upon the bromination of an alkene to create stillbene dibromide 2.89g of the pro
ID: 870161 • Letter: U
Question
upon the bromination of an alkene to create stillbene dibromide 2.89g of the product was collected and has a melting point of 209c(once it started melting) and 211c (when the product finsihed melting.
? Compare the measured melting point with the literature value, and the melting point of the crude product. Explain your findings. What can you conclude about the purity of your product?
below is the experimental procedure and all the information that i have.
Place 2.0 g of (E)-stilbene, boiling chips, and 40 mL of ethanol in a 100 mL round-bottom flask. Heat the flask to dissolve the stilbene. Add 4.0 g of pyridinium tribromide (if solid material adheres to the interior walls of the flask, use few milliliters of ethanol to rinse it down). Continue to heat for additional 5 minutes after the addition of reagents is complete, and then remove the flask from the hot plate. The product dibromide should quickly begin to precipitate or crystallize. Let the reaction cool to room temperature, and then chill the mixture in an ice bath. Collect the product by vacuum filtration. Set aside a small amount of this crude product so that you can measure its melting point. Wash the isolated solid with a small amount of ice-cold methanol to remove any adsorbed pyridinium salts. Set the product aside for a week. Measure the mass of your purified solid, record its melting point, and calculate the yield of the reaction. Compare the measured melting point with the literature value, and the melting point of the crude product. As a reference record the melting point of (E)-stilbene to check the calibration of your melting point apparatus
Explanation / Answer
According to literature, the value for stillbene dibromide melting point is 238oC. As the sample melted completely at 211oC (27oC below the temp expected), it means that the sample is not pure.
As the melting point of the crude product was 209oC (once it started melting), and the finish melting point was 211oC, the sample's melting point range is 2oC. Melting point range for pure crystalline organic compounds usually has a characteristic value from 0.5 to 1oC. Impurities tend to increase the range of melting, then again, the sample can not be considered as a pure organic compound.
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