CH 2270
Addition of A Halogen To An Alkene – Synthesis of Stilbene Dibromide
Materials
From the Chemicals Hood:
Nothing from the Stockroom
Glacial acetic acid
trans-stilbene
1 M Br2 in dichloromethane solution
Methanol
Saturated sodium bisulfite aqueous solution
In this experiment you will synthesize stilbene dibromide by reacting the alkene
trans-stilbene with elemental bromine (Br2).
The mechanism of the reaction proceeds through a cyclic bromonium ion
intermediate and this can get a little complicated, as you could imagine four different yet
similar mechanisms for the formation of stilbene dibromide. Because trans-stilbene is a
symmetrical compound the first addition of a bromine atom can occur from either the
“top” or the “bottom” of the alkene. There is no chemical (or geometrical) difference.
Notice that the bromine molecule, though formally non-polar, has a weak Br-Br bond and
is polarizable, especially when approached by a nucleophilic alkene (Figures 1 and 4).
Figure 1. Addition of Bromine to the “top” of the Alkene of trans-Stilbene
Subsequent addition of the bromide anion to the still-symmetrical bromonium ion can
occur from the left- or right-side of the carbon-carbon bond (Figures 2, 3, 5, and 6).
Figure 2. Attack of Bromide Anion to the “Left Side” Carbon of a Bromonium Ion
Figure 3. Attack of Bromide Anion to the “Right Side” Carbon of a Bromonium Ion
Figure 4. Addition of Bromine to the “bottom” of the Alkene of trans-Stilbene
Figure 5. Attack of Bromide Anion to the “Left Side” Carbon of a Bromonium Ion
Figure 6. Attack of Bromide Anion to the “Right Side” Carbon of a Bromonium Ion
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 ).
Procedure - Preparation of Stilbene Dibromide from trans-Stilbene
Rinse a clean 250 mL Erlenmeyer flask with acetone to remove traces of water. To
this Erlenmeyer flask, add 40 mL of glacial acetic acid and 4.0 g of trans-stilbene. Add
25 mL of a 1 M solution of elemental bromine in CH2Cl2 (dichloromethane, DCM)
incrementally, adding a few mL at a time and swirling gently between additions. Heat the
flask in a steam bath with occasional swirling for 5 – 10 minutes. The orange bromine
color may persist, faintly. Allow the flask to cool to room temperature at which time solid
should form. Vacuum filter (Buchner funnel with suction) the solid and wash with 20 mL
of ice-cold methanol.
CAUTION
1. Bromine is a strong oxidizer, causes eye and skin burns as well as digestive and respiratory tract
burns. May be fatal if inhaled. Is corrosive to metal. Use sodium thiosulfate to neutralize Br2 spills.
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 if 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 the eyes, skin, digestive and respiratory tracts.
Wear goggles and gloves when handling all these chemicals. Keep them in the hood when possible.
Dry the product by spreading it on a sheet of filter paper. Let it dry until next lab
period. Once dried, weigh it, calculate the yield and take its melting point. Using the
melting point, determine which stereoisomer(s) you have synthesized, the meso
compound or the racemic mixture. Check your conclusion with the TA.
Save your compound until next lab period when you will use it as the starting
material for the synthesis of diphenylacetylene.
CLEANUP
All liquid wastes go into the liquid halogenated organic waste container in the hood.