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Org. Synth. 1956, 36, 83
DOI: 10.15227/orgsyn.036.0083
α-SULFOPALMITIC ACID
[Hexadecanoic acid, 2-sulfo-]
Submitted by J. K. Weil, R. G. Bistline, Jr., and A. J. Stirton1.
Checked by James Cason and Gerhard J. Fonken.
1. Procedure
Caution! Rubber gloves and a protective face shield should be worn while handling liquid sulfur trioxide, and the reaction should be carried out in a hood.
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.
2. Notes
1. A purified fatty acid is recommended for the preparation of a pure α-sulfo acid. Purified palmitic acid (m.p. 60.8–61.4°, neutralization equivalent 256.2) is prepared by twice recrystallizing a good commercial grade of palmitic acid from acetone at 0°, using a solvent ratio of 10 ml. to 1 g. However, the reaction may be applied to commercial saturated higher fatty acids, if the iodine number is sufficiently low. The checkers obtained similar results with recrystallized Neo-Fat 1-56 (Armour and Company, Chicago, Ill.) or Eastman white label grade palmitic acid.
2. Other chlorinated solvents such as tetrachloroethylene or chloroform may be used in place of carbon tetrachloride. Caution! The reaction of sulfur trioxide with chlorinated solvents has been reported3 to give phosgene and other toxic products. Adequate venting of all by-product gases is essential.
3. The mixture which is a slurry at 15° may be used or it may be warmed to about 30° to give a clear solution. If the slurry is warmed before sulfur trioxide addition, it is usually not necessary to heat after all of the sulfur trioxide has been added.
4. Liquid sulfur trioxide may be purchased in stabilized form as Sulfan B,4 m.p. 17°, b.p. about 45°. Caution must be exercised in handling sulfur trioxide. The liquid is highly corrosive to the skin and the vapor may cause injury if inhaled. The powerful oxidizing and dehydrating effects of sulfur trioxide should not be underestimated. The liquid may be stored in a glass-stoppered bottle if the stopper is lubricated with a heavy silicone grease. Because the liquid reacts violently with water the bottle lip should be wiped free of any moisture to avoid spattering when the liquid is poured. If solidification of sulfur trioxide should occur as the result of hydration, the vented (Hood!) solid may be liquefied by gentle application of heat from an infrared lamp.
5. Shorter refrigerating periods at temperatures up to +5° give slightly lower yields.
6. Since the mixture filters rather slowly, even after storage overnight, a Büchner funnel of at least 20 cm. diameter should be used, and a hard grade of filter paper (such as Whatman No. 50) is recommended. Use of a rubber dam helps to express solvent and exclude moisture from the hygroscopic product.
7. The monosodium salt CH3(CH2)13CH(SO3Na)CO2H may be prepared by neutralizing only the sulfonic acid group or by adding aqueous sodium sulfate to a hot aqueous solution of the crude α-sulfo acid and cooling to room temperature. The monosodium salt crystallizes in white plates, leaving most of the color in the filtrate. The disodium salt is formed by further neutralization with sodium hydroxide.
8. α-Sulfolauric, α-sulfomyristic, α-sulfostearic, and α-sulfobehenic acids have been prepared by this procedure. The table shows the melting point and neutralization equivalent after at least four recrystallizations and a 12-hour drying period at 76° under 1 mm. pressure. There is some uncertainty in the melting points because of the hygroscopic character of the α-sulfo acids.

Neutralization Equivalent

Found

Theoretical

M.P.


α-Sulfolauric acid

142

140.4

86.5–88°

α-Sulfomyristic acid

154

154.2

85–86.5°

α-Sulfopalmitic acid

171

168.2

90–91°

α-Sulfostearic acid

182

182.2

96–97°

α-Sulfobehenic acid

212

210.3

95–97°


3. Discussion
Other direct methods for the sulfonation of the higher fatty acids are by the use of sulfur trioxide vapor5 or by the use of chlorosulfonic acid.6 Indirect methods are also available for the preparation of α-sulfo fatty acids and their salts from an α-bromo fatty acid made by the Hell-Volhard-Zelinsky reaction. The bromo compound may be converted directly to the sodium salt of a sulfonic acid through the Strecker reaction7 or may be converted to the mercaptan and oxidized to the sulfonate.8 Sulfonation of the lower fatty acids has been studied by Backer and co-workers.9
α-Sulfonation with sulfur trioxide appears to be generally applicable to carboxylic acids having an α-methylene group.

References and Notes
  1. Eastern Regional Research Laboratory, Philadelphia 18, Pennsylvania, a laboratory of the Eastern Utilization Research Branch, Agricultural Research Service, U. S. Department of Agriculture, Philadelphia 18, Pennsylvania.
  2. Weil, Bistline, and Stirton, J. Am. Chem. Soc., 75, 4859 (1953).
  3. "Reactions of SO3," Tech. Service Bull. SF-2, General Chemical Division, Allied Chemical and Dye Corporation, 40 Rector Street, New York 6, N. Y.
  4. "Storage, Handling and Use of Sulfan," Tech. Service Bull. SF-3, General Chemical Division, Allied Chemical and Dye Corporation.
  5. Günther (1932), PB30081, Office of Tech. Services, U. S. Department of Commerce; Bibliography of Scientific and Industrial Reports, 4, 662 (1947).
  6. Günther and Hetzer (to I. G. Farbenindustrie A.-G.), U. S. pat. 1,926,442 [C. A., 27, 6001 (1933)].
  7. Mehta and Trivedi, Melliand Textilber., 21, 117, 288 (1940) [C. A., 34, 6087 (1940)].
  8. Weil, Witnauer, and Stirton, J. Am. Chem. Soc., 75, 2526 (1953).
  9. For example, de Boer, Rec. trav. chim., 71 814 (1952).

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

monosodium salt CH3(CH2)13CH(SO3Na)CO2H

sodium hydroxide (1310-73-2)

chlorosulfonic acid (7790-94-5)

sulfur trioxide (7446-11-9)

chloroform (67-66-3)

sodium sulfate (7757-82-6)

carbon tetrachloride (56-23-5)

acetone (67-64-1)

phosgene (75-44-5)

tetrachloroethylene (127-18-4)

palmitic acid (57-10-3)

α-Sulfopalmitic acid,
Hexadecanoic acid, 2-sulfo- (1782-10-1)

α-Sulfolauric acid

α-Sulfomyristic acid

α-Sulfostearic acid

α-Sulfobehenic acid