Methods of Preparation of Fatty Acids and Their Derivatives: Laboratory Exercises

The Author: Gary R. List, Formerly of - National Center for Agricultural Utilization Research, ARS, U.S. Department of Agriculture, Peoria, IL, USA

Preface and Foreward

The information reported here was assembled by Dr. Howard Teeter (1915-1993) who took his Ph.D. under Professor Roger Adams at the University of Illinois in 1938. Teeter spent most of his career at the Northern Regional Research Laboratory (NRRL) in Peoria, IL. By 1950, when this manual was assembled, he was in charge of the Industrial Oil section at NRRL. It was intended to be a resource for both experienced organic chemists and as a training tool for young researchers beginning their careers in fats and oils.

Introduction

Most food science programs do not include laboratory courses in the preparation of fatty acids and their derivatives. Thus, while much of the traditional analysis and characterization of lipids has been replaced by instrumental methods including gas chromatography, mass spectrometry, nuclear magnetic resonance, high-performance liquid chromatography, etc., wet chemistry still has a place in fats and oils. For example, a large supply of high-purity oleic acid is needed for research purposes.  Some would find this to be a challenging problem. What and where are the starting materials? How do I go about preparing and purifying the preparation? A step by step procedure would be an excellent solution to the problem. The procedures given here allow the preparation and purification of numerous fatty acids and their derivatives, as well as experience in organic synthesis, purification by crystallization and distillation. It should be pointed out that these are possibly not the best procedures available but experience has shown them to be satisfactory for the purposes intended. The physical properties of triglycerides and their component fatty acids are of interest because their structures and composition affect the texture, solid fat content and melting behavior of food fat blends.

Contents

*Most of the material given here was taken from the literature but in a few cases, procedures developed at the authors’ laboratories are included.

Saponification and Purification of Free Fatty Acids

Procedure
Reflux for 15 minutes a mixture of 200 gr. oil and 400 ml. of a 20% alcoholic potassium hydroxide. (Note 1) Cool the flask under tap water and acidify the solution to Congo red paper with dilute sulfuric acid. (Note 2) Cool again if necessary; extract the fat acids repeatedly with a total of 1500 ml. of petroleum ether (bp 30°-60° C) and transfer the washings to a 2-liter separatory funnel  3  times, with 200 ml portions of distilled water or until the last wash is neutral to Congo red paper.  (Notes 3, 4) Then dry the washings with 50 gr. anhydrous calcium sulfate, filter and remove the solvent from a water bath. Final traces of solvent are removed by passing a stream of nitrogen or carbon dioxide through the fat acids in vacuo. The neutralization equivalent should be about 280, acid value 200. (Note 5)

Notes:

  1. The alkali is prepared by dissolving 50grof potassium hydroxide in 50 ml of water and then adding 350 ml of commercial absolute ethanol. Sodium hydroxide should not be used because the sodium soaps formed on saponification will not be soluble in the volume of alcohol used.
  2. The dilute sulfuric acid is prepared by pouring 50 ml onto 200 gr cracked ice.
  3. Emulsions may be broken by adding salt to the funnel so that it falls through the interface.
  4. Litmus paper should not be used since it frequently turns red due to the acidity of the fat acids.
  5. If the neutralization equivalent is too high, the saponification should be repeated.

Preparation of Methyl Esters by Transesterification with Methanol

Procedure A:
The apparatus consists of a round-bottomed flask fitted with a reflux condenser and a drying tube to protect the contents of the flask from moisture. Either of the two procedures may be used. (Note 1) The oil, an equal weight of methanol, and 0.5% of sodium methoxide (Note 2) are placed in the flask. The mixture is then refluxed. (Note 3)  Transesterification is essentially complete when the refluxing mixture becomes homogeneous. This change usually occurs at or very shortly after the time of refluxing begins (Note 4) The mixture is then refluxed for 2 hours. After inactivation of the catalyst by addition of glacial acetic acid until a drop of the reaction mixture give a neutral or slightly acid reaction with moist litmus paper, the excess methanol is removed by distillation. Upon cooling the, the glycerol appears as a separate layer (lower) with a separatory funnel. The residue of methyl esters is dissolved in petroleum ether, washed thoroughly with water, and dried. The solution is then evaporated and the methyl esters are purified by distillation at reduced pressure.
Procedure B:
The oil and an equal weight of methanol are placed in the flask and brought to reflux. (Note 3) Pellets of sodium or potassium hydroxide are then added one at a time through the condenser. After each pellet is added the contents of the flask are refluxed until the pellet is dissolved.  Pellets are added in this manner until the mixture becomes homogeneous during the period of refluxing. (Note 4) The preparation is the completed in the manner described in part A.

