Typical procedure: Bromine is slowly added dropwise to a round bottom flask containing a suspension of iron and 1,2,3,4-tetrahydronaphthalene. In the early stage of the reaction, the flask is cooled with water. The reaction is warmed to 30 to 40 ℃ allowing gas evolution. The gas is passed through a wash bottle containing 1,2,3,4-tetrahydronaphthalene in order to eliminate any bromine in the hydrogen bromide. For optimum yields of hydrogen bromide, dry the 1,2,3,4-tetrahydronaphthalene over anhydrous sodium sulfate and then distill it under a dry inert atmosphere (such as argon or nitrogen). If any moisture is introduced into the reaction, the yield of hydrogen bromide becomes remarkably poor.