Two-step bromine attack

By 2010-11-01T00:00:00

FEATURE-Bromine-p180-testtubes-150

Source: © ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY

An experiment for the classroom to show that bromine adds to an alkene by two-step electrophilic addition

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 textbooks2,3 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. 

Thanks for using Education in Chemistry. You can view one Education in Chemistry article per month as a visitor. 

A photograph of a teacher standing in a white lab coat, speaking with a class of children in a laboratory, is superimposed on a colourful background. Text reads "Teach Chemistry means support for classroom and staff room".

Register for Teach Chemistry for free, unlimited access

Registration is open to all teachers and technicians at secondary schools, colleges and teacher training institutions in the UK and Ireland.

Get all this, plus much more:

  • unlimited access to resources, core practical videos and Education in Chemistry articles
  • teacher well-being toolkit, personal development resources and online assessments
  • applications for funding to support your lessons

Set up your Teach Chemistry account

Already a Teach Chemistry member? Sign in now.

Not eligible for Teach Chemistry? Sign up for a personal account instead, or you can also access all our resources with Royal Society of Chemistry membership.