Bromination of Trans- Stilbene to Form
The Br–Br bond becomes polarized and the more positively charged Br atom is transferred to the alkene to yield a bromonium ion and a bromonium anion. The bromonium anion attacks a carbon atom to open the three-membered ring and produce vicinal dibromide. The net result of this reaction is anti addition of bromine as shown in Figure 3. [pic] Figure 3: The anti addition of bromine to an alkene The traditional procedure utilizes a solvent, such as methylene chloride or carbon tetrachloride, which are both carcinogens. Typically, elemental bromide is also used but it is corrosive and can cause chemical burns.
This experiment is a green bromination of trans-stilbene because it reduces the need to use hazardous chemicals. It utilizes ethanol as a solvent and bromide is generated in situ from the reaction of hydrobromic acid and hydrogen peroxide. Materials and Methods First, a water bath was prepared and heated to 90-100oC on a hot plate. A 100mL round-bottom flask was equipped with a magnetic stir bar. A Mettler PJ360 DeltaRange balance was used to measure out 0. 500g of 96% trans-stilbene (Sigma-Aldrich Batch# 00807JD), which was added to the 100mL flask. Next, 15mL of the 95% (general lab reagent) ethanol were also added to the flask.
A reflux condenser was set up and the solution of ethanol and 96% trans- stilbene was allowed to dissolve. After the stilbene dissolved, 0. 8mL of 48% hydrobromic acid (A. C. S. reagent Batch# 06722CC, HBr content 47. 0-49. 0%) was added in drops while the solution was heated and stirred. The mixture appeared clear and colorless. Then, 0. 8mL of 30% hydrogen peroxide (Sigma Aldrich), was also added drop-wise to the refluxing reaction while it was being heated and stirred. The color of the mixture became dark yellow. The solution was then heated and stirred of approximately 20 minutes until the solution turned cloudy white.
The mixture was rinsed with 95% ethanol (GLR) and became light yellow. The solution was heated and stirred for an additional 7 minutes until the color appeared cloudy white. The round-bottom flask was removed from the heat and cooled to room temperature. Once the solution had cooled to room temperature, The solution was then neutralized with drops of NaHCO3 (GLR) until its pH was 7. After the solution was neutralized, the flask was put on ice so the crystals would precipitate. The crystals were then collected using vacuum filtration, rinsing with cold ethanol. The crystals were placed in a glass vial and allowed to dry for 24 hours.
The next day the micro-melting point was taken using a Stanford Research System Optimelt Automated Melting Point System. Finally, a small amount of crystals were ground up with a mortar and pestle, mixed with one drop of Nujol mull (GLR), and placed on a sodium chloride window which was used to obtain a FT-IR spectrum on a Perkin-Elmer Paragon 1000 FT-IR. Table 1: Parameters for Perkin-Elmer Paragon 1000 FT-IR |First X |4400cm-1 | |Last X |500cm-1 | |Min Y |5. 88476 | |Max Y |84. 54895 | |Resolution |4. 000000cm-1 | |Points |3901 | |Interval between points |-1 | |Data format in X |Wavenumbers (cm-1) | |Data format in Y |%T | |Spectral Range |4400-500cm-1 | |Number of Scans |4 |
Results At the beginning of the reaction, the solution was a yellow, gold color. When the hydrobromic acid was added to the refluxing solution, the solution turned burnt orange. Upon addition of the hydrogen peroxide, the solution went from a dark, golden yellow color to an egg-yolk color. At the midpoint of the refluxing, approximately 15 minutes after the hydrogen peroxide was added, the solution was a cloudy, light yellow color. While refluxing, there was a slight accumulation of flaky crystals on the flask.
A layer of crystals lined the bottom of the flask. Once the solution cooled to room temperature, litmus paper was then used to test the acidity of the solution. NaHCO3 was added to the solution drop wise and the pH was retested until the litmus paper indicated a pH of 7. The melting point of the dried crystals was measured to be 243. 3oC which was close to the literature value of 241 oC. The crystalline product was then analyzed using FT-IR, which is shown in Figure 4.
The FTIR obtained in lab closely matches the FTIR obtained from the SBDS database. [pic] Figure 4: FTIR of dibromo stilbene obtained in lab The percent yield was also calculated and found to be 45. 54 percent. A Proton NMR was performed on the crystals and can be found in Appendix 1 (attached to back). Three peaks appear on the Proton NMR between 7-8ppm. These peaks correspond to the hydrogens on the benzene ring. In addition a single peak is found near 2ppm. These peaks verify the formation of dibromo stilbene.