt or acidum salis) released vaporous hydrogen chloride, which was called marine acid air. In the 17th century, Johann Rudolf Glauber used salt (sodium chloride) and sulfuric acid for the preparation of sodium sulfate, releasing hydrogen chloride gas (see production, below). In 1772, Carl Wilhelm Scheele also reported this reaction and is sometimes credited with its discovery. Joseph Priestley prepared hydrogen chloride in 1772, and in 1810 Humphry Davy established that it is composed of hydrogen and chlorine.

During the Industrial Revolution, demand for alkaline substances such as soda ash increased, and Nicolas Leblanc developed a new industrial-scale process for producing the soda ash. In the Leblanc process, salt was converted to soda ash, using sulfuric acid, limestone, and coal, giving hydrogen chloride as by-product. Initially, this gas was vented to air, but the Alkali Act of 1863 prohibited such release, so then soda ash producers absorbed the HCl waste gas in water, producing hydrochloric acid on an industrial scale. Later, the Hargreaves process was developed, which is similar to the Leblanc process except sulfur dioxide, water, and air are used instead of sulfuric acid in a reaction which is exothermic overall. In the early 20th century the Leblanc process was effectively replaced by the Solvay process, which did not produce HCl. However, hydrogen chloride production continued as a step in hydrochloric acid production.

Historical uses of hydrogen chloride in the 20th century include hydrochlorinations of alkynes in producing the chlorinated monomers chloroprene and vinyl chloride, which are subsequently polymerized to make polychloroprene (Neoprene) and polyvinyl chloride (PVC), respectively. In the production of vinyl chloride, acetylene (C2H2) is hydrochlorinated by adding the HCl across the triple bond of the C2H2 molecule, turning the triple into a double bond, yielding vinyl chloride.

The "acetylene process", used until the 1960s for making chloroprene, starts out by joining two acetylene molecules, and then adds HCl to the joined intermediate across the triple bond to convert it to chloroprene as shown here:

This "acetylene process" has been replaced by a process which adds Cl2 to one of the double bonds in 1,3-butadiene instead, and subsequent elimination produces HCl instead, as well as chloroprene.

## Production

Most hydrogen chloride produced on an industrial scale is used for hydrochloric acid production.

### Direct synthesis

In the chlor-alkali industry, salt solution is electrolyzed producing chlorine (Cl2), sodium hydroxide, and hydrogen (H2). The pure chlorine gas can be re-combined in an HCl forming hydrogen chloride gas.
Cl2 + H2 → 2HCl
As the reaction is exothermic, the installation is called an HCl oven. 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.

### Organic synthesis

The largest production of hydrochloric acid is 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.
R-H + Cl2 → R-Cl + HCl
R-Cl + HF → R-F + HCl
The resulting hydrogen chloride gas is either reused directly, or absorbed in water, resulting in hydrochloric acid of technical or industrial grade.

### Laboratory methods

Small amounts of HCl gas for laboratory use can be generated in a HCl generator by dehydrating hydrochloric acid in two ways:

• Liberation from concentrated hydrochloric acid by the addition of sulfuric acid.
• Liberation from concentrated hydrochloric acid by the addition of anhydrous calcium chloride.

Alternatively, HCl can be generated by the reaction of sulfuric acid with sodium chloride:

2NaCl + H2SO4 $longrightarrow$ Na2SO4 + 2HCl↑
Clandestine laboratories commonly use HCl gas generated in this way to convert a freebase drugs into the corresponding hydrochloride salts which are more conveniently distributed.

HCl can also be prepared by the hydrolysis of certain reactive chloride compounds such as phosphorus chlorides, thionyl chloride (SOCl2), and acyl chlorides. Adding more water would absorb the HCl gas forming hydrochloric acid. For example, cold water can be gradually dripped onto phosphorus pentachloride (PCl5) to give HCl in this reaction:[4]

PCl5 + H2O $longrightarrow$ POCl3 + 2HCl
Hot water could liberate more HCl by hydrolyzing PCl5 all the way to ortho-phosphoric acid.[4] Reaction of water with phosphorus trichloride (PCl3) also yields HCl.[4] Reaction of thionyl chloride with water would give sulfur dioxide (SO2) gas as well as HCl. For the reactions of thionyl chloride or acyl chlorides with water, see thionyl chloride or acyl halide.

HCl can also be made using sodium bisulfate and sodium chloride: NaHSO4 + NaCl --> Na2SO4 + HCl

## Applications

These are some of the uses for hydrogen chloride gas:

• Most hydrogen chloride is used in the production of hydrochloric acid.
• Hydrochlorination of rubber
• Production of vinyl and alkyl chlorides
• Chemical intermediate in other chemical production
• Used in toilet bowl cleaner (The Works)
• Use as babbitting flux
• Treatment of cotton
• Delinting
• Separation from wool
• Used in semiconductor industry (in pure grade)
• Etching semiconductor crystals
• Converting silicon to SiHCl3 for purification of silicon

Hydrogen chloride usually comes in compressed gas cylinders that are either red and brown or grey with a yellow band.