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Expt 07 - Williamson Ether Synthesis of Butyl Methyl Ether

Expt 07 - Williamson Ether Synthesis of Butyl Methyl Ether...

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Reflux Column EXPERIMENT SEVEN Williamson Ether Synthesis of Butyl Methyl Ether DISCUSSION A Williamson ether synthesis consists of two separate reactions: the preparation of an alkoxide, and the reaction of this reagent with the alkyl halide. Sodium methoxide is prepared by the addition of sodium metal to methanol. In the reaction, sodium metal is oxidized to sodium cations and the hydrogen atoms of the -OH groups are reduced to hydrogen gas. A large excess of methanol is used to act as a solvent for the sodium methoxide. The exothermic nature of the reaction causes the methanol to boil. To prevent the methanol from boiling away, an upright condenser, called a reflux condenser, is attached to the reaction flask. Methanol vapors condense in the condenser and the liquid runs back into the flask. Because of the vigor of the reaction, the sodium must be added slowly; otherwise, the methanol will boil violently, overwhelm the capacity of the reflux condenser, and spew out the top of the condenser. An uncontrolled reaction of this type, called a runaway reaction, may erupt like a volcano, throwing flammable solvent and corrosive chemicals over laboratory workers and the work area. There are a number of techniques by which a solid can be added to a reaction mixture. In this experiment, small pieces of sodium are added to the flask through the reflux condenser tube. Most solids (generally powders) should not be added to reaction vessels in this manner because they tend to stick to the sides of the condenser. Should the sodium stick to the side of the condenser, a long glass rod or tubing can be used to push it down into the methanol. (It is prudent to bend an L at one end of the glass rod so that it cannot drop completely through the condenser and puncture the flask.) After all the sodium has reacted, excess methanol is removed by distillation. Decreasing the volume of solvent increases the rate of reaction of sodium methoxide with 1-bromobutane and permits the entire reaction sequence to be carried out in a single laboratory period.
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It is possible to stop the synthesis at this point until the next laboratory period. However, it is not desirable to do this because sodium methoxide is strongly basic. It absorbs moisture from the air and is converted to methanol and NaOH. CH 3 O - + H 2 O ----> CH 3 OH + OH - If it is necessary to store the sodium methoxide, the flask must be sealed with a well-greased glass stopper. After the excess methanol has been distilled, the flask containing the sodium methoxide is refitted with the reflux condenser, and 1-bromobutane is added in small portions through the condenser. The vigorous spontaneous reaction should be allowed to subside before each addition. As the reaction proceeds, sodium bromide precipitates. After all the 1-bromobutane has been added, the reaction mixture is heated at reflux (heated to boiling with a reflux condenser attached to the flask) to complete the reaction.
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