Bromous acid is the inorganic 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]
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| Names | |
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| IUPAC names
hydroxy-λ3-bromanone
hydroxidooxidobromine bromous acid | |
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3D model (JSmol)
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PubChem CID
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CompTox Dashboard (EPA)
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| Properties | |
| HBrO2 | |
| Molar mass | 112.911 g/mol |
| Conjugate base | Bromite |
| Related compounds | |
Other anions
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Hydrobromic acid; hypobromous acid; bromic acid; perbromic acid |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Discovery
editIn 1905, Richards A. H. proved the existence of bromous acid through a series of experiments involving silver nitrate (AgNO3) and bromine.[2] The reaction of excess cold aqueous to form hypobromous acid (HBrO), silver bromide (AgBr) and nitric acid (HNO3):
- Br2 + AgNO3 + H2O → HBrO + AgBr + HNO3
Richards discovered that the effect of adding excess liquid bromine in a concentrated silver 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]
- Br2 + AgNO3 + H2O → HBrO + AgBr + HNO3
- 2 AgNO3 + HBrO + Br2 + H2O → HBrO2 + 2 AgBr + 2 HNO3
Isomerism
editThe 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]
Synthesis
editA oxidation reaction between hypobromous acid (HBrO) and hypochlorous acid (HClO) can be used to produce bromous acid (HBrO2) and hydrochloric acid (HCl).[citation needed]
- HBrO + HClO → HBrO2 + HCl
A redox reaction of hypobromous acid (HBrO) can form bromous acid (HBrO2) as its product:[citation needed]
- HBrO + H2O − 2e− → HBrO2 + 2H+
The disproportionation reaction of two equivalents hypobromous acid (HBrO) results in the formation of both bromous acid (HBrO2) and hydrobromic acid (HBr):[citation needed]
- 2 HBrO → HBrO2 + HBr
A rearrangement reaction, which results from the syn-proportion of bromic acid (HBrO3) and hydrobromic acid (HBr) gives bromous acid (HBrO2):[citation needed]
- 2 HBrO3 + HBr → 3 HBrO2
Salts
edit
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.[1]
Belousov–Zhabotinsky reaction
editBromous acid is a product of the Belousov–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]
- BrO−
3 + 2 Br− → HBrO2 + HBrO
Other relevant reactions in such oscillating reactions are:
- HBrO2 + BrO−
3 + H+ → 2 BrO•
2 + H2O - 2 HBrO2 → BrO−
3 + HOBr + H+
Bromites reduce permanganates to manganates (VI):[1]
- 2 MnO−
4 + BrO−
2 + OH− → 2 MnO2−
4 + BrO−
3 + H2O
pKa measurement
editThe 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 this disproportionation reaction on [H+] in a pH range of 4.5–8.0. The value of acid dissociation constant measured by this method is Ka = (3.7±0.9)×10−4 M and pKa = 3.43±0.05.[8]
Reactivity
editIn 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.
References
edit- ^ a b c Wiberg, Nils; Holleman, A. F.; Wiberg, Egon, eds. (2001). "Oxygen Acids of Bromine". Inorganic Chemistry. Academic Press. pp. 449–451. ISBN 978-0-12-352651-9.
- ^ a b c Richards, A. H. (15 January 1906). "The existence of bromous acid (HBrO
2)". Journal of the Society of Chemical Industry. 25 (1 Suppl): 4–5. hdl:2027/mdp.39015030318508. - ^ Glaser, Rainer; Jost, Mary (16 August 2012). "Disproportionation of Bromous Acid HOBrO by Direct O-Transfer and via Anhydrides O(BrO)2 and BrO-BrO2. An Ab Initio Study of the Mechanism of a Key Step of the Belousov–Zhabotinsky Oscillating Reaction". The Journal of Physical Chemistry A. 116 (32): 8352–8365. Bibcode:2012JPCA..116.8352G. doi:10.1021/jp301329g. PMID 22871057.
- ^ de Souza, Gabriel L. C.; Brown, Alex (July 2016). "The ground and excited states of HBrO2 [HOOBr, HOBrO, and HBr(O)O] and HBrO3 (HOOOBr and HOOBrO) isomers". Theoretical Chemistry Accounts. 135 (7). doi:10.1007/s00214-016-1931-8.
- ^ Vassalini, Irene; Alessandri, Ivano (30 December 2015). "Spatial and Temporal Control of Information Storage in Cellulose by Chemically Activated Oscillations". ACS Applied Materials & Interfaces. 7 (51): 28708–28713. doi:10.1021/acsami.5b11857. PMID 26654462.
- ^ Field, Richard J.; Koros, Endre; Noyes, Richard M. (December 1972). "Oscillations in chemical systems. II. Thorough analysis of temporal oscillation in the bromate-cerium-malonic acid system". Journal of the American Chemical Society. 94 (25): 8649–8664. Bibcode:1972JAChS..94.8649F. doi:10.1021/ja00780a001.
- ^ Massagli, A.; Indelli, A.; Pergola, F. (1970). "Kinetic investigation of the decomposition of bromite". Inorganica Chimica Acta. 4: 593–596. doi:10.1016/S0020-1693(00)93357-7.
- ^ Faria, R. B.; Epstein, Irving R.; Kustin, Kenneth (January 1994). "Kinetics of Disproportionation and pKa of Bromous Acid". The Journal of Physical Chemistry. 98 (4): 1363–1367. doi:10.1021/j100055a051.
- ^ Mayer, Robert J.; Ofial, Armin R. (18 May 2018). "Nucleophilic Reactivities of Bleach Reagents". Organic Letters. 20 (10): 2816–2820. doi:10.1021/acs.orglett.8b00645. PMID 29741385.
Further reading
edit- Ukeles, S.D.; Freiberg, M. (2002). "Bromine, Inorganic Compounds". Kirk-Othmer Encyclopedia of Chemical Technology. doi:10.1002/0471238961.0218151321110512.a01. ISBN 978-0-471-48494-3.