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 crude product and calculate the percent yield. Save a small sample of the crude for a mp test to be carried out later.

Recrystallize (see Mohrig, Chapter 15) the product from xylenes: Add the crude solid to a 100-mL round bottom flask along with 8 mL of xylenes. Add a few boiling chips, attach a reflux condenser and heat the solution to boiling on a sand bath. Add more xylenes if necessary, maintaining boiling, until all of the solid has dissolved and then cool slowly to room temperature. Once a good amount of crystals have formed you should then chill on ice to maximize crystal growth. Collect the crystals on a Buchner funnel and wash with 2 mL of hexanes followed by 2 mL of pentane. Leave the vacuum on for a few minutes so as to speed the removal of traces of hexanes and pentane from the solid.

Characterization of Product

Obtain the melting point of both your crude and final product. Also obtain a ¹H NMR spectrum of the final product. One group will asked to be obtain a C-13 NMR spectrum and we will also have a group run NMRs of the crude, unrecrystallized product.

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. If the reaction were carried out using cis-stilbene as the reactant which stereoisomer of the product would be formed?
   
3. The proton NMR of the crude shows clear evidence for the presence of a minor product, which is most likely 2-bromo-1-ethoxy-1,2-diphenylethane. Identify the peaks due to this compound (both by labeling them on the spectra and including them in your results tables) Propose a likely mechanism for how this product forms and predict its stereochemistry.
4. Challenge question - The class storage folder also includes a proton NMR spectrum of an experiment run by a student team in Chem 350 last year. (File name = "dibromostilbene crude product CDCl3-1") For this experiment, trans-stilbene was dissolved in CDCl₃ in an NMR tube and then treated with a Br₂/CDCl₃ solution followed by acquiring the NMR of the entire solution.  What does this NMR say about your answers to questions 1 and 2 and the accepted mechanism of alkene bromination? (Note that in our experiment we analyzed the precipitate, which may only represent a portion of the product formed by the actual reaction.)