ADDITION OF BROMINE TO AN ALKENE
Please watch the bromination experiment video before working on this lab. In this experiment we will synthesize stilbene dibromide by reacting the alkene, trans-stilbene with elemental bromine, Br2.
As discussed in the halogenation of alkene material, the mechanism of the reaction proceeds through a cyclic bromonium ion intermediate. Because trans-stilbene is a symmetrical molecule the first addition of a bromine atom to make the cyclic bromonium ion can occur either from the "top" or the "bottom" of the alkene. There is no chemical or geometrical difference. Subsequent addition of the bromide anion to the symmetrical, cyclic bromonium ion can occur from the left- or right-hand side of the carbon-carbon bond. The product of this reaction has two stereogenic centers, and since all three substituents on each stereogenic center are the same (bromine, phenyl, hydrogen), one of the isomers of stilbene dibromide is a meso compound (mp 241-243 °C). The other two isomers of stilbene dibromide are the (+) and (-) enantiomers. If these are formed it will be as the racemic mixture (mp 114-115 °C). If you were to actually do this experiment, you would find by determination of the melting point of the product, that only the meso-stilbene dibromide is formed.
CAUTIONS FOR THIS EXPERIMENT
1. Bromine is a strong oxidizer, causes eye and skin burns as well as digestive and respiratory tract burns. May be fatal if inhaled. It is corrosive to metal.
2. Dichloromethane is irritating to eyes and skin. It may be harmful if inhaled or swallowed.
3. Trans-Stilbene causes eye and skin irritation and is harmful if ingested or inhaled.
4. Stilbene dibromide is corrosive and a lachrymator. It may cause irritation to skin, the digestive tract (if swallowed), and respiratory tract (if inhaled).
5. Glacial acetic acid can cause severe burns to eyes, skin, digestive and respiratory tracts.
6. Goggles and gloves are required while performing this experiment.