Editing Bromine (section)

===Hydrogen bromide===
The simplest compound of bromine is [[hydrogen bromide]], HBr. It is mainly used in the production of inorganic [[bromide]]s and [[alkyl bromide]]s, and as a catalyst for many reactions in organic chemistry. Industrially, it is mainly produced by the reaction of [[hydrogen]] gas with bromine gas at 200–400&nbsp;°C with a [[platinum]] catalyst. However, reduction of bromine with [[red phosphorus]] is a more practical way to produce hydrogen bromide in the laboratory:<ref name="Greenwood809">Greenwood and Earnshaw, pp. 809–12</ref>
: 2 P + 6 H{{sub|2}}O + 3 Br{{sub|2}} → 6 HBr + 2 H{{sub|3}}PO{{sub|3}}
: H{{sub|3}}PO{{sub|3}} + H{{sub|2}}O + Br{{sub|2}} → 2 HBr + H{{sub|3}}PO{{sub|4}}

At room temperature, hydrogen bromide is a colourless gas, like all the hydrogen halides apart from [[hydrogen fluoride]], since hydrogen cannot form strong [[hydrogen bond]]s to the large and only mildly electronegative bromine atom; however, weak hydrogen bonding is present in solid crystalline hydrogen bromide at low temperatures, similar to the hydrogen fluoride structure, before disorder begins to prevail as the temperature is raised.<ref name="Greenwood809" /> Aqueous hydrogen bromide is known as [[hydrobromic acid]], which is a strong acid (p''K''{{sub|a}} = −9) because the hydrogen bonds to bromine are too weak to inhibit dissociation. The HBr/H{{sub|2}}O system also involves many hydrates HBr·''n''H{{sub|2}}O for ''n'' = 1, 2, 3, 4, and 6, which are essentially salts of bromine [[anion]]s and [[hydronium]] [[cation]]s. Hydrobromic acid forms an [[azeotrope]] with boiling point 124.3&nbsp;°C at 47.63&nbsp;g HBr per 100&nbsp;g solution; thus hydrobromic acid cannot be concentrated beyond this point by distillation.<ref name="Greenwood812">Greenwood and Earnshaw, pp. 812–6</ref>

Unlike [[hydrogen fluoride]], anhydrous liquid hydrogen bromide 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}}Br{{sup|+}} and {{chem|HBr|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 bromine, though its salts with very large and weakly polarising cations such as [[caesium|Cs{{sup|+}}]] and [[quaternary ammonium cation|{{chem|NR|4|+}}]] (R = [[methyl group|Me]], [[ethyl group|Et]], [[butyl group|Bu{{sup|''n''}}]]) may still be isolated. Anhydrous hydrogen bromide 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.<ref name="Greenwood812" />