Notes:

  1. The procedure using sodium methoxide is advantageous because the free fatty acid present in the oil is neutralized with the formation of water and because moisture present in the oil is destroyed with the formation of sodium hydroxide, itself an effective catalyst. The principal disadvantage is the difficulty of storing sodium methoxide without deterioration. The procedure using sodium hydroxide has the advantage of greater convenience but is less satisfactory if appreciable amounts of free fatty acid or moisture are present as partial saponification of the oil may occur. With refined bleached and deodorized oils from commerce (the usual type encountered in preparative work), the convenience of the sodium hydroxide outweighs the advantages of the sodium methoxide method.
  2. Fresh, un-deteriorated sodium hydroxide should be used. Sodium methoxide is commercially available (BASF industries) and may be preserved for reasonable periods in sealed containers. For occasional trans esterification preps freshness of the sodium methoxide may be ensured by first placing freshly cut sodium metal in the flask equivalent to the amount of methoxide required. Methanol is then added and after the solution of the sodium is complete, the oil is added and refluxing commenced.
  3. The contents of the reaction flask should be swirled occasionally during heating to avoid superheating of the methanol layer.
  4. Failure of the mixture to become homogeneous indicates that the catalyst has been inactivated by free fatty acid (or by moisture in the sodium hydroxide method). The remedy is to add an additional catalyst. If excessive amounts of catalyst appear to be required the oil should be recovered, de-acidified, and thoroughly dried before proceeding.

Esterification and Deacidification

Procedure:
Fat acids are refluxed eight hours with 4 times their weight of methanol containing 1-2% of concentrated sulfuric acid (based on the weight of fat acids). The mixture is the poured into water and extracted with ether. The ether solution is washed with dilute (5%) potassium carbonate and evaporated. (Note 1) Conversion to methyl esters should be 97-98%. The product may be distilled in vacuo to remove color if desired. The acid number of the product should be determined. If low the product should be dissolved in ether and rewashed with potassium carbonate solution. If high the best method for deacidification is to follow the esterification procedure above. These methods may be applied to glycerides, etc.

Notes:

  1. Remove traces of water by bubbling an inert gas through the product in vacuo.

Alkali Isomerization

Procedure:
Linseed oil fatty acids (625 gr) (Notes 1, 2) were refluxed under nitrogen for 2 hours with an equal weight of potassium hydroxide dissolved in 2.5 liters of ethylene glycol. (Notes 3, 4) The mixture was then cooled, several hundred milliliters of water was added and refluxing resumed for a brief period. (Note 5) The soap solution was acidified and the fat acids isolated by the usual procedure. (Note 6)

Notes:

  1. The procedure may be applied to any desired non-conjugated polyunsaturated fat acid.
  2. In the experiment described, solid acids had been removed by 2 crystallizations of the linseed fat acids from lignoin at -18° C. This step was necessary because pure pseudo eleostearic acid was to be isolated.  In many cases, this preliminary purification is not essential.
  3. Conduct all operations with ethylene glycol in the hood, the vapor is toxic.
  4. Anhydrous ethylene glycol may be obtained from good grade commercial products by refluxing in an open vessel until the temperature of the vapor reaches 197° C.
  5. This step is necessary to saponify esters as a result of the anhydrous conditions prevailing during isomerization.
  6. See procedure for saponification and isolation of free fat acids.

Reference:

  1. Kass and Burr, J. Am. Chem. Soc. 61 3292, 1939

Preparation of Oleic Acid by Low-Temperature Crystallization of Olive Oil Fatty Acids

Procedure:
In a 4 liter flask, a solution of 225 g of olive oil fatty acids is dissolved in 3450 cc of acetone and cooled to 20° C overnight. The mixture was filtered, and the filtrate cooled to -60° C in a dry ice-alcohol bath (Note 1) and again filtered. The resulting crystal fraction was subjected to 3 further crystallizations at -60° C each time from 2000 cc acetone. By this procedure, the more soluble linoleic acid was removed in the several filtrates. After these 4 crystallizations at 60° C the product was made up to 1250 cc with acetone and cooled slowly on the first appearance of crystals, about -35° C. Although the crystal crop was largely oleic acid, it contained some additional saturated acids not removed by the first separation at -20° C. The product remaining in the filtrate was oleic acid. The acetone was removed by warming under reduced pressure. The residue was distilled at 15mm pressure in all-glass apparatus. It was water white and odorless. Constants yield 90 gr, iodine value 90.03 calc 89.93, neutralization eq. 282.2, calc 282.3, n20d 1.4585, purity 96%.

