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Saturday, December 22, 2012

Synthesis of n-Butyl Methyl Ether - SN2 Reactions - Lu Le Laboratory

The Williamson ether synthesis is an organic reaction, forming an ether from an organohalide and an alcohol.

A Williamson Ether Synthesis consists of two separate reactions:

1.      The preparation of an alkoxide
2.      The reaction of alkoxide with the alkyl halide

Sodium methoxide could be 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 reaction is an exothermic reaction, it would make the methanol boil. To prevent the methanol from boiling away, we set a condenser upon the reaction flask, called a reflux condenser. Methanol vapors condenser 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 andthe work area.
        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.

5.5g 1-bromobutane
8 g anhydrous calcium chloride
3 g 25% aqueous calcium chloride
40 mL methanol
1.4g diced sodium metal


Step 1: Preparation of Sodium Methoxide

1.      Place 40 mL of methanol in a 50-mL round-bottom flask and fit the flask with a reflux condenser. Add 1.4 g of diced sodium metal through the top of condenser, allowing the reaction to subside before adding the next piece. If the sodium sticks to the inside of the condenser tube, push it into the reaction flask with a long glass tube or rod.

 Reflux Apparatus

 Sodium Metal under Ether

 Sodium is Reacting with Methanol

2.      After the sodium has completely reacted, fit the flask for a simple distillation, put a stir bar into the reaction flask, and distil 25mL of methanol into a graduated cylinder. If it is necessary to stop the experiment at this point, store the sodium methoxide in the round bottom flask with a heavily greased glass stopper.

After the sodium has completely reacted

Distill 25mL Methanol out from the mixture

The Boiling Point of Methanol
Some Sodium Methoxide Precipitate on the Wall of the Round-Bottom Flask

Step 2: Reaction of Sodium Methoxide and 1-Bromobutane

1.      Fit the 50-mL round-bottom flask containing the sodium methoxide with a reflux condenser.

2.      Cool the flask to room temperature with an ice bath. Weigh 5.5 g of 1-bromobutane into a flask. Using a dropper, add the 1-bromobutane into the reaction vessel through the top of the condenser in 1~2mL aliquots over about a 2minute period. Although warming may be required to start the reaction, the reaction is quite exothermic.

 Sodium Bromide Precipitate from the Solvent

3.      Do not add all the 1-bromobutane in one portion. Cool the reaction flask with an ice bath, during the addition. Cork the Erlenmeyer flask between additions so that the
1-bromobutane does not evaporate.

4.      After the addition has been completed, let the reaction vessel stand at room temperature until the exothermic reaction has subsided and the methanol ceases to reflux. Then heat the mixture at a gentle reflux, or "simmer," for one-half hour with a heat plant. 
5.      The mixture will bump because of the precipitated solid; therefore, do not attempt a vigorous reflux.
6.      After the reflux, cool the reaction vessel with a water or ice bath and add 4~6 mL of water to the mixture through the reflux condenser. If all the sodium bromide does not dissolve, add a few additional milliliters of water. Dissolving the sodium bromide prevents bumping in the next step.

 A Huge of Sodium Bromide Formed

7.      Equip the flask for simple distillation. Distil the two-layered mixture, collecting the material that boils up to 64°C. The volume of distillate should be about  8mL.

 Add Some Water into the Mixture to Dissolve NaBr

 The Clean Liquid is the Distillate of n-Butyl Methyl Ether

8.      Transfer the distillate to a 25mL separatory funnel and extract it with three 3mL portions of 25% aqueous calcium chloride. (The calcium chloride solution is the lower layer in this extraction, and the interface is difficult to see.)

 The upper layer is the ether

9.      Pour the product from the separatory funnel into a 25mL round-bottom flask, add  a gram of anhydrous calcium chloride, stopper the flask, and let it stand overnight.

 Here is the final product

Experimental Record

Weight of n-Butyl Methyl Ether
2.315 g
Weight of n-Butyl Methyl Ether(Theory)
3.520 g
65.8 %

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