To a suspension of
37.4 g. (0.2 mole) of o-chlorophenylthiourea (p. 180) in 300 ml. of water at 100°, contained in a
3-l. beaker, is added a boiling solution of
132 g. (2 moles) of 85% potassium hydroxide in 300 ml. of water. The resulting solution is immediately treated with a hot saturated solution of
83.5 g. (0.22 mole) of lead acetate trihydrate, added as rapidly as possible and with good stirring (Note
1). The reaction mixture, from which large quantities of
lead sulfide separate instantly, is boiled for 6 minutes and cooled to 0°, and the
lead sulfide is filtered with suction by means of a
large Büchner funnel (Note
2). The colorless filtrate is acidified at 0–5° (Note
3) by the slow addition with stirring of
120–140 ml. of glacial acetic acid. The white crystalline precipitate of nearly pure
o-chlorophenylcyanamide which separates is collected by filtration on a
suction filter and is washed with six 150-ml. portions of ice water. The crystalline mass of white plates is filtered, drained, and dried (Note
4). The yield of product, melting at
100–104°, is
26–28 g. (
85–92%).