Notes:

  1. This and subsequent crystallizations were conducted with continuous stirring.
  2. To obtain the melting point the liquid was frozen in the melting point tube and held overnight before measurement.

References:

  1. Brown and Shinowara, J. Am. Chem. Soc. 59 6-8 1937
  2. Brown, Chem. Rev. 29 336 1941.

Preparation of Methyl Oleate by Fractional Distillation and Low-Temperature Crystallization of Methyl Esters of Olive Oil Fatty Acids

Procedure:
One thousand gr of USP olive oil was converted into methyl esters (Note 1) and fractionally distilled in vacuo to remove palmitic and lower esters. (Note 2) The esters of the C18 acids (701 gr) were crystallized (Note 3) from acetone, 15 ml/gr ester at -60° C. The precipitate (522 gr was dissolved in acetone, 10ml/gr ester crystallized at -37° C and filtered. The material in the filtrate (463 gr) was crystallized twice from acetone (15 ml/gr ester at -60° C). The precipitate (399 gr) was then fractionally distilled in vacuo and the distilled fraction was twice crystallized  (Note 4) from redistilled petroleum ether (bp 30°-45° C) (6cc/gr ester at -65° C). The final precipitate was warmed in vacuo to remove the solvent. Yield 298 gr, Constants (Note 5) mp -19.9 to -19.6 C, Iodine value 85.7 calc 85.7, nD 1.4522 (20 deg, 1.4502 (25 deg), purity 99% or better.

Notes:

  1. See procedure for transesterification.
  2. The still used was comprised of an electrically heated column (600x36 mm) packed with glass helices and equipped with a total reflux, partial take off the head.
  3. The mixture was cooled by direct addition of solid carbon dioxide (dry ice).
  4. The mixture was stirred mechanically and cooled externally with a solid CO2 alcohol bath.
  5. Oleic acid was prepared by saponification of this product and had the following constants: Iodine Value 90, mp -13 - 13.2 (-16, -16.3), nD 1.4599 (20), 4581 (25).

Reference:

  1. Wheeler and Reimenschneider, Oil and Soap, 16 207 1939.

Preparation of Pure Oleic Acid, Methyl Oleate, and Oleyl Alcohol, by Distillation and Fractional Crystallization of Red Oil, Methyl Esters of Red Oil and Commercial Oleyl Alcohol

Procedures:
A. Distillation
Distillations were conducted with a column 3 feet long and 1 inch in diameter (approx. 0.5 TP) electrically heated and insulated with magnesia pipe covering. A still head with automatic control of reflux ratio was employed during fractionation, a ratio of about 4:1 being employed. 1100 gr of material were distilled in each run at a pressure of 2mm Hg. The residue did not exceed 5% of the charge.
B. Fractional crystallization (Note 2)
Six hundred grams of the C18 fraction obtained in the fractional distillation of red oil (methyl esters of red oil or commercial oleyl alcohol) was dissolved in 6 liters of acetone, and the solution cooled to -20° C (methyl oleate -37° C < oleyl alcohol -10° C) with stirring. (Note 3)  The mixture was maintained at this temperature for 1 hour with stirring, and then filtered through a jacketed funnel (Note4) and precooled to the proper temperature. (Note 5) The filtrate was diluted with 3 liters of acetone and cooled to -50° C (methyl oleate, 6-oleyl alcohol, -45° C) with stirring. After being maintained at this temperature, the mixture was filtered through the jacketed funnel pre-cooled to the proper temperature. The white crystalline cake of oleic acid (methyl oleate, oleyl alcohol) was sucked and pressed as dry as possible and then washed on the funnel with 1400 cc of acetone cooled to -70° C. The crystal cake was well mixed with the wash liquid and then sucked dry as possible. The oleic acid (methyl oleate, oleyl alcohol) was then cut out of the funnel and the occluded acetone was recovered by distillation.
Yields: Oleic acid 511 gr, methyl oleate 482 gr., oleyl alcohol 588 gr
Constants: Iodine value: oleic acid 88.8, methyl oleate 84.4, oleyl alcohol 89.6
Purity: Oleic acid 92.8 %, methyl oleate 95.7%, oleyl alcohol 95%

