Chemistry 350 Laboratory - Fall Semester 2010 – Professor T. Nalli, Winona State University

Expt #6. Synthesis of 1,2-Dibromo-1,2-diphenylethane

Relevant textbook readings – Mohrig, Chapter 9. Smith, Chapter 10.13-10.14

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 ones actually form in the reaction. From this information you will determine if the addition of Br₂ is anti, syn, or nonstereoselective.

Procedures

Because Br₂ is extremely hazardous, we will generate it in situ in small amounts by the reaction of potassium bromate (KBrO₃) with HBr (eq 2).

KBrO₃ + 6 HBr —› 3 Br₂ + KBr + 3 H₂O (2)

Running the reaction.

· Add 0.60 g of trans-stilbene and 10 mL of hexanes 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.

· Add 0.40 g KBrO₃ to the stirring mixture.

· Attach a reflux condenser and make sure water is running through it before going on to the next step.

· Quickly add 1.6 mL 48% hydrobromic acid through the top of the reflux condenser to the reaction flask.

· Stir the solution for 40 min at room temperature.

· To destroy any left-over bromine, add 1-pentene dropwise until there is no trace of the red color of bromine remaining. (No more than 5 or 10 drops of should be necessary).

Work-up procedures.

· Collect the crystals by vacuum filtration on a pre-weighed Buchner funnel washing with a small amount of ice-cold hexanes. (Hint: use cold hexanes to rinse any crystals that remain in the flask into the funnel.)

· Dry the crystals somewhat by using the vacuum filtration apparatus to pull air through them for a few minutes.

· Weigh the funnel with the crystals and calculate the yield of crude product.

· Recrystallize (see Mohrig, Chapter 9) the product from xylenes: Add the crystals to a 100-mL round bottom flask along with 10 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 slowly, maintaining boiling, until all of the solid has dissolved. Cool slowly to room temperature and then on ice and collect the crystals on a Buchner funnel. Wash the crystals with 1-2 mL of cold hexanes.

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

Characterization of Product

· Obtain the melting point. Look up literature values for the different stereoisomers of 1,2-dibromo-1,2-diphenylethane. Which stereoisomer do you have?

· Obtain the ¹H NMR and ¹³C NMR spectra using CDCl₃ as the solvent.

· Obtain the IR spectrum using mineral oil (Nujol) as a carrier fluid. This type of IR sample is commonly referred to as a "Nujol mull".

Report

Please try to integrate the answers to the assigned questions into your results and conclusions section for this report (rather than answering each sequentially).

Assigned Questions:

1. Which stereoisomer was formed in this reaction? Explain this result by using the accepted mechanism for bromination of an alkene by Br₂.

2. What was the purpose of washing the crystals in each vacuum filtration step with cold hexanes? Why does the wash solvent have to be cold?