Editing Chlorine (section)

===Hydrogen chloride===
[[File:DCl Neutron powder.png|thumb|upright=1.8|right|Structure of solid deuterium chloride, with D···Cl hydrogen bonds]]
The simplest chlorine compound is [[hydrogen chloride]], HCl, a major chemical in industry as well as in the laboratory, both as a gas and dissolved in water as [[hydrochloric acid]]. It is often produced by burning hydrogen gas in chlorine gas, or as a byproduct of chlorinating [[hydrocarbon]]s. Another approach is to treat [[sodium chloride]] with concentrated [[sulfuric acid]] to produce hydrochloric acid, also known as the "salt-cake" process:<ref name="Greenwood809" />
:NaCl + H<sub>2</sub>SO<sub>4</sub> {{overunderset|⟶|150&thinsp;°C|&thinsp;}} NaHSO<sub>4</sub> + HCl
:NaCl + NaHSO<sub>4</sub> {{overunderset|⟶|540–600&thinsp;°C|&thinsp;}} Na<sub>2</sub>SO<sub>4</sub> + HCl
In the laboratory, hydrogen chloride gas may be made by drying the acid with concentrated sulfuric acid. Deuterium chloride, DCl, may be produced by reacting [[benzoyl chloride]] with [[heavy water]] (D<sub>2</sub>O).<ref name="Greenwood809">{{harvnb|Greenwood|Earnshaw|1997|pp=809–812}}</ref>

At room temperature, hydrogen chloride is a colourless gas, like all the hydrogen halides apart from [[hydrogen fluoride]], since hydrogen cannot form strong [[hydrogen bond]]s to the larger electronegative chlorine atom; however, weak hydrogen bonding is present in solid crystalline hydrogen chloride at low temperatures, similar to the hydrogen fluoride structure, before disorder begins to prevail as the temperature is raised.<ref name="Greenwood809" /> Hydrochloric acid is a strong acid (p''K''<sub>a</sub> = −7) because the hydrogen-chlorine bonds are too weak to inhibit dissociation. The HCl/H<sub>2</sub>O system has many hydrates HCl·''n''H<sub>2</sub>O for ''n'' = 1, 2, 3, 4, and 6. Beyond a 1:1 mixture of HCl and H<sub>2</sub>O, the system separates completely into two separate liquid phases. Hydrochloric acid forms an [[azeotrope]] with boiling point 108.58&nbsp;°C at 20.22&nbsp;g HCl per 100&nbsp;g solution; thus hydrochloric acid cannot be concentrated beyond this point by distillation.<ref name="Greenwood812">{{harvnb|Greenwood|Earnshaw|1997|pp=812–816}}</ref>

Unlike hydrogen fluoride, anhydrous liquid hydrogen chloride is difficult to work with as a solvent, because its boiling point is low, it has a small liquid range, its [[dielectric constant]] is low and it does not dissociate appreciably into H<sub>2</sub>Cl<sup>+</sup> and {{chem|HCl|2|-}} ions – the latter, in any case, are much less stable than the [[bifluoride]] ions ({{chem|HF|2|-}}) due to the very weak hydrogen bonding between hydrogen and chlorine, though its salts with very large and weakly polarising cations such as [[caesium|Cs<sup>+</sup>]] and [[quaternary ammonium cation|{{chem|NR|4|+}}]] (R = [[methyl group|Me]], [[ethyl group|Et]], [[butyl group|Bu<sup>''n''</sup>]]) may still be isolated. Anhydrous hydrogen chloride is a poor solvent, only able to dissolve small molecular compounds such as [[nitrosyl chloride]] and [[phenol]], or salts with very low [[lattice energy|lattice energies]] such as tetraalkylammonium halides. It readily protonates [[nucleophile]]s containing lone-pairs or π bonds. [[Solvolysis]], [[ligand]] replacement reactions, and oxidations are well-characterised in hydrogen chloride solution:<ref name="Greenwood818">{{harvnb|Greenwood|Earnshaw|1997|pp=818–819}}</ref>
:Ph<sub>3</sub>SnCl + HCl ⟶ Ph<sub>2</sub>SnCl<sub>2</sub> + PhH (solvolysis)
:Ph<sub>3</sub>COH + 3 HCl ⟶ {{chem|Ph|3|C|+|HCl|2|-}} + H<sub>3</sub>O<sup>+</sup>Cl<sup>−</sup> (solvolysis)
:{{chem|Me|4|N|+|HCl|2|-}} + BCl<sub>3</sub> ⟶ {{chem|Me|4|N|+|BCl|4|-}} + HCl (ligand replacement)
:PCl<sub>3</sub> + Cl<sub>2</sub> + HCl ⟶ {{chem|PCl|4|+|HCl|2|-}} (oxidation)