Greener Brominations
Greener Brominations
Abstract
In this experiment, trans-stilbene was brominated and debrominated with the use of three methods: bromination with pyridinium tribromide, bromination with hydrogen peroxide and hydrobromic acid, and debromination with zinc. The yields of parts A, B, and C are as follows: 68.6 %, 54.6 %, and 55.9 %.
Introduction
In the chemical process of bromination, an alkene is halogenated with bromine. Solvents that are typically used include methylene chloride and carbon tetrachloride along with iodine, the traditional reagent. However, because of the carcinogenic properties of the solvents and the corrosiveness of iodine, other alternatives are utilized : bromination with pyridinium tribromide, bromination with hydrogen peroxide and hydrobromic acid, and ethanol replaces the iodine. The figure above demonstrates the equilibrium of pyridinium tribromide with pyridinium hydrobromide, which allows gradual release of bromine into the reaction. Some of the benefits of using pyridinium tribromide are (1) weight can be determined more easily as opposed to bromine in liquid form, (2) a safer solvent, such as ethanol may be used as opposed to a solvent containing chlorine.
The reaction above shows the bromination of stilbene with hydrobromic acid and hydrogen peroxide. This method may be used in place of the pyridinium tribromide, which has less atom economy and can be corrosive to metal. Ethanol is used as a solvent. Zinc is used in the process of debromination with ethanol as the solvent. Zinc is used due to vicinal properties of stilbene dibromide, which allows an easier debromination an easier pathway. 1
Experimental Procedure
A. Bromination of trans-stilbene with pyridinium tribromide
A 40 mL of ethanol and 2.0 g of trans-stilbene mixture was prepared in a 125-mL Erlenmeyer flask with a magnetic stirrer.The flask was clamped to a variac, heated using a heating mantle, and then stirred until the stilbene dissolved. Next,
Abstract
In this experiment, trans-stilbene was brominated and debrominated with the use of three methods: bromination with pyridinium tribromide, bromination with hydrogen peroxide and hydrobromic acid, and debromination with zinc. The yields of parts A, B, and C are as follows: 68.6 %, 54.6 %, and 55.9 %.
Introduction
In the chemical process of bromination, an alkene is halogenated with bromine. Solvents that are typically used include methylene chloride and carbon tetrachloride along with iodine, the traditional reagent. However, because of the carcinogenic properties of the solvents and the corrosiveness of iodine, other alternatives are utilized : bromination with pyridinium tribromide, bromination with hydrogen peroxide and hydrobromic acid, and ethanol replaces the iodine. The figure above demonstrates the equilibrium of pyridinium tribromide with pyridinium hydrobromide, which allows gradual release of bromine into the reaction. Some of the benefits of using pyridinium tribromide are (1) weight can be determined more easily as opposed to bromine in liquid form, (2) a safer solvent, such as ethanol may be used as opposed to a solvent containing chlorine.
The reaction above shows the bromination of stilbene with hydrobromic acid and hydrogen peroxide. This method may be used in place of the pyridinium tribromide, which has less atom economy and can be corrosive to metal. Ethanol is used as a solvent. Zinc is used in the process of debromination with ethanol as the solvent. Zinc is used due to vicinal properties of stilbene dibromide, which allows an easier debromination an easier pathway. 1
Experimental Procedure
A. Bromination of trans-stilbene with pyridinium tribromide
A 40 mL of ethanol and 2.0 g of trans-stilbene mixture was prepared in a 125-mL Erlenmeyer flask with a magnetic stirrer.The flask was clamped to a variac, heated using a heating mantle, and then stirred until the stilbene dissolved. Next,