Since the Pollution Prevention Act was passed in 1990, chemists have had to find ways to deal with their procedures and reactions in a more eco-friendly manner. Innovative and creative chemists now abound and this procedure is a fruit of their efforts. Elemental bromine (Br2) used in organic chemistry is a toxic and volatile substance and therefore difficult to work with and does not meet green chemistry standards. For this procedure aqueous hydrobromic acid (HBr) is used in its stead. It is much less corrosive and it helps the reaction proceed in a much more effective and safe manner.
The bromination process is now carried out in situ (within the reaction mixture) with the assistance of another less caustic solvent, ethanol. This makes observing the reaction straightforward as well as safe as the bromine is contained within the reaction area. One characteristic of green chemistry is to generate environmentally friendly byproducts when the experiment is complete. An acceptable byproduct is water, which is the byproduct produced in the bromination of stilbene. Include the reaction, written out with structures, to top off this lovely introduction.
During addition of hydrogen peroxide, the color changed from clear and transitioned from yellow to orange.
After 6 minutes, the color faded to a cloudy white.
Melting Point
Onset: 234.4°C
Clear Point: 235.2°C
Analysis
calculation OK, sig figs OK.
The melting point of a substance is one way to determine purity of a substance. The product has a literature value melting point at 241ºC, so the observed melting point range of near 235°C implies a reasonably pure substance recovered. did it melt normally or did it decompose?
Name: dibromostilbene
Alternate Name: Stilbene dibromide
IUPAC Name: (1,2-dibromo-2-phenyl-ethyl)benzene
CAS #: 5789-30-0
Molecular Formula: C6H5CBr=CBrC6H5
Compound Formula: C14H12Br2
Structure:
Discussion
As hydrogen peroxide was added drop wise to the mixture of (E)-stilbene (trans-stilbene), ethanol and hydrobromic acid, a color change occurred immediately. This color change from yellow to orange was the indication of the discharge of bromine in the mixture during the addition of hydrogen peroxide. Color change also represents the electrophilic addition reaction being demonstrated in this lab and the in situ formation of bromine by the oxidation of hydrobromic acid with hydrogen peroxide.
Mass results may have been more accurate and potentially yielded more product had time been taken to let the product dry naturally. Instead, after the vacuum filtration process was complete the product was placed between two towels and compressed in order to expedite the drying process. This may have resulted in loss of product by either sticking to, or falling out from betwixt the two pieces of filter paper.
The pH strips were used to determine when the solution had reached a pH of ≈5. NaHCO₃ was added dropwise and used to buffer the solution. The room for error during this part of the procedure was potentially significant as the test strips did not always reach the product, therefore 185 drops were added until there was a noticeable pH change.
Melting point of the final product, dibromostilbene, may have been less than accurate due the precision of the measurements of solvents being added during the bromination process. This inaccuracy may have been the cause of the product having a melting point lower than the estimated value of 241º C.
Conclusion
A successful preparation, isolation, purification and characterization of dibromostilbene was completed in the laboratory. Successful addition of bromine to and alkene was achieved and noted from the color change from yellow-orange to an opaque white almost colorless solution. It should be noted that the experiment fits inside the domain of characteristic constraints for green chemistry.
A characteristic of green chemistry and that of the bromination of stilbene experiment, is the formation of an environmentally friendly byproduct such as water. The bromination process was successfully carried out within the reaction through the oxidation of hydrobromic acid with hydrogen peroxide. This process has practical applications in the world of chemistry as it can be used to identify degrees of unsaturation. The test only works for double and triple carbon to carbon bonds. A positive test will yield an orange to red color generation, thus revealing the presence of several bonds or ring structures. This lab earned the following scores for: format (2/2) style (2/2) data (2.5/3) quality of result (1/1) quality of reported data (1/1) conclusion (2/2) error (1/1) post-lab question (2/2) for a total of 13.5/14.
Introduction
Since the Pollution Prevention Act was passed in 1990, chemists have had to find ways to deal with their procedures and reactions in a more eco-friendly manner. Innovative and creative chemists now abound and this procedure is a fruit of their efforts. Elemental bromine (Br2) used in organic chemistry is a toxic and volatile substance and therefore difficult to work with and does not meet green chemistry standards. For this procedure aqueous hydrobromic acid (HBr) is used in its stead. It is much less corrosive and it helps the reaction proceed in a much more effective and safe manner.
The bromination process is now carried out in situ (within the reaction mixture) with the assistance of another less caustic solvent, ethanol. This makes observing the reaction straightforward as well as safe as the bromine is contained within the reaction area. One characteristic of green chemistry is to generate environmentally friendly byproducts when the experiment is complete. An acceptable byproduct is water, which is the byproduct produced in the bromination of stilbene.
Include the reaction, written out with structures, to top off this lovely introduction.
Procedure
The procedure for this experiment can be found at University of Oregon's Greener Education Materials for Chemists.
Chemicals used in the lab:
Data
Analysis
The melting point of a substance is one way to determine purity of a substance. The product has a literature value melting point at 241ºC, so the observed melting point range of near 235°C implies a reasonably pure substance recovered. did it melt normally or did it decompose?
Discussion
As hydrogen peroxide was added drop wise to the mixture of (E)-stilbene (trans-stilbene), ethanol and hydrobromic acid, a color change occurred immediately. This color change from yellow to orange was the indication of the discharge of bromine in the mixture during the addition of hydrogen peroxide. Color change also represents the electrophilic addition reaction being demonstrated in this lab and the in situ formation of bromine by the oxidation of hydrobromic acid with hydrogen peroxide.
Mass results may have been more accurate and potentially yielded more product had time been taken to let the product dry naturally. Instead, after the vacuum filtration process was complete the product was placed between two towels and compressed in order to expedite the drying process. This may have resulted in loss of product by either sticking to, or falling out from betwixt the two pieces of filter paper.
The pH strips were used to determine when the solution had reached a pH of ≈5. NaHCO₃ was added dropwise and used to buffer the solution. The room for error during this part of the procedure was potentially significant as the test strips did not always reach the product, therefore 185 drops were added until there was a noticeable pH change.
Melting point of the final product, dibromostilbene, may have been less than accurate due the precision of the measurements of solvents being added during the bromination process. This inaccuracy may have been the cause of the product having a melting point lower than the estimated value of 241º C.
Conclusion
A successful preparation, isolation, purification and characterization of dibromostilbene was completed in the laboratory. Successful addition of bromine to and alkene was achieved and noted from the color change from yellow-orange to an opaque white almost colorless solution. It should be noted that the experiment fits inside the domain of characteristic constraints for green chemistry.
A characteristic of green chemistry and that of the bromination of stilbene experiment, is the formation of an environmentally friendly byproduct such as water. The bromination process was successfully carried out within the reaction through the oxidation of hydrobromic acid with hydrogen peroxide. This process has practical applications in the world of chemistry as it can be used to identify degrees of unsaturation. The test only works for double and triple carbon to carbon bonds. A positive test will yield an orange to red color generation, thus revealing the presence of several bonds or ring structures.
This lab earned the following scores for: format (2/2) style (2/2) data (2.5/3) quality of result (1/1) quality of reported data (1/1) conclusion (2/2) error (1/1) post-lab question (2/2) for a total of 13.5/14.
Very nice, a pleasure to read!
Notes
The chemical structures, formulas, and vital statistics of each compound was researched using Wolfram Alpha computational knowledge engine and confirmed using the CRC Handbook of Chemistry and Physics [90th Edition].