Although there are several ways of making hydrogen bromide in the literature, I have successfully used the method below when I needed the reagent for anti-Markovnikov hydrobromination of alkenes. It has turned out to be very convenient during cases where the aqueous reagent could not be used since the generated gas can be bubbled through the reaction solutions as needed.
Procedure:[1] A two-neck round-bottom flask was charged with iron powder, and enough tetrahydronaphthalene (tetralin) to completely submerge the iron. The round bottom flask was fitted with a dropping funnel, and the second neck was fitted with an appropriate setup to bubble the product gas into the reaction of choice. Bromine was added to the dropping funnel and the round-bottom flask cooled with a water bath. Bromine was then added dropwise, being careful to keep the reaction from becoming too vigorous. Once the reaction slows down, the flask is heated to 40degC to keep it going. Bromine addition is continued until no more HBr is needed.
Notes:
- Reagent quantities were all estimated, none measured. Cowboy chemistry.
- Residual Br2 can be scrubbed from product HBr gas by bubbling through pure tetrahydronaphthalene.
- Residual moisture can be scrubbed from product HBr gas by passing through anhydrous CaBr2 column.[2] Don’t use CaCl2, P2O5, or H2SO4 as they will contaminate the product gas.
- HBr can be bubbled into air/moisture sensitive reactions by carrying the same setup under a protective argon atmosphere
[1] M. Schmeisser “Chlorine, Bromine, Iodine” in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer. Academic Press, NY, 1963, 1, 282-286.
[2] Baxter, G. P.; Warren, R. D. J. Am. Chem. Soc. 1911, 33, 3, 340-344.