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

Relevant textbook readings – Mohrig, Chapter 15. Smith, Chapter 10

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.

This lab will also provide an introduction to the process known as "recrystallization". Recrystallization is a standard method for the purification of an organic solid and it is discussed in detail in Chapter 15 in Mohrig.

Because Br₂ is very 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)   

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 0.60 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 Br₂fumes 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 cyclohexene dropwise until there is no trace of the red color of bromine remaining. (No more than 10 drops of should be necessary).

Work-up procedures.

Collect the solid product 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.) Leave the vacuum on for a few minutes so as to promote evaporation of the solvent from the solid.

Weigh the funnel with the collected solid and record the yield of crude product in your lab notebook. 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 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 the great majority of solid has dissolved. Because KBr is a byproduct of the reaction and will remain insoluble in hot xylenes you will need to carry out a hot gravity filtration using a fluted filter paper. Reheat the filtered solution to redissolve solid that forms during the filtration and then cool slowly to room temperature. Once a good amount of crystals have formed you can then chill on ice to maximize crystla growth. Collect the crystals on a Buchner funnel. Wash the crystals with 2 mL of cold hexanes followed by 2 mL of pentane. Again leave the vacuum on for a few minutes so as to speed the removal of traces of hexanes and pentane from the solid.

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

Characterization of Product

Obtain the melting point of the crystalline final product as well as the crude.

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. Note that the mp of the product is quite high (over 100 degrees higher than the mp of the other stereoisomer) at the same time the solubility in organic solvents (e.g., hexane and chloroform) is very low. Explain how these facts are related to the structure of the compound.