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| Names | |
|---|---|
| IUPAC names hydroxy-λ3-bromanone hydroxidooxidobromine bromous acid | |
| Identifiers | |
3D model (JSmol) | |
| ChEBI | |
| ChemSpider |
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| Properties | |
| HBrO2 | |
| Molar mass | 112.911 g/mol |
| Conjugate base | Bromite |
| Related compounds | |
Otheranions | Hydrobromic acid;hypobromous acid;bromic acid;perbromic acid |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Bromous acid is theinorganic compound 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.[1]
In 1905, Richards A. H. proved the existence of bromous acid through a series of experiments involvingsilver nitrate (AgNO3) and bromine.[2] The reaction of excess cold aqueous to formhypobromous acid (HBrO),silver bromide (AgBr) andnitric acid (HNO3):
Richards discovered that the effect of adding excess liquid bromine in a concentratedsilver nitrate (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.[2]
According to Richards,hypobromous acid (HBrO) arises by the reaction of bromine and silver nitrate solution:[2]
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.[3]
Another study identified three isomers: HOOBr, HOBrO, and HBr(O)O.[4]
A oxidation reaction betweenhypobromous acid (HBrO) andhypochlorous acid (HClO) can be used to produce bromous acid (HBrO2) andhydrochloric acid (HCl).[citation needed]
A redox reaction ofhypobromous acid (HBrO) can form bromous acid (HBrO2) as its product:[citation needed]
Thedisproportionation reaction of two equivalentshypobromous acid (HBrO) results in the formation of both bromous acid (HBrO2) andhydrobromic acid (HBr):[citation needed]
A rearrangement reaction, which results from the syn-proportion ofbromic acid (HBrO3) andhydrobromic acid (HBr) gives bromous acid (HBrO2):[citation needed]
The saltsNaBrO2·3H2O andBa(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.[1]
Bromous acid is a product of theBelousov–Zhabotinsky reaction 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 (BrO−
3 ) and bromine (Br−):[5][6]
Other relevant reactions in such oscillating reactions are:
Bromites reducepermanganates tomanganates (VI):[1]
The acid dissociation constant of bromous acid,Ka =[H+][BrO−
2]/[HBrO2], was determined using different methods.
The value of the pKa 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 pKa value for bromous acid was 6.25.[7]
Using another method, the pKa 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 thisdisproportionation reaction on [H+] in a pH range of 4.5–8.0. The value of acid dissociation constant measured by this method isKa =(3.7±0.9)×10−4 M and pKa =3.43±0.05.[8]
In comparison to other oxygen-centered oxidants (hypohalites, anions of peroxides) and in line with its low basicity, bromite is a rather weak nucleophile.[9] Rate constants of bromite towards carbocations and acceptor-substituted olefins are by 1–3 orders of magnitude lower than the ones measured with hypobromite.
