The apparatus (Fig. 14) is similar to that described on p. 795 and in a previous volume.² It consists of a Pyrex glass reaction tube, 90 cm. long by 45 mm. in outside diameter, mounted vertically in an electric furnace about 50 cm. long. Attached to the top of the tube are a graduated dropping funnel (Note 1), an inlet tube for nitrogen, and a thermocouple well extending to the bottom of the heated section and holding a movable thermocouple. The entire heated section, which begins 10 cm. from the top of the tube, is packed with Pyrex glass rings 10 mm. in outside diameter by about 10 mm. in length held in place by a plug of glass wool supported by indentations in the tube. The lower end of the tube is attached to a 1-l. round-bottomed flask immersed in an ice bath and having a side arm from which vapors pass successively through a trap immersed in an ice bath and a trap immersed in a bath of Dry Ice and acetone. Each trap is capable of holding about 200 ml. of liquid. The temperature of the hottest part of the tube, which is located near the middle of the heated section, is raised to 575° ± 10° while nitrogen (Note 2) is passed successively through a flowmeter and the tube at a rate of 4–6 1./hr. Under these conditions (Note 3), 658 g. (645 ml., 3.5 moles) of 1,5-pentanediol diacetate (Note 4) is added to the tube over a period of 3.5 hours. The contents of the three receivers are combined and distilled at atmospheric pressure through a 15-cm. indented Claisen-type still head; the condenser is cooled with ice water, and the receiver is immersed in an ice bath. The fraction boiling at 25–55°, wt. 170–190 g., is redistilled through a 60-cm. column packed with glass helices or a column of similar efficiency to give 150–170 g. (63–71%) of 1,4-pentadiene, b.p. 26–27.5° /760 mm., n_D25 1.3861–1.3871 (Note 5).

A. α-Chloroethyl ethyl ether. A mixture of 200 g. (201 ml.) of redistilled paraldehyde, b.p. 121–122.5° (equivalent to 4.54 moles of acetaldehyde), and 200 g. (254 ml., 4.34 moles) of absolute ethanol is placed in a 1-l. three-necked flask fitted with a mechanical stirrer and a gas inlet tube reaching to the bottom of the flask. The mixture is cooled to −5° in a mixture of Dry Ice and acetone, and dry hydrogen chloride (Note 1) is passed into the stirred reaction mixture maintained at about −5° until 200 g. (5.48 moles) has been absorbed. During this operation, which requires about 2 hours, the reaction mixture separates into two layers. The upper layer of crude α-chloroethyl ethyl ether is removed, and the dissolved hydrogen chloride is swept out by bubbling dry nitrogen gas through the mixture. The product is dried overnight with 25–50 g. of anhydrous calcium sulfate (Note 2). The yield of crude α-chloroethyl ethyl ether (Note 3) is 411–432 g. (87–92% based on ethanol). The product is not distilled, since it decomposes readily.

B. α,β-Dibromoethyl ethyl ether. Four hundred and twenty-five grams (3.92 moles) of the crude α-chloroethyl ethyl ether (part A) is placed in a dry 1-l. three-necked flask, fitted with a dropping funnel, mechanical stirrer, and an outlet tube leading to a gas absorption trap⁸ to dispose of the hydrogen chloride evolved. The flask is cooled in an ice bath, and 625 g. (200 ml., 3.92 moles) of bromine is added in small portions from the dropping funnel with stirring. The reaction mixture is allowed to become almost colorless after each addition. When all of the bromine has been added (5–6 hours), a slow current of dry nitrogen is bubbled through the reaction mixture to sweep out the hydrogen chloride (Note 4). The product is dried overnight with 25–50 g. of anhydrous calcium sulfate and then distilled at reduced pressure (Note 5). The yield of colorless α,β-dibromoethyl ethyl ether is 599–663 g. (66–73% based on the α-chloroethyl ethyl ether), b.p. 70–75° /27 mm., n_D20 1.5097–1.5102.

C. Allylmagnesium bromide (Note 6). In a dry 5-l. three-necked flask, equipped with a sealed stirrer, a reflux condenser and drying tube, a pressure-equalized dropping funnel, and a nitrogen inlet tube, are placed 195 g. (8.0 g. atoms) of dry magnesium turnings and 2.4 l. of anhydrous ether. The flask is cooled in an ice bath, a small crystal of iodine is added, and a solution of 400 g. (287 ml., 3.31 moles) of allyl bromide (redistilled, b.p. 69–71°) in an equal volume of anhydrous ether is added dropwise over a period of 17 hours. During the reaction a slow stream of dry, oxygen-free nitrogen is passed through the flask. After the addition is complete, the reaction mixture is stirred for 30 minutes. The Grignard reagent is decanted into a dry, graduated storage bottle; the residue in the flask is washed with 150–200 ml. of dry ether; and the wash liquid is added to the storage bottle. Samples of the clear supernatant solution are analyzed by acidimetric titration.⁹ The yield of allylmagnesium bromide is 2.62–2.95 moles (79–89%).

D. α-Allyl-β-bromoethyl ethyl ether. The same apparatus is used as in the preparation of allylmagnesium bromide. The flask is charged with an amount of the Grignard solution (part C) equivalent to 2.78 moles of allylmagnesium bromide (or chloride) and cooled in an ice bath. A solution of 580 g. (2.5 moles) of α,β-dibromoethyl ethyl ether (part B) in an equal volume of anhydrous ether is added slowly with stirring over a period of 3–4 hours. The mixture is allowed to stand overnight and is then hydrolyzed with 75 ml. of 20% acetic acid followed by 500 ml. of water. The ether layer is separated, washed with four 100-ml. portions of 10% aqueous sodium bicarbonate solution followed by four 100-ml. portions of saturated aqueous sodium chloride solution, dried over 100 g. of anhydrous calcium sulfate, and distilled under reduced pressure. The yield of colorless α-allyl-β-bromoethyl ethyl ether is 370–396 g. (77–82% based on the α,β-dibromoethyl ethyl ether), b.p. 72–75° /21 mm., n_D20 1.4600–1.4606.

E. 1,4-Pentadiene. A 2-l. three-necked flask is equipped with a sealed stirrer and a 28-cm. reflux condenser, at the top of which is a 2.5 × 35 cm. Vigreux fractionating column attached to an efficient condenser arranged for distillation. Water at 35–40° is pumped through the reflux condenser, ice water is pumped through the downward condenser, and the receiver is ice-cooled and attached to a trap cooled by Dry Ice. The flask is charged with 380 g. (1.97 moles) of α-allyl-β-bromoethyl ethyl ether (part D) in 550 ml. of n-butyl alcohol; then 550 g. (8.4 g. atoms) of zinc dust and 2 g. of anhydrous zinc chloride are added. The mixture is stirred vigorously and heated gradually to the point where the pentadiene distils at a rate of about one drop every 2 seconds. The reflux condenser and column return most of the butyl alcohol to the flask. The reaction takes 5–6 hours for completion. The distillate is washed with five 100-ml. portions of ice water to remove most of the butyl alcohol and is then dried overnight with 15 g. of calcium chloride. The crude dry product is distilled through a 2.5 × 35 cm. Vigreux column to give 97–102 g. (72–76%) of colorless 1,4-pentadiene, b.p. 26–27° /740 mm., n_D20 1.3887–1.3890.