Notes:

  1. The procedure described apply to the preparation of oleic acid, methyl oleate or to oleyl alcohol. The distillation step is the same in each case. Temperatures in the fractional crystallizations, however, are different for the different substances. The necessary changes are indicated in parentheses. Red oil is the liquid fraction obtained by hydrogenation of fats followed by pressing. Originally Candles were made from the stearine fraction until about 1920 when electric lights became common. However red oil production extended beyond then. Today red oil is known as commercial oleyl  alcohol
  2. Crystallizations were conducted in a 3.5 gallon Pyrex bottle equipped with an efficient mechanical stirrer and a thermometer. The bottle was placed in a 6-gallon stoneware crock which in turn was placed in a wooden box. The free space between the crock and the box was filled with powdered 85 percent magnesia pipe covering compound .the average thickness of the insulation was ¾ in. Cooling was affected by surrounding the bottle with an acetone-dry ice bath. Dry ice should not be added to the solution of the fat acid or derivative because of the possible introduction of impurities.
  3. Stirring should be just rapid enough to keep any material from adhering to the sides of the bottle.
  4. All filtrations were carried out in a 10 in Buchner funnel which was jacketed by a galvanized iron pail wrapped with several thicknesses of asbestos cloth. The funnel was supported by a tight-fitting rubber stopper inserted through a hole in the bottom of the pail.an additional rubber stopper was attached to the protruding stem of the funnel for insertion into the filter flask. The funnel was surrounded by an acetone-dry ice bath maintained at the proper temperature.  In general, all filtrations were very rapid.
  5. The solid material removed consistently consisted of stearic acid (methyl stearate, octadecyl alcohol).
  6. Although the improvement seldom justifies the time and effort, further purification can be obtained by a final distillation through the column.

Yields: Oleic acid 420 gr, methyl oleate 412 gr, oleyl alcohol 450 gr.
Constants: Oleic acid iodine value 90, methyl oleate iodine value 84.3, oleyl alcohol Iodine value 91.5.
Purity: Oleic acid 94.8%, methyl oleate 96.8%, oleyl alcohol 97.7%.

Reference:

  1. Swern, et al., Oil and Soap 21 133 1944.

Preparation of Elaidic Acid by Isomerization of Oleic Acid with Nitrogen Oxides

Procedure:
A mixture of 9.6 gr of oleic acid and a solution of 0.5 gr sodium nitrite in 2 ml water is heated to 58°-62° C on a water bath when 5 ml conc nitric acid and water 50/50 is added. The mixture was stirred for 15 minutes, removed from the bath and allowed to stand for 1 hour. The fat acid layer was then solid at room temperature was taken up in 200 ml ether, washed free of mineral acid, dried and filtered. The solution was chilled to-25° for 0.5 hr, and filtered giving a crop of crystals weighing about 5 grams. This was dissolved in 100ml  ether and two fractions were obtained by 1 - chilling to -30° C, mp 43.5-44.5 C; 2 - chilling to -40° C, 1 gr mp 43°-43.5° C. The fractions were combined and crystallized from 100 ml Shellysolve F at -10° C, yield 3.1 gr, mp 44°-45° C.

Reference:
Sol Radlove, Internal preparation notebook, 1732, p 11 NRRL.

Preparation of Methyl Ricinoleate and Ricinoleic Acid

Procedure:
Methyl Ricinoleate  
One kilogram of castor oil (iodine no 87.9, Sap. no 179.9) was transesterified with 1500 ml of methanol containing  2% dry hydrogen chloride by refluxing on a hot water bath for 24 hrs. The crude esters were then distilled at a pressure of 2-3 mm, the fraction boiling at 185-212 being collected. This material is sufficiently pure for most purposes. If further purification is required the ester is dissolved in acetone (50cc/gr) and cooled to -55° C. Any precipitate which forms is removed by filtration and the filtrate is then chilled to -65° C. The precipitate is collected acetone is removed by warming in a vacuum and the precipitate is again recrystallized from acetone (25cc/gr) at -65° C. The product is finally distilled at reduced pressure to give methyl ricinolate of about 99.5% purity.
Ricinoleic Acid (Note 1)
Castor oil fat acids were dissolved in acetone (20cc/Gr) and cooled successively to -20° C and -70° C. (Note 2) The precipitate formed was removed by filtration and the last traces of solvent were removed from the product by warming to below 60° in vacuo.  The purity of the product was 95.6-97%.

