A
1-l., three-necked, round-bottomed flask with ground-glass fittings is charged with a mixture of
36 ml. of boron trifluoride diethyl etherate,
72 ml. of glacial acetic acid, and
120 ml. of chloroform (Note
1). The flask is equipped with a
spiral reflux condenser, an
efficient mechanical stirrer, and a
dropping funnel (Note
2). The
chloroform solution is heated and maintained at reflux with vigorous stirring, and a solution of
32 g. (30 ml., 0.30 mole) of 1,3-propanedithiol and
25 g. (29 ml., 0.33 mole) of methylal (dimethoxymethane) in
450 ml. of chloroform (Note
1) is added at a constant rate over 8 hours. The mixture is allowed to cool to room temperature, washed successively with four 80-ml. portions of water, twice with
120 ml. of 10% aqueous potassium hydroxide, and twice with 80-ml. portions of water. The
chloroform solution obtained is dried over
potassium carbonate and concentrated in a
500-ml., round-bottomed flask under reduced pressure with a
rotating flask evaporator (Note
3). The residue, which crystallizes on cooling to room temperature, is dissolved in
60 ml. of methanol by heating to the boiling point. The hot solution is filtered rapidly through a
prewarmed funnel, allowed to cool slowly to room temperature, then kept overnight at −20°. The colorless crystals are collected by filtration through a
prechilled Büchner funnel, washed with
cold methanol (−20°), and dried under reduced pressure (Note
3) and (Note
4), yielding
28–29 g. of product, m.p.
53–54°. Solvent is removed from the mother liquor, and the residue is recrystallized as described, giving an additional
1.5–2.0 g.; the total yield of recrystallized
1,3-dithiane is
29.5–31.0 g. (
82–86% based on
propane-1,3-dithiol). A purer sample (Note
5) can be prepared by subsequent sublimation of the recrystallized product at 0.1–0.5 mm. (45–48° bath temp.) (Note
6). The yield of pure product is
28–30 g. (
78–84%), m.p.
53–54°. The residue from the sublimation is a brown syrup weighing less than 1 g.