SYNTHESIS OF 1,2-DIBROMO-1,2-DIPHENYLETHANE AND OPTICAL ROTATION

 

Purpose:  Trans-1,2-diphenylethene is commonly known as trans-stilbene.  This experiment consists of three parts.  The first part of the experiment is the bromination of trans-stilbene.    You will determine the mode of addition of Br2, syn or anti to the double bond by analyzing the product melting point.  The second part is to determine the optical rotations of (+)-carvone and (-)-carvone.  Finally, complete the problem set on stereochemistry.

 

Required reading:  Mayo 137-142 and B&F  199-202,  101-112, 1073-1075, 1015.

 

Part I

            Overall reaction:

trans-stilbene                         1,2-dibromo-1,2-diphenylethane

 

Bromine can approach the double bond in two possible ways either syn or anti, which lead to four possible stereoisomeric products I, II, III, and IV.

 

As shown above, compounds I and II are mirror images of each other, a pair of enantiomers, which results from the syn mode of addition of Br2.  Bromine can approach either the top or the bottom of the double bond, and it has an equal chance of attack from either direction yielding 50% of compound I and 50 % of compound II; this is a racemic mixture.  Anti mode of addition of Br2 to the double bond gives the two possible structures III and IV.  These two drawings depict the same product; it is a meso compound.  A meso compound has at least two more chiral centers, but the molecule itself is achiral, not optically active, because it has a plane of symmetry. 

 

Physical properties:  the meso 1,2-dibromo-1,2-diphenylethane has a melting point of 238°C.  The racemic mixture of 1,2-dibromo-1,2-diphenylethane has a melting point of 113°C. 

 

 

 

 

Part II

 

                                   

 

            Determine the optical rotations of the two carvones.  Use the equation below to calculate the specific rotations.

 

           

               a

[a]tl  =  ___

             c l

[a]tl  =  specific roration which has been corrected from the following terms.

a = observed rotation in degrees, c = concentration in grams/mL (use density for pure liquid), l = length of sample tube in decimeters, l = wavelength of light (usually indicated as “D” for the sodium D line), t = temperature in degrees Celsius.

 

            Part III  Use the website http://science.csustan.edu/nhuy/chem3012/spartan/set3-stereochem/stereochemistry.htm to complete the stereochemistry problem set.                                

Procedure:

 

Part I

            To a 25 mL Erlenmeyer flask add 0.2 g trans-stilbene, 6 mL of ether and a stir vane.  Stir to dissolve the solid stilbene (warm it on the steam bath if necessary) and then add dropwise 2 mL of 1M Br2 in CHCl3 with stirring (stir the solution using the stirred plate).  After 15 minutes, cool the flask in an ice bath for an additional 5 minutes.  Using a shortened pipette, transfer the fine crystals to a prepared Hirsch funnel with suction.  Transfer the remaining ether, then add 2 mL fresh cold ether to rinse the flask, and transfer the rinsings with any solids remaining to the Hirsch funnel.  Repeat the rising process once more with fresh cold ether.  Press the solid on the filter to remove any remaining ether, then transfer the solid to a small preweighed beaker, and determine the weight of the 1,2-dibromo-1,2-diphenylethane isolated. 

·        Determine the melting point of this product. 

·        Which product was formed? 

·        What was the mode of addition, syn or anti of Br2 to the double bond? 

·        Calculate the % yield (Set up a reaction table as shown in “Common Practices….”this will help you to calculate the % yield.

·        Assign R or S configuration for the central carbon atoms in compounds I, II, III, and IV.

·        Convert structures I, II, III and IV to the Fisher projections.  Which two of the four drawings represent the racemic mixture and which of the other two represent the same compound, meso compound?

·        Write the mechanism for the reaction using curved arrows to show the electron flows.

·        The mode of addition you determined from the experiment is typical for all additions of Br2 to alkenes.  Make stereochemical drawings as shown in structures I, II, etc or sawhorse for the product(s) of bromination to cis-stilbene (cis-1,2-diphenylethene).  Is this product a racemic mixture or a meso product?

 

Part II

 

            Determine the optical rotations of the two enantiomers of carvones in tube A and B.  Your instructor will show you how to use the polarimeter.   Calculate the specific rotations for the two compounds.

            Do the two carvones have different odors?  Record your impressions.  Determine the contents of tubes A and B using the information above.  True or false, if the compound has a chiral center with a R configuration it always rotate polarized light countered-clockwise (-) or vice versa.

 

Part III

 

Use the website to answer the following questions:  stereochemistry

1)      Draw three -dimensional orientations for all of the compounds (use wedged and dash).

 

2)      Which one of the six compounds is the meso compound?

 

3)      Draw a diastereomer for one of the compounds excluding compound B.

 

5)  Name or identify all six compounds.  Don’t forget to include their configurations.