Organic Syntheses, CV 5, 734
Submitted by H. B. Stevenson and W. H. Sharkey
1.
Checked by R. D. Birkenmeyer, W. E. Russey, and F. Kagan
2.
1. Procedure
A
"Vycor" tube 550 mm. long and 25 mm. in outside diameter is packed for
500 mm. of its length with 6-mm. × 6-mm. quartz rings and mounted vertically so that the upper section is encased in a tube furnace 150 mm. long and the lower section is encased in a tube furnace 300 mm. long. The upper furnace, which serves as a preheater, is monitored by a thermocouple placed between the tube and the furnace heating elements. The temperature of the lower furnace is monitored by a
thermocouple located in the center of the lower packed section. The upper end of the pyrolysis tube is fitted with a
Y-tube carrying the thermocouple well and a graduated addition funnel with a pressure-equalizing arm. The lower end is attached through one
500-ml. trap and two
200-ml. traps, each immersed in a mixture of solid
carbon dioxide and
acetone, to a regulated vacuum source. One gram of
hydroquinone is placed in the first trap.
The pyrolysis tube is flushed with
nitrogen, the lower section is heated to 600° and the upper section to 300° (Note
1), and the pressure is regulated at 25 mm. Then
184 g. (1.00 mole) (Note 2) of dimethyl 3-methylenecyclobutane-1,2-dicarboxylate3 is admitted over a period of 3 hours (Note
3). The product, which amounts to 172–177 g., collects in the traps. It is distilled through a
13-mm. × 1.2-m. Nester spinning-band still.
4 First there is
41–47 g. (
48–55%) of
methyl acrylate,
n25D 1.4010, b.p.
34–36°/150 mm., then
39–43 g. (
40–44%) of
methyl butadienoate,
n25D 1.4635 (Note
4), b.p.
59–60°/52 mm. or
48–49°/26 mm. By continuing the distillation, 20–30 g. of starting material, b.p.
125–150°/25 mm., can be recovered.
2. Notes
1. The temperature of the lower section is quite critical and should be maintained within the range 590–610°. However, the preheater section is needed only to volatilize the ester, so any temperature between 200° and 400° is satisfactory.
2. Since an estimated
3–5 g. of carbon is deposited in the tube during the pyrolysis, it is advisable to pyrolyze only 1 mole of ester at a time and to burn out the
carbon with a slow stream of air at 600° between pyrolyses.
3. The space velocity is approximately 125 l. of standard gas per l. of free space per hour, and the contact time is approximately 0.3 second.
4. The checkers used a 10-mm. × 0.76-m. Nester spinning band still and obtained material having
nD25 1.4620 that could not be purified by redistillation. Analysis by vapor-phase chromatography (
silicone gum rubber, 20% w/w on firebrick, 120-cm. × 6-mm. outside diameter column at 125°) showed this material to be
95% methyl butadienoate contaminated by small amounts of two other materials.
3. Discussion
4. Merits of the Preparation
This procedure gives a product free of acetylenic groups. It illustrates the synthesis of an olefinic compound by cracking a
cyclobutane into two fragments.
This preparation is referenced from:
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