Notes

  1. Pure ricinoleic acid cannot be obtained by distillation of castor oil fat acids or by hot saponification of pure methyl ricinoleate. Under these conditions dehydration to octadecandienoic acids and condensation to polyricinoleic acids occur.  Cold saponification does not proceed to completion.
  2. If a precipitate appears at -20° C, it should be removed by filtration and discarded.
  3. More elaborate (and inconvenient) methods of preparing ricinoleic acid have been reported. (Refs 2, 3) It is doubtful that they lead to products more pure that obtained by crystallization.

References:  

  1. Brown and Green, J. Am. Chem. Soc. 62 738 1940.
  2. Rider, ibid, 53 4139 1931.
  3. Straus, Hein, and Salzmann, Ber. 66 631 1933.

Preparation of Tetrabromo Stearic Acid

Procedure:
The free fatty acids from 200 gr safflower oil (Note 1) prepared as directed under “Saponification of free fatty acids” are made up with Skellysolve F (Note2) to a total of 2 liters. The solution is placed in 3 liters, 3 necked round bottomed flask equipped with a low-temperature thermometer and a separatory funnel. Place the entire assembly in a low-temperature bath (Note 3) and cool to 5° C. Cautiously transfer 75 ml (230gr) (Note 4) of bromine to the separatory funnel. With stirring add the bromine slowly to the solution and dry ice to the low-temperature bath at such a rate as to keep the temperature of the reaction in the 5°-10° C range. Stopper the flask and allow it to stand overnight in the refrigerator. Filter the precipitate through a fine mesh cloth (Note 5) on a Buchner funnel and wash the precipitate with 2 200 ml portions of cold Skellysolve F. Transfer the filter cake to a glass or enameled tray and air dry the product overnight in the hood. Dissolve the crude tetrabromides (175 gr) in 1000 ml of fresh hot Skellysolve C (Note 6) and cool to X C without filtering. Filter the precipitate on a Buchner funnel and without drying redissolve in 1000 ml of fresh hot Skellysolve E, add 2 gr filter cel and 6 gr decolorizing carbon and filter hot through a coarse fritted glass funnel under suction. Cool the solution to 0 C and keep it overnight at that temperature. Filter the precipitate on a Buchner funnel and wash the cake with 2 100ml portions of cold Skellysolve F. Dry the light brown product (Note7) under vacuum. Melting point 114.5°-115.5° C, Yield is 127 gr, 42.5% of theory.

Notes:

  1. Corn oil may be employed.
  2. Skellysolve F, petroleum ether bp 30°-60° C.
  3. A 5-gallon stoneware crock with ethanol or acetone as a bath has operated very satisfactorily using dry ice as the coolant.
  4. The quantity of bromine used was calculated from the IV of the fat acids.
  5. Filter paper cannot withstand the action of the corrosive reaction liquors.
  6. Any high boiling aliphatic hydrocarbons may be used such as Skellysolve C (bp 90°-97° C) or Skellysolve E (bp 110°-120° C) for crystallization. In continuous operation, the other liquor from the second crystallization of a given batch is used as the solvent for the crystallization of the next batch.
  7. The color disappears during the debromination of methyl linoleate. The tetrabromides prepared from corn oil by this procedure are pure white.

Reference:

  1. Sol Radlove, Internal Report, NRRL, May 1945. 

Preparation of Methyl Linoleate, Linoleic Acid by Debromination

Procedure:
In a 2 L round-bottomed flask equipped with a reflux condenser are placed 200gr purified tetrabromo stearic acid, 400 ml of absolute methanol, and 200gr mossy zinc. The mixture is warmed to start the reaction to start the debromination reaction.  The reaction is exothermic and it may need to be necessary to cool the reaction flask by dipping in cold water.  After the vigorous reaction subsides (in about 5 minutes) the mixture is refluxed for 30 minutes. A solution of 10ml concentrated sulfuric acid in methanol is added through the condenser, and refluxing is continued for 3 hours. The mixture is decanted from the remaining zinc (Note1) into a 4 L separatory funnel.  The zinc is washed with a few milliliters of methanol to complete the transfer.  To the funnel are added 500 ml of Skelleysolve F and 1500 ml of water.  The funnel is shaken, the layers allowed to separate, and the aqueous layer is withdrawn and extracted again with 200 ml of Skellysolve F.  The combined Skellysolve extracts are transferred to a 2 L separatory funnel and washed once with 500 ml of water twice with 100ml portions of dilute (4-5%) aqueous sodium carbonate and then 3 times with 10% aqueous methanol. The solution is then slightly acidified and washed with water until neutral to Congo red paper. The solution is shaken with anhydrous sodium sulfate.  It is then filtered and evaporated, and the residue distilled under reduced pressure. BP 156°-159° C at 1 mm Hg, Yield 98.6 gr Wijs IV (30 minutes) 173 Calc 172, 3.

