A
2-l. three-necked round-bottomed flask, with standard-taper ground-glass joints, is equipped for heating with an electric mantle or an
oil bath, and fitted with a
ball joint-sealed mechanical stirrer, a
thermometer well, a
graduated, pressure-equalizing dropping funnel, and a small vent. The dropping funnel, which should have a
close-fitting stopcock well lubricated with heavy silicone grease, is placed so as to discharge
sulfur trioxide well above the surface of the reaction mixture.
Palmitic acid (200 g., 0.78 mole) (Note
1) and
600 ml. of carbon tetrachloride (Note
2) are added to the flask. Solution of the
palmitic acid is endothermic and causes the temperature of the mixture to fall 5 to 10° below room temperature (Note
3). Stabilized liquid
sulfur trioxide, 53 ml. (100 g., 1.25 moles) (Note
4), is added dropwise from the dropping funnel to the stirred mixture. Solution of
palmitic acid becomes complete, the solution darkens, and the temperature rises to 45° as the
sulfur trioxide is added over a 30-minute period. The reaction mixture is finally heated for 1 hour at 50–65° with continued stirring and is then chilled in an
ice bath before the accessories are removed from the flask. The necks are closed with
glass stoppers, and the reaction mixture is refrigerated at about −15° overnight (Note
5).
Crystallized solids are filtered by suction (Note
6), washed with cold
carbon tetrachloride, and dried at room temperature in a
vacuum desiccator to constant weight (1 to 2 days). The crude dark product weighs
197–223 g. (
75–85%), and is usually satisfactory for the preparation of derivatives such as salts (Note
7) and esters.
2 One crystallization from
acetone (7 ml./g.) at −20° yields a light gray solid, with neutralization equivalent within 1–2% of the theoretical value of 168, in a yield of
178–197 g. (
68–75%). Two or three additional crystallizations (leaving a yield of only
25–30%) are required to give an almost colorless crystalline solid, m.p.
90–91° (Note
8). Once crystallized material is moderately stable to storage in a container protected from moisture, but slow darkening occurs, especially in presence of sunlight.