Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

204 ACID CHLORIDES.

provide an air condenser as in (a) and fit a thermometer in the

neck of the flask by means of asbestos cord. On distilling the

liquid the dissolved hydrogen chloride escapes first. Save the

distillate between 150° and 165° and purify it by redistilling.

Yield, about 170 g. of sulphuric acid monochloride, boiling-point

153°.

Dependent preparation: Pyrosulphuric Acid Chloride No. 150.

150. Pyrosulphuric Acid Chloride (Disulphuryl Chloride),
     Q/SO 2 C1
     N SO
     2 C1
     The action of phosphorus pentachloride on sulphuric acid monochloride
     results in the formation, not of sulphuric acid dichloride, but of pyrosul-
     phuric acid chloride, with the splitting out of a molecule of water:
     Cl SO 2 OH Cl SO 2.
     ^ O + H 2 O.
     Cl SO 2 OH Cl SO 2 /
     For the purpose of withdrawing the water, however, phosphorus pentoxide
     serves better than the pentachloride. It is upon this mode of forming the
     acid chloride that the customary constitutional formula of pyro- or di-sul-
     phuric acid is based.
     Introduce 20 g. of phosphorus pentoxide and then 30 g. of
     sulphuric acid monochloride into a small retort, and close the

tubulus with asbestos. Distil slowly without a condenser and

catch the distillate in a flask that rests in a bath of cold water.

Redistil the first product from a small flask with a side-arm con-

denser and with a thermometer made tight with asbestos cord.

The liquid boils at 146°. Yield, 20 to 25 grams. Preparations

of entirely pure pyrosulphuric acid chloride boil at 152.5-153°.

151. Sulphurous Acid Chloride (Thionyl Chloride) SOCV
     Provide a liter, round-bottomed flask with a stopper through

which a delivery tube and the lower end of a return condenser are
inserted, the latter so as to just pass through the stopper and the

former so as to reach to the bottom of the flask. Place this

apparatus under the hood, and add 500 g. of phosphorus penta-

chloride (not less) to the flask. Introduce sulphur dioxide
(from 200 g. copper and 400 g. concentrated sulphuric acid),

purifying it by bubbling it through sulphuric acid and then pass-

ing it through a tube filled with crystals of potassium sulphate.

(^1) Another method for preparing thionyl chloride depends on the reac-
tion: SO 3 + SC1 2 = SO 2 + SOC1 2.