Williamson Ether Synthesis - Introduction to Organic Chemistry - Lecture Notes, Study notes of Organic Chemistry

Download Williamson Ether Synthesis - Introduction to Organic Chemistry - Lecture Notes and more Study notes Organic Chemistry in PDF only on Docsity! 49 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. Reflux Column Docsity.com 50 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. CH3O - + H2O CH3OH + 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. After a reaction has been carried out, the product must be isolated from the reaction mixture and purified. The general procedure is termed a work-up. A specific work-up procedure is dictated by the physical and chemical properties of the products and by-products in the mixture. In this experiment, the first step in the work-up is the addition of water to dissolve the sodium bromide. Two phases result: an aqueous-methanol phase and an organic phase. These two phases could be separated in a separatory funnel; however, the separation would be poor because the product n- butyl methyl ether is partly soluble in the methanol-water phase. Because n-butyl methyl ether and methanol form an azeotrope that boils at 56°C, they are easily separated from the mixture by a distillation. The distillate contains n-butyl methyl ether and methanol, while the residue contains water, sodium bromide, and the bulk of the methanol. The next step in the work-up procedure is to remove the methanol from the product ether. Methanol is water-soluble, but the ether is not. An aqueous extraction of the methanol can be used at this point because most of the solvent methanol was left behind in the distillation step. An aqueous solution of calcium chloride instead of pure water is used to extract the methanol from the ether. The presence of the salt in the aqueous layer "salts out" the ether so that it is not carried into the water solution by the methanol. After extraction, the ether is dried with calcium chloride. Calcium chloride forms solid molecular complexes with both methanol and water; therefore, any residual methanol and water are removed from the product. Finally, the product is purified by distillation. If the earlier distillation and drying steps were carried out carefully, only the clear product (bp 65-68°C) will distil. If the distillate is cloudy, it contains water and must be redried and redistilled. If part of the material boils at 56°C, the boiling point of the ether-methanol azeotrope, then all the methanol was not removed. In this case, the product must be rewashed with aqueous calcium chloride, redried, and redistilled. Docsity.com 53 Decant the dried product into a 50-mL round-bottom flask. Use a dropper to transfer the final portion, being careful not to transfer any solid. Add 2-3 boiling chips to the flask; fit it for simple distillation; and distil the product, collecting the material boiling at 65-68°C. You should obtain about 10 g (57%) of n-butyl methyl ether. Measure the refractive index of the product, and calculate your per cent yield. Transfer the product to a correctly labeled vial and hand it in to your instructor. EXPERIMENTAL NOTES 1) A reflux condenser is an ordinary condenser arranged in an upright position, as shown in the figure, so that vapors from a boiling liquid are condensed and returned to the flask. A reflux condenser can be used during spontaneous exothermic reactions or when a liquid is being boiled by a heat source. The purposes of reflux are twofold: the reaction temperature can be maintained and the solvent is not lost to the atmosphere. 2) Sodium is stored under mineral oil to protect it from air and moisture. If the sodium has been diced by the storeroom personnel, remove the pieces with tweezers, blot off excess mineral oil with a laboratory tissue, and weigh the sodium on a tared watch glass. Do not allow it to sit in the air for any length of time. If the sodium has not been diced, use the following procedure. Remove a lump from the jar and blot off the oil. Dip your fingers in fresh mineral oil (not the sodium jar!) to provide a protective coating. (Using latex gloves is unwise. If a fire should occur, the latex can catch fire and cause severe burns.) Cut the crust from the sodium with a razor blade or knife to expose the shiny metal. Cut a wedge of the fresh metal and transfer it to a watch glass for dicing into pieces about the size of small peas, smaller than the inside diameter of the condenser. After the sodium has been cut, return all small slivers of the metal back to the main jar. Clean all utensils, desk, and watch glasses with a tissue dampened with isopropyl alcohol or methanol. Soak the tissues used for cleaning in a beaker with methanol in the hood for one hour before disposing of them (or before allowing them to come into contact with water). Clean your hands with a tissue soaked in isopropyl alcohol, then wash them thoroughly with soap and water. Docsity.com 54 Name Date Williamson Ether Synthesis Butyl methyl Ether Grams % Yield Ref. Index Experimental CRC/Theoretical 1a) Why is sodium metal stored under mineral oil? 1b) What is the crust that forms on sodium metal? 2) A chemist desires to wash the mineral oil off some sodium with a solvent. Which solvent or solvents would be appropriate? a) water b) ethanol c) pentane d) petroleum ether Docsity.com 55 3) What would be the expected results of each of the following actions by a student while carrying out this experiment? [Use equations in your answers to (d) and (e).] (a) Removing the reflux condenser to add Na to the methanol. (b) Adding the Na rapidly through the condenser. (c) Distilling 175 mL of methanol from the sodium methoxide instead of the 125 mL called for. (Give the immediate results and the results in subsequent reaction with 1- bromobutane.) (d) Using 95% ethanol instead of methanol in the experiment. (e) Leaving the sodium methoxide solution exposed to the air until the next laboratory period before adding the 1-bromobutane. (f) Pouring the 1-bromobutane through the condenser in one portion. Docsity.com