Checked by William S. Johnson and Kenneth L. Williamson.
1. Procedure
A
250-ml. three-necked flask fitted (glass joints) with a
sealed mechanical glass stirrer, an
addition funnel, and a
reflux condenser capped with a
calcium chloride drying tube is charged with
30.0 g. (0.140 mole) of 2-methyldodecanoic acid (Note
2) and
13.7 ml. (0.144 mole) of phosphorus tribromide (Note
3). Stirring is commenced, and
14.6 ml. (0.284 mole) of dry bromine (Note
4) is introduced slowly from the addition funnel until the reaction mixture retains a deep bromine coloration. The addition requires about 10 minutes to this stage (Note
5). The remainder of the
bromine is then added all at once, and the flask heated in a bath maintained at 85–90° (Note
6) for 1.5 hours. An additional
3.6 ml. (0.07 mole) of bromine is then added and the heating at 85–90° continued for 18 hours. The mixture is cooled to room temperature and poured into a
1-l. separatory funnel containing about 150 ml. of water and 200 g. of cracked ice. The transfer is completed with the addition of
150 ml. of benzene, and the separatory funnel is shaken vigorously for about 10 minutes, during which time most of the ice melts and the originally denser organic phase becomes the upper phase. The aqueous layer is separated and washed with
100 ml. of benzene, while the organic layer is vigorously shaken with another 200-ml. portion of ice water. This ice-water wash is also shaken with the
100-ml. portion of benzene and is then discarded. The combined
benzene extracts are filtered through anhydrous
sodium sulfate to remove suspended water; then the
benzene and residual
bromine are removed under reduced pressure (
water aspirator) at a bath temperature of 70° or below (Note
7). The crude bromoacyl bromide is added slowly, at room temperature, to a solution of
13.7 g. (0.35 g. atom) of potassium metal (Note
1) in
300 ml. of dry tert-butyl alcohol (Note
1) contained in a well-dried
1-l. flask fitted with a reflux condenser that is protected from moisture with a calcium chloride drying tube. The resultant suspension is heated at reflux for 1 hour, cooled, and diluted with three volumes of water. The mixture, containing insoluble
tert-butyl 2-methylenedodecanoate, is extracted with two
100-ml. portions of low-boiling petroleum ether (Note
8) and (Note
9). The combined
petroleum ether extracts are washed with water and filtered through anhydrous
sodium carbonate into a distilling flask from which the solvent is flash-distilled. The residual
tert-butyl 2-methylenedodecanoate is then distilled through a
61-cm. Podbielniak-type column (Note
10) at
129–130°/3.0 mm.; the yield is
18.5–21 g. of semipurified ester,
nD25 1.4405–1.4413 (Note
11) and (Note
12).
The
tert-butyl 2-methylenedodecanoate is hydrolyzed by heating under reflux with ethanolic
potassium hydroxide for 6 hours;
40 ml. of 95% ethanol and
3.7 g. (0.056 mole) of 85% potassium hydroxide are used for each 10 g. (0.037 mole) of the ester. The hydrolysis mixture is cooled, diluted with three volumes of water, and extracted with two
100-ml. portions of petroleum ether which are discarded. The aqueous phase is acidified to Congo red with
5N sulfuric acid, and the
2-methylenedodecanoic acid is extracted with two
150-ml. portions of low-boiling petroleum ether (Note
8). These extracts are combined, washed with three 100-ml. portions of water to ensure complete removal of
tert-butyl alcohol, and dried by filtration through anhydrous
sodium sulfate. The solvent is removed by flash distillation, and the acid is subjected to rapid distillation from a Claisen flask (Note
13).
2-Methylenedodecanoic acid containing less than
5% of 2-methyl-2-dodecenoic acid is collected at
149–151°/1.7 mm., m.p. about
32°, λ
max 209 mμ, ε 7800, in
hexane (Note
14). The yield is
10.5–12 g. (
35–40% from
2-methyldodecanoic acid), not including the acid obtained on acidification of the dehydrohalogenation mixture (Note
9).
2. Notes
2.
2-Methyldodecanoic acid was prepared as follows according to the method of Cason, Allinger, and Williams.
