Two-step bromine attack

In 1925, Francis¹ showed that some chlorobromoethane is produced when aqueous bromine reacts with ethene in the presence of sodium chloride, indicating that the bromine is added to the double bond by slow electrophilic addition before the rapid addition of the chloride ion:

C₂ H₄ + Br₂ → C₂ H₄ Br⁺ + Br^-

C₂ H₄ Br⁺ + Cl^- → C₂ H₄ BrCl 

While older A-level textbooks^2,3 include this evidence for the mechanism, it is often omitted in more up to date revision guides that focus on examination requirements. This means that pupils may be taught the mechanism, often using the curly arrow notation, quite divorced from any experimental support. Even advanced textbooks ^4,5 often concentrate on the stereochemistry of the reaction and omit the more elementary work. We believe that it is wrong not to present pupils with experimental evidence for what they are being taught and have modified the experiment to allow it to be performed in a double period.