Electrophilic addition in alkenes (2) – the bromine water test

The classic test for an alkene is to shake your sample with orange bromine water, and if the molecule contains a C=C double bond the bromine water is decolourised.

A chemical reaction has clearly occurred, but could you draw out the mechanism?

CH₂CHCH₃ + Br_2(aq) ⇾ CH₂BrCHOHCH₃ + Br^–_(aq) + H₃O⁺_(aq)

If we want a dibromo-product we need to change the conditions of the reaction, with bromine dissolved in an organic solvent at room temperature.

Practice questions

1. Oleic acid is a fatty acid derived from plant oils. Describe a test you could carry out to show that this fatty acid is unsaturated and state what you would observe.

2. Draw a full mechanism for the reaction between cyclohexene and bromine in an organic solvent.  Include relevant charges, dipoles and curly arrows.

3. Cycloocta-1,3,5,7-tetraene has the molecular formula C₈H₈. 

(a) Draw the skeletal formula for the molecule.

(b) Draw the mechanism for the addition of bromine to the molecule, forming C₈H₈Br₂.

Answers

1. Shake a sample of oleic acid with bromine water  / bromine dissolved in CCl₄. The colour will change from orange to colourless. 
   

3.