Expt #5. Synthesis of 1,2-Dibromo-1,2-diphenylethane

Relevant textbook readings – Mohrig, Chapter 21.9. Klein, Chapter 9

Overview

The stereoselectivity of alkene bromination will be examined in this experiment. trans-1,2-Diphenylethene (common name trans-stilbene) will be brominated to form 1,2-dibromo-1,2-diphenylethane (eq 1). This product exists as three stereoisomers. You will determine which isomer(s) actually form in the reaction. From this information you will determine if the addition of Br₂ is anti, syn, or nonstereoselective.

Because Br₂ is very hazardous, we will generate it in situ in small amounts by the reaction of hydrogen peroxide with HBr (eq 2).

          H₂O₂      +    2 HBr     —›  Br₂     +    2 H₂O     (2)   

Pre-Lab - For your plan for this experiment make sure to combine equations 1 and 2 in order to come up with overall balanced chemical equation. Then use this equation to calculate the theoretical yield of 1,2-dibromo-1,2-diphenylethane.

Procedures

Running the reaction.

Add 0.50 g of trans-stilbene and 10 mL of ethanol to a 25-mL round bottom flask.

Clamp the flask up to a ring stand well back into the fume hood so as to prevent any accidental exposure to Br₂ fumes that may escape the reaction apparatus. Begin stirring the solution using a magnetic stir bar.

Attach a reflux condenser and heat to reflux either using a hot water bath or a sand bath..

Add 1.2 mL 48% hydrobromic acid followed by 0.8 mL of 30% H₂O₂. Use a Pasteur pipet to do both additions dropwise through the top of the condenser.

Continue to stir and reflux the solution until the color of Br₂ has fully dissipated.

Work-up procedures.

Cool to room temperature and then add saturated NaHCO₃ until the pH is nearly neutral (pH = 5-7).

Cool on an ice bath and then collect the product by vacuum filtration washing with a 1 mL of ice cold ethanol.

Dry the product on the Buchner funnel for 10 min.

Weigh the final product and calculate the yield and percent yield.

Characterization of Product

Obtain the melting point, IR, and ¹H NMR of the product. One group will asked to be obtain a C-13 NMR spectrum and we may also have a group prepare a sample for GC-MS.

Questions:

1. Based on the mp as compared to literature values, which stereoisomer of 1,2-dibromo-1,2-diphenylethane was formed? Explain how this result is consistent with the accepted mechanism for bromination of an alkene by Br₂.
2. Draw Newman projections for all possible staggered conformers of the meso product. Do the same for one of the enantiomers of racemic dibromodiphenylethane. Predict the most stable conformer of each isomer and note the torsion angle between the Hs. Predict the ¹H-¹H coupling constant between the Hs using the Karplus relationship (see Mohrig page 352).