Bromous acid is the

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemis ...

with the formula of HBrO₂. It is an unstable compound, although salts of its conjugate base – bromites – have been isolated. In acidic solution, bromites decompose to bromine.Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemistry'', Elsevier

Discovery

In 1905, Richards A. H. proved the existence of bromous acid through a series of experiments involving

silver nitrate Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar causti ...

(AgNO₃) and bromine. The reaction of excess cold aqueous to form

hypobromous acid Hypobromous acid is a weak, unstable acid with chemical formula of HOBr. It is mainly produced and handled in an aqueous solution. It is generated both biologically and commercially as a disinfectant. Salts of hypobromite are rarely isolated as ...

(HBrO),

silver bromide Silver bromide (AgBr) is a soft, pale-yellow, water-insoluble salt well known (along with other silver halides) for its unusual sensitivity to light. This property has allowed silver halides to become the basis of modern photographic materials. A ...

(AgBr) and

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

(HNO₃): :Br₂ + AgNO₃ + H₂O → HBrO + AgBr + HNO₃ Richards discovered that the effect of adding excess liquid bromine in a concentrated

(AgNO₃) resulted in a different reaction mechanism. From numbers of equivalent portions of acid bromine formed from the previous reaction, the ratio between oxygen and bromine was calculated, with the exact value of O:Br (0.149975:0.3745), suggesting the acid compound contains two oxygen atom to one bromine atom. Thus, the chemical structure of the acid compound was deducted as HBrO₂. According to Richards,

(HBrO) arises by the reaction of bromine and silver nitrate solution: :Br₂ + AgNO₃ + H₂O → HBrO + AgBr + HNO₃ :2 AgNO₃ + HBrO + Br₂ + H₂O → HBrO₂ + 2 AgBr + 2 HNO₃

Isomerism

The molecule HBrO₂ has a bent structure with ∠(H−O−Br) angles of 106.1°. HOBrO also adopts a non-planar conformation with one isomer structure (2a) adopting a dihedral angle ∠(H−O−Br−O) of 74.2°. Moreover, the planar structures of two other isomers (2b-''cis'' and 2c-''trans'') are transition state for fast enantiomerization. Another study identified three isomers: HOOBr, HOBrO, and HBr(O)O.

Synthesis

A oxidation reaction between

(HBrO) and

hypochlorous acid Hypochlorous acid (HClO, HOCl, or ClHO) is a weak acid that forms when chlorine dissolves in water, and itself partially dissociates, forming hypochlorite, ClO−. HClO and ClO− are oxidizers, and the primary disinfection agents of chlorine sol ...

(HClO) can be used to produce bromous acid (HBrO₂) and

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...

(HCl). :HBrO + HClO → HBrO₂ + HCl A redox reaction of

(HBrO) can form bromous acid (HBrO₂) as its product: :HBrO + H₂O − 2e⁻ → HBrO₂ + 2H⁺ The disproportionation reaction of two equivalents

(HBrO) results in the formation of both bromous acid (HBrO₂) and

hydrobromic acid Hydrobromic acid is a strong acid formed by dissolving the diatomic molecule hydrogen bromide (HBr) in water. "Constant boiling" hydrobromic acid is an aqueous solution that distills at and contains 47.6% HBr by mass, which is 8.77 mol/L. ...

(HBr): : 2 HBrO → HBrO₂ + HBr A rearrangement reaction, which results from the syn-proportion of

bromic acid Bromic acid, also known as hydrogen bromate, is an oxoacid with the molecular formula HBrO3. It only exists in aqueous solution.''The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals''. 14th Edition. 2006.''Van Nostrand's Scienti ...

(HBrO₃) and

(HBr) gives bromous acid (HBrO₂): :2 HBrO₃ + HBr → 3 HBrO₂

Sodium bromite

The salts NaBrO₂·3H₂O and Ba(BrO₂)₂·H₂O have been crystallized. Upon treatment of these aqueous solutions with salts of Pb²⁺, Hg²⁺, and Ag⁺, the corresponding heavy metal bromites precipitate as solids.

Belousov–Zhabotinsky reaction

Bromous acid is a product of the

Belousov–Zhabotinsky reaction A Belousov–Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of non-equilibrium thermodynamics, resulting in the establishment of a nonlinear chemical oscillator. The only common element in ...

resulting from the combination of potassium bromate, cerium(IV) sulfate, propanedioic acid and citric acid in dilute sulfuric acid. Bromous acid is an intermediate stage of the reaction between bromate ion ( ) and bromine (Br⁻): * + 2 Br⁻ → HBrO₂ + HBrO Other relevant reactions in such oscillating reactions are: * HBrO₂ + + H⁺ → 2 + H₂O * 2 HBrO₂ → + HOBr + H⁺ Bromites reduce

permanganate A permanganate () is a chemical compound containing the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom is in the +7 oxidation state, the permanganate(VII) ion is a strong oxidizing agent. The ion is a ...

s to manganates (VI): * 2 + + OH⁻ → 2 + + H₂O

p''K''_a measurement

The acid dissociation constant of bromous acid, ''K''_a = , was determined using different methods. The value of the p''K''_a for bromous acid was estimated in research studying the decomposition of bromites. The research measured the rate of bromite decomposition as a function of hydrogen and bromite ion concentrations. The experimental data of the log of the initial velocity were plotted against pH. Using this method, the estimated p''K''_a value for bromous acid was 6.25. Using another method, the pK_a for bromous acid was measured based on the initial velocity of the reaction between sodium bromites and potassium iodine in a pH range of 2.9–8.0, at 25 °C and ionic strength of 0.06 M. The first order dependence of the initial velocity of this disproportionation reaction on ⁺in a pH range of 4.5–8.0. The value of acid dissociation constant measured by this method is ''K''_a = and p''K''_a = .

Reactivity

In comparison to other oxygen-centered oxidants (hypohalites, anions of peroxides) and in line with its low basicity, bromite is a rather weak nucleophile. Rate constants of bromite towards carbocations and acceptor-substituted olefins are by 1–3 orders of magnitude lower than the ones measured with hypobromite.

References

{{Hydrogen compounds Bromites Halogen oxoacids Hydrogen compounds Oxidizing acids Oxidizing agents