Hydrobromic acid serves as a strong mineral acid and brominating agent in biochemical protocols, particularly for selective DNA/RNA hydrolysis, phosphonium salt cleavage, and regioselective glycosidic bond cleavage due to its larger Br⁻ ion size compared to HCl. It functions as a versatile strong acid (pKa -9) in nucleic acid chemistry and protecting group manipulation.
Chemical Properties
Hydrobromic acid (HBr, MW 80.91 g/mol) exists as a colorless-to-yellow fuming liquid (48% w/w aqueous, density 1.49 g/cm³, bp 126°C azeotrope) that completely dissociates per HBr → H⁺ + Br⁻, forming constant boiling 47.6% HBr (8.8 M). It is less oxidizing than HCl but exhibits higher nucleophilicity, forming air-stable alkyl bromides versus reactive chlorides. It also produces volatile fumes (bp -67°C gas).
Biochemical Applications
In nucleic acid work, 48% HBr (1–2 h, 37°C) is used for selective cleavage of benzylidene acetals from ribonucleosides (Br⁻ displaces PhCH(OMe)₂⁺ intermediates quantitatively). In peptide chemistry, 30% HBr/AcOH allows simultaneous Asp(OtBu)/Boc deprotection (3 h, RT) without Trp oxidation, unlike TFA. Carbohydrate laboratories apply anhydrous 33% HBr/AcOH for regioselective 1,6-anhydroglucose hydrolysis to free glucose, milder than HI. Phospholipid analysis uses HBr to cleave phosphonium salts to bromoalkanes for GC-MS applications.
