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Narrative
Three test tubes contain bromine. Cyclohexane is added to the first tube, cyclohexene to the second, and benzene to the third. The red color of bromine vapor disappears from the tube containing cyclohexene. pH paper shows that hydrogen bromide is not produced in this reaction.

Discussion
This sequence demonstrates that alkanes and benzene do not react with bromine in the absence of a catalyst. Alkenes like cyclohexene undergo addition reactions with bromine to produce dibromoalkanes.