B.
Diethyl mercaptoacetal. A
12-l. three-necked flask, equipped with an
efficient Hershberg stirrer, a
Dry Ice condenser, and a
stopper, is placed in an
enameled container (Note
6). The flask is charged with
500 g. (1.38 moles, calculated as tetrasulfide) of 1,1,1',1'-tetraethoxyethyl polysulfide and
500 ml. of anhydrous ether. The condenser is filled with
acetone and Dry Ice, and the contents of the flask are cooled by means of
acetone and Dry Ice kept at −35° to −45°.
Liquid ammonia (Note 7) is introduced into the stirred solution until 4.5–5.0 l. has been added (Note
8). The solution is stirred for several minutes to ensure homogeneity, and
200 g. (8.7 g. atoms) of sodium is added in 2- to 5-g. lumps to the mixture over a period of 30–45 minutes. The last
2–10 g. of sodium causes the solution to turn deep blue. The mixture is stirred for 30 additional minutes, and
ammonium chloride (about 200 g.) is cautiously added with stirring until the blue color is discharged. The cooling bath is removed and replaced by a bath of hot water into which steam may be introduced. The mixture is stirred while the
ammonia is removed by distillation or evaporation (the distillation should be continued until the residue can no longer be stirred or is difficult to stir). The soft, semisolid mass containing small amounts of
ammonia is cooled in ice, and 1 l. of ice-cold water is added with stirring. When most of the solid has dissolved, ice-cold
hydrochloric acid is added cautiously, with frequent checks of the pH, until a pH of 8.0–8.5 is reached. The pH is then adjusted to 7.8–8.0 (preferably by a pH meter) by passing
carbon dioxide into the cold solution (Note
9). The oil is separated from the aqueous layer, which is extracted with two
300-ml. portions of ether. The combined organic layers are washed successively with 100 ml. of water and two
100-ml. portions of saturated sodium chloride, and are dried over
magnesium sulfate. The
ether is removed by distillation, and the residue is distilled from a
500-ml. flask using a
small Vigreux column or a
Claisen head. The product (Note
10) weighs
320–372 g. (
77–90% yield, based on polysulfide;
71–83% yield, based on bromoacetal), boils at
62–64°/12 mm., and has
n25D 1.4391–1.4400. The residue,
23–27 g.,
n25D 1.4702–1.4730, consists mainly of
1,1,1',1'-tetraethoxyethyl sulfide.