Results and Discussion

1 2 3

Figure 1. Addition reaction of Hydrobromic Acid with trans-stilbene to form 1,2-dibromo-1,2-diphenylethane.

Figure 2. Half reactions showing the oxidation of Bromine and the Reduction of Hydrogen Peroxide and then the overall oxidation-reduction reaction.

The bromination of compound 1 is an oxidation reaction that took place in a solution of ethanol.

When Hydrobromic acid and Hydrogen Peroxide were added to the solution successively, a

bright yellow precipitate formed to indicate the formation of the diatomic bromine in the

solution. Hydrogen Peroxide can be both a strong reducing and oxidizing agent. In this case it

was the catalytic oxidizing agent and was used as a Green reagent and was itself reduced to give

a byproduct of water.

Table I. Melting Point Analysis and Percent Recovery

Compound MP(C) Range Percent Recovery

(1S, 2R) 1,2-Dibromo- 232 C 230-231C 50.9% 1,2-diphenylethane

6309! Bromination of trans-stilbene11-13-13AMDG

1

After the experiment resulting in the bromination of compound 1, melting point analysis

was done to test the purity of the compound. When the dry product was observed it was a

crystalline white color but in melting the compound, a brown tinge was observed as the

temperature reached 200C. The brown tinge in the sample was attributed to the high

temperatures at which the compound was tested. The compound completely melted at 232C

(having a literature value of 241C). Although it could be concluded that the the compound was a

Meso product (by means of anti addition) the chemical reaction contains two products possible

due to the possibility anti addition. Even though the overall mechanism is indicative of anti

addition, the lower melting point could be attributed to a small period of syn addition that could

have taken place during the reaction process. The low yield of the compound (50.9%) was

determined from the theoretical yield calculated from stoichiometric values of the reactants in

the chemical equation. The low yield can be attributed to the experimental procedure error. The

procedure takes time and there were many rooms for error.

Figure 3. 13C NMR Spectral Data for compound 2

2

13C NMR data of the compound displayed 4 peaks due to the line of symmetry that exists.

The most downfield peak correlates to the Carbons on the aromatic rings adjacent to the Carbon

bonded to the Bromine. The peaks upfield of this signal (~127-129 ppm) are indicative of the rest

of the Carbons on the aromatic ring, and the most upfield peak (~56ppm) correlates to the the

Carbon bonded directly to the Bromine. In comparing the NMR data of the Meso compound and

the dl Pair compounds, no difference was found. The signals were the same and should be

because there is no change in position of any of the substituents on the molecules, the orientation

of the Bromine and the Hydrogen is simply switched.

Figure 4. 1H NMR Spectral Data for Compound 2

1H NMR presented 5 peaks (also due to its line of symmetry) but the most upfield peak

was disregarded as it was the peak that indicated the water in the deuterochloroform. The

downfield peaks (~7.40, 7.29, 7.27ppm) present downfield complex splitting due to their

location on the aromatic ring and their position next to the double bonds. The peak upfield of

those signals (~5.51ppm) correlate to the Hydrogen on the Carbon bonded to the Bromine.

3

In comparing both NMR spectral data for the Meso compound and the dl pair

compounds, it was seen that there was no difference in chemical shifts and it was concluded that

NMR spectral data could not, by itself, determine the stereochemical properties of enantiomeric

compounds. However, for diastereotopic protons a Deuterium test can be done by replacing a

diastereotopic with Deuterium and it should give different NMR signals.

Figure 5. The stepwise polar reaction mechanism for the addition of Br2 to cis-stilbene.

The above reaction does not occur in this experiment, but it can be seen that if cis-

stilbene was used in the reaction, rotation around the Carbon bonds would allow the same

compounds to be formed.

Conclusion

Out of the three experimental methods for analyzing the compounds, melting point was the most

useful. Carbon 13 and Proton NMR were not the best tools for identification of our product due

to the similar signals presented by enantiomeric products in the racemic mixture. Melting point

gave a very reliable indication of the identity of the compound because of the literature value of

melting point.Finally, Hydrogen Peroxide is a good Greene reagent to use. Instead of getting a

volatile byproduct, the byproduct was water.

4

Experimental

A round bottom 25-mL flask, equipped with a jacketed condenser and stir bar was assembled in

the fume hood. trans-stilbene (51g) was dissolved in ethyl alcohol (25-mL). Cool water was run

through the condenser and the flask was heated in a sand bath on medium heat until reflux. The

solution was removed from the heat and HBr (2.0 mL) was added dropwise with Pasteur pipette

and the solution. A yellow tint resulted due to the color of HBr. Hydrogen Peroxide (1mL) was

added dropwise and a bright yellow precipitate formed. The condenser was reattached and heated

in the sand bath (75 minutes). The flask was removed from heat and was left to cool to RT.

Sodium Carbonate (48 drops) was added to the cooled solution until neutral and the flask was put

into an ice bath (3 minutes) to cool. The precipitate was vacuum filtrated and titurated with hot

ethanol (20mL). When the ethanol started to boil the flask was removed from the hot plate and

refiltered to get the solid di-bromo-product. The sample was left to dry and the mass was

determined to be 0.49g. Melting points were taken in the Thomas-Hoover Melting point

apparatus and were determined to have a range of 230-231C and a melting point of 232C.

Some of the sample was weighed out (0.0142g) and prepared for NMR analysis. 1HNMR 7.40

(4H, dd), 7.29 (4H dd), 7.27 (2H, dd), 5.51 (2H, s), 1.4 (H2O and CDCl3). 13CNMR 138.3 (2C),

129.3 (8C), 128.9 (2C), 53.3 (2C).

References

Slough, Greg; Chemistry 210L Organic ChemistryI Lab Manual. 2013, pp. 51-55

ChemDraw, structures and 1HNMR analysis.

www.sigma-aldrich.com

5