Published TCIMAIL newest issue No.196
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Precursors for Preparation of Highly Reactive Reagents
The highly reactive reagents are generally unstable and frequently arrive to the customer in a decomposed state. Consequently, these compounds should be prepared immediately prior to use.
The precursors required for such preparations are shown below. For details, please refer to references.
Preparation of Cyanamide
Glacial acetic acid 57 g (54 mL, 0.75 mol) and water 135 mL are placed in a large sized agate mortar and while thoroughly stirring, calcium cyanamide 40 g (0.5 mol) is slowly added and the mixture is grounded. Acetylene gas is generated, yielding a dull grayish to dark colored paste. Subsequently, the paste is dried at 40 - 50°C for 12 - 18 hours under reduced pressure 30 mmHg to yield a gray powder which is placed in a Soxhlet extractor. The powder is extracted with 400 mL of ether containing several drops of dilute acetic acid for 2 - 3 hours (2×). The ether extraction is dried over anhydrous sodium sulfate (30 g). Concentration under a reduced pressure gives 10.5 - 15.8 g (50 - 70% from CaCN2) colorless viscous oil.
Reference
Preparation of Cyclopentadiene
The reaction apparatus (Figure) is assembled by attaching a thermometer, Vigreaux distilling column and Liebig condenser to a 500 mL two necked flask. The receiver is cooled by means of a refrigerants or freezing mixtures. Dicyclopentadiene (195 g) is placed in the two necked flask and heated to about 160°C with an electric heating mantle or oil bath. Thermal decomposition begins at about 150°C and distillates at 38 ~ 46°C can be obtained (addition of iron powder expedites the thermal decomposition rate of cyclopentadiene). Heating is applied slowly, because rapid temperature increases cause dicyclopentadiene to distill prior to undergoing thermal decomposition. In such a case, re-distillation allows narrow boiling-point distillates. Cyclopentadiene dimerzes rapidly at room temperature and should be used immediately.
Reference
Preparation of Diazomethane
A reaction and distillation apparatus is assembled by connecting an addition funnel and condenser to a 100 mL long-neck distillation flask. Two in series receiving flasks are connected to the apparatus with the second flask containing an induction tube. A solution containing 6 g potassium hydroxide in 10 mL water, and a solution containing 35 mL carbitol (diethylene glycol monoethyl ether) and 10 mL of ether are placed in the distillation flask. 20 ~ 30 mL ether is placed in the second receiving flask making sure the induction tube is immersed into the ether. Both receiving flasks are cooled to 0°C. A solution containing N-methyl-N-nitroso-p-toluenesulfonamide (21.5 g, 0.1 mol) in 140 mL ether is placed in the addition funnel. Heat the distillation flask to 70°C in a warm water bath. As the ether begins to boil, start adding the N-methyl-N-nitroso-p-toluenesulfonamide solution dropwise over a 20 minute period. Swirl distillation flask from time to time. About 3 g of diazomethane (0.07 mol) is dissolved in ether distillate. The reagent should be used at once without storing it.
*Note: Diazomethane is explosive and highly toxic. Wear protective gloves, protective goggles, protective mask and work in a well ventilated fume hood. Also place a safety shield in front of distillation apparatus. Diazomethane may explode upon contact with ground glass joints or other scratched glass surfaces, so avoid handling or preparing this compound with such surfaces. Ordinarily, diazomethane is used as a solution in ether.
*We have TMS diazomethane available. This reagent is analogous in reactivity to diazomethane, but less explosive and less toxic. Please consider using this reagent prior to preparing diazomethane.
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Reference
Preparation of Dimethyl Ketene
Tetramethyl-1,3-cyclobutanedione is placed in the ketene generation apparatus, heated at 120°C, and sublimed, then thermally decomposed by means of the red heated nichrome wire under the nitrogen atmosphere. Subsequently, the evolved gas is captured by means of a trap cooled in dry ice-acetone to obtain dimethylketene in about 60% yield. Dimethylketene is a yellowish liquid (bp 34°C) with characteristic odor, remarkable reactivity, and transforms into a solid dimer when allowed to stand at room temperature. On exposure to air it generates white solid peroxides and explodes with even the slightest stimulus. For this reason, its monomer is stored under nitrogen at -78°C and it is used immediately.
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Reference
Preparation of Ethyl Diazoacetate
A solution containing 140 g glycine ethyl ester hydrochloride (1 mol) in 250 mL water is placed in a 2 L four-neck flask. 600 mL dichloromethane is added and the mixture is cooled to -5°C under a nitrogen atmosphere. A solution containing 83 g sodium nitrite in 250 mL water is cooled to 0°C and added with stirring to the reaction mixture. While keeping the reaction temperature below -9°C, 95 g 5% sulfuric acid is added dropwise to the reaction over a 3 minute period. After ten minutes addition stirring the reaction is transferred to a 2 L separatory funnel while cold. The isolated yellow-greenish organic phase is poured into a cooled 1 L 5% sodium carbonate solution, then placed into a separatory funnel and shaken. The isolated aqueous phase is extracted with 75 mL dicholoromethane, dried over 15 g anhydrous sodium sulfate, and concentrated under reduced pressure (350 mmHg). Futher concentration at 20 mmHg and a bath temperature of 35°C gives 90 - 100 g (79 - 88%) yellow oily substance. The product is pure enough to be used for ordinary synthesizing purposes. It is explosive, and distillation under even reduced pressure is hazardous.
*Note: Ethyl diazoacetate is explosive and very toxic. Wear protective gloves, protective goggles, protective mask, and use a safety screen. Work in a well ventilated fume hood.
Reference
Preparation of Anhydrous Hydrogen Bromide
Bromine is slowly added dropwise to a round bottom flask containing a suspension of iron and 1,2,3,4-tetrahydronaphthalene. In the early stage of the reaction, the flask is cooled with water. The reaction is warmed to 30 to 40°C allowing gas evolution. The gas is passed through a wash bottle containing 1,2,3,4-tetrahydronaphthalene in order to eliminate any bromine in the hydrogen bromide. For optimum yields of hydrogen bromide, dry the 1,2,3,4-tetrahydronaphthalene over anhydrous sodium sulfate and then distill it under a dry inert atmosphere (such as argon or nitrogen). If any moisture is introduced into the reaction, the yield of hydrogen bromide becomes remarkably poor.