2 A three-necked flask fitted with a sealed mechanical stirrer,
dropping funnel, and
efficient reflux condenser is charged with
1.4 l. of absolute ethanol; then, while stirring,
48.3 g. (2.1 g. atoms) of sodium is added gradually in small pieces. The dropping funnel is charged with
383 g. (2.2 moles) of diethyl methylmalonate (Matheson, Coleman and Bell) which is added to the
sodium ethoxide solution over a period of about 20 minutes; then the mixture is boiled under reflux for 5 minutes. The dropping funnel is charged with
442 g. (2.0 moles) of n-decyl bromide (Eastman Kodak Company, white label brand) which is added to the mixture as rapidly as is allowed by the exothermic reaction. After the addition is complete (about 20 minutes), the mixture is boiled under reflux for 2 hours, then neutralized with a few drops of
glacial acetic acid. About two-thirds of the alcohol is removed by distillation, and 2 l. of water is added to the residue. The organic phase is separated, and the aqueous phase extracted with three
250-ml. portions of benzene. The organic phase and extracts are combined, washed with water, and dried over anhydrous
sodium sulfate. The residue obtained upon evaporation of the solvent is treated with a solution of
447.5 g. of 85% potassium hydroxide pellets in 3.5 l. of 95% ethanol and the mixture heated at reflux, with stirring, for 4 hours. About two-thirds of the solvent is removed by distillation, 3 l. of water is added, followed by sufficient (about
2 l.) 6N sulfuric acid (cooling is necessary) to bring the pH of the solution to 1–2. The organic phase is separated, and the aqueous phase (containing some precipitated sulfates) is extracted with two portions of
ether. The organic phase and extracts are combined, washed with water, then with saturated
sodium chloride solution, and finally dried over anhydrous
sodium sulfate. The residue obtained upon evaporation of the
ether is heated at 180–190°, at which temperature decarboxylation occurs smoothly over a 20-minute period. The material is then distilled from a
modified Claisen flask. The yield of product, b.p.
159–161°/4.4 mm., is
262–318 g. (
61–74%),
nD25 1.4404–1.4408.
3.
Phosphorus tribromide (Eastman Kodak Company, white label brand) was freshly distilled before use. The full molar equivalent accelerates the desired α-bromination.
5. The first mole equivalent of
bromine reacts with the
phosphorus tribromide to form the solid pentabromide which, in turn, is rapidly consumed in the formation of the acyl bromide.
6. The temperature is critical. At lower temperatures the reaction is very slow, and at higher temperatures partial dehydrobromination and subsequent allyic bromination result in contamination of the product with dienoic acid, as evidenced by the characteristic intense absorption at 275 mμ in
hexane.
7. Higher temperatures at this point tend to promote dehydrobromination under the acidic conditions with the formation of
2-methyl-2-dodecenoic acid.
8. Commercial
hexane from petroleum fraction (b.p.
65–68°) is satisfactory.
9. Acidification of the alkaline aqueous layer to Congo red with
5N sulfuric acid liberates
2–4 g. of crude
2-methylenedodecanoic acid which can be isolated by extraction with
petroleum ether as described below. Distillation of such material gave a product which did not crystallize readily.
10. A simplified Podbielniak column
3 was employed. Other columns of comparable efficiency should be suitable.
11. If the distillation is carried out in a Claisen flask, the final product is impure, apparently contaminated with about
10% of 2-methyl-2-dodecenoic acid.
12. Pure
tert-butyl 2-methylenedodecanoate may be isolated by refractionation of this product through an
efficient column. The yield in such an experiment was
15–17 g. of material with
nD25 1.4400.
13. Fractionation through a column results in some isomerization.
14.
2-Methylenedodecanoic acid, free from any isomeric impurities, can be obtained by recrystallization from
acetone. The pure acid is recovered in over
70% yield, m.p.
33.3–34.2°, λ
max 210 mμ, ε 7500, in
hexane. It is very difficult to obtain pure methylene acid from Claisen-distilled
tert-butyl ester (cf. (Note
11)).
3. Discussion
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved