Hydrobromic acid is a diatomic molecule that is a colorless gas at room temperature.
It is also called hydrogen bromide and its chemical formula is “HF.”
The compound has a molar mass of 80.91 g/mol.
Chemically it is an acid with an acrid odor and is very soluble in water. When this HF gas is dissolved in water, it forms a hydrobromic acid solution.
Uses of Hydrobromic acid
Hydrogen bromide is a popular reagent used to make many commercial chemicals.
- It is used in the production of inorganic bromides like sodium bromide.
- It is also used to make organic bromides by substitution.
- Hydrogen bromide is used in the reaction with alkynes to form bromoalkenes by peroxide reaction or anti-Markovnikov rule. This reaction forms bromides where the bromine is attached to the less substituted carbon.
- It is used as a reagent in opening epoxides and lactones.
- HBr is used as a catalyst for oxidations and alkylations and condensations in organic chemistry.
- Highly pure hydrogen bromide is used in precision plasma etching processes by the electronics industry (in this process, reactive ions created within the plasma react with surface atoms, thus removing the surface metal)
- Hydrogen bromide is used as a catalyst in the preparation of medicines.
- It is also soluble in organic solvents like alcohol.
Chemical structure and properties
- The hydrogen end has a lower attraction for electrons than the bromine end.
- Therefore, the molecule is polar with a net dipole moment of 820 mD (Milli debye).
- Anhydrous hydrogen bromide (HBr) is corrosive and irritant in a gaseous state.
- Hydrobromic acid is a hydrogen halide. The molecule is linear in shape, and the bond length is 141.4 pm.
- Anhydrous and aqueous solutions are commonly used reagents to prepare the bromide compounds.
- It has a melting point of −86.9 °C and a boiling point of −66.8 ° C.
- It is represented as HBr. Hydrobromic acid formed by dissolving HBr in water is a strong acid.
- It releases hydrogen ions, thus behaving like an acid. It acts as a strong acid.
- It ionizes into the following two ions.
HBr = H+ + Br –
Synthesis of Hydrobromic acid
Preparing hydrobromic acid from phosphorus tribromide(PBr3) and water
- In a flask fitted with a funnel, the clean sand is placed. Over this, a mixture of 25 g of red phosphorus and more sand is placed.
- This mixture is moistened by putting about 40 ml of water and 50 ml of bromine in the funnel.
- The outlet tube is connected to a U-shaped tube that contains glass beads coated with moist red phosphorus. These coated beads increase the surface area)
- Two wash bottles filled with 100 ml and 50 ml of water are kept consecutively.
- Both bottles are kept in an ice bath.
- The bromine is added very slowly to the original flask (kept in ice to cool)
- As the reaction proceeds, the ice bath can be removed often to control the gas evolution.
- When all the bromine is added, the reaction flask is warmed slightly to drive off the residual acid vapors to give a higher yield.
Chemical reactions of hydrobromic acid
It occurs when peroxide is not used. The bromine gets bonded to the more substituted carbon in the aliens.
- Anti-markovnikov rule
It can only be done with HBr, and Hydrogen Peroxide needs to be present.
Hydrogen Peroxide is an unstable molecule and breaks into two free radicals of OH, which attack HBr, thus creating a Bromine radical.
This attacks the least substituted carbon of the alkene molecule.
Once the bromine radical attacks the alkene molecule, a carbon radical will be formed.
So the radical is formed at the more substituted carbon as it is more stable due to hyper-conjugation. The bromine gets bonded to the less substituted carbon.
- Substitution reactions
HBr releases Bromide ion easily and can be used in substitution reactions. The reaction may follow both SN1 and SN2 mechanisms.
It behaves like hydrochloric acid in its activity toward metals and their oxides and hydroxides.
- With metals
Hydrobromic acid reacts with metals to give bromide salt and hydrogen gas
2Na + 2HBr -> 2NaBr + H2
- With metal oxides
With metal oxides, it gives salt and water
Na2O + 2HBr -> 2NaBr + H2O
- With metal hydroxides
Hydrobromic acid reacts with metal hydroxides to give salt and water
Zn(OH)2 + 2HBr -> ZnBr2 + 2H2O
- With epoxides
It helps in ring-opening in epoxides forming halohydrins.
- With ethers
In a nucleophilic substitution reaction, alkyl ethers are cleaved by strong acids like HBr.
This reaction produces an alcohol product and an alkyl halide. This mechanism is possible because the bromide ion is a good nucleophile.