Hydrobromic Acid | It Formula, Synthesis, reactions and Uses

Hydrobromic acid is a diatomic molecule. It is also called as hydrogen bromide and is present in the form of a colorless gas at room temperature. It has an acrid odor. Its other physical characteristics include the gas being very soluble in water. On dissolving the gas in water, it forms hydrobromic acid solution. It is also soluble in organic solvents like alcohol. The hydrogen end has a different attraction for electrons than the bromine end. Therefore, the molecule is polar in nature.It has a net dipole moment of 820 mD (millidebye).

Anhydrous hydrogen bromide (HBr) is corrosive in a gaseous state and an irritant.

Hydrobromic acid is a hydrogen halide. The molecule is linear in shape and the bond length is of 141.4 pm.  Anhydrous and aqueous solutions are both commonly used reagents in the preparation of bromide compounds. The compound has a molar mass of 80.91 g/mol. 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 in the following two ions

HBr = H+ + Br

Synthesis of Hydrobromic acid

Preparing hydrobromic acid from phosphorus tribromide(PBr3) and water

  1. In a flask fitted with a funnel, clean sand is placed. Over this, a mixture of 25 g of red phosphorus and more sand is placed.
  2. This mixture is moistened by putting about 40 ml of water, and 50 ml of bromine in the funnel.
  3. The outlet tube is connected to a U-tube which contains glass beads(to increase the surface area) coated with moist red phosphorus.
  4. Two wash bottles filled with 100 ml and 50 ml of water are kept consecutively.
  5. Both bottles are kept in an ice bath.
  6. The bromine is added very slowly to the original flask (kept in ice to cool)
  7. As the reaction proceeds the ice bath may be removed as the evolution of gas can be controlled easily.
  8. When all the bromine is added the reaction flask is warmed slightly to drive off the residual acid vapors to give higher yield.

Chemical reactions of hydrobromic acid


Occurs when peroxide is not used. The bromine gets bonded to the more substituted carbon in the aliens.

Anti-markovnikov rule

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. This is because 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 or SN2 mechanism.

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.

epoxide rings opened by HBr
Image by wikipedia

With ethers

Alkyl ethers are cleaved by the strong acids like HBr in a nucleophilic substitution reaction.

bydrobromic acid breaks the alkyl ether
Credit: University of Calgary

This reaction produces an alcohol product and an alkyl halide. This mechanism is possible because bromide ion is a good nucleophile.

Uses of Hydrobormic 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 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 in alkylations and condensations in organic chemistry.
  • High purities of hydrogen bromide are 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.

Leave a Comment