The free acid (Note2) is prepared by cold saponification with 5% alcoholic sodium hydroxide. The volume employed should contain a weight of hydroxide equal to the weight of the ester to be saponified. The solution is allowed to stand overnight.it is then dissolved in equal volume of warm water, a current of carbon dioxide is introduced below the surface of the liquid, and it is acidified with dilute (1:X) by weight of sulfuric acid. The stream of carbon dioxide is maintained during subsequent operations. The waste is siphoned off, the acid is washed with hot water and dried over anhydrous sodium sulfate and preserved under carbon dioxide. The yield of product is practically quantitative. (MP -8°-9° C)

Notes:

  1. The unreacted zinc should be preserved for subsequent debrominations.
  2. The free acid is much less stable toward oxidation than the ester. It is, therefore, preferable to store the material as ester and to prepare free acid only as needed. Unlike the ester, the free acid cannot be distilled without some decomposition.

Reference:

  1. Organic Synthesis, 22 pp. 75-81 1942.

Preparation of cis-9, cis-12-octadeanoic acid (linoleic)

Procedure:
Two kg of the unsaturated acids (Note1) from corn oil was dissolved in acetone (75 g/L) and crystallized in 2 tier batches at -50° C in a bath of dry ice and alcohol. The crystals were filtered rapidly and the filtrate cooled to -70° C.  The resulting crystals were warmed under reduced pressure to remove acetone and distilled at 2-3mm Hg. About 70-80 g of undistilled acids was allowed to remain in the distilling flask. The yield was 380gr. The iodine value indicated a purity of 93.3%.

Notes:

  1. Saturated acids were removed from the corn oil by crystallizing from acetone (10 cc/g at -20° C) and discarding the crystals formed.

References:

  1. Brown and Frankel, J. Am. Chem. Soc. 60 54 1938.
  2. Brown and Stoner, ibid, 59 3 1937.

Preparation of Hexabromostearic Acid

Procedure:
The procedure given for tetrabromo stearic acid is followed with the following exceptions.

  1. Use perilla oil fat acids.
  2. Use 100 g bromine
  3. Use ether to wash the crude hexabromides.
  4. Dioxane (6cc/g) is used for crystallization. Solutions need not be chilled below 15°-20° C. Crystallization is reasonably complete in several hours, but it is preferable to allow standing overnight. The product melts at 181° C.

Reference:

  1. McCutcheon, Org. Syn. 22 82 1942.

Preparation of Methyl Linolenate

Procedure:
Equal weights of powdered hexabrocstearic acid are suspended in 2.5 parts of methanol. The mixture is refluxed and an equal volume of 7.5 N alcoholic hydrogen chloride is added gradually. The mixture should become clear within 10 minutes after the addition of the acid is begun. Refluxing is continued for 3 hours after which the methy linolenate is isolated in the manner described for methyl linoleate. The boiling point is about 174/2.5 mm. Linolenic acid is prepared by cold saponification.  See procedure for debromination linoleic acid and notes therto.

Reference:

  1. Norris, Kass and Burr, Oil and Soap 17 123 1040.

Preparation of cis-9, cis-12, cis-15 -octadecatrienoic acid (Linolenic)

Procedure:
Enough acetone was added to 500gr of the fat acids of linseed or perilla oil to make a volume of 6 Liters. The solution was successively chilled to -20°, -45°, -60° and -75° C. The crystals formed at each temperature were removed by filtration and discarded. Acetone was removed from the final filtrate by warming under reduced pressure. The residue was distilled at 2 mm Hg. The yield was 65gr of material containing 74-79% of linolenic acid. 250gr of the crude linolenic acid was made up to 4 liters with petroleum ether. Six successive crystallizationswere made at -60° C, the crystal crop each time being employed. The final yield was 60gr, purity 88%. (Data for linseed acids) Periila acids yielded a product of 83% purity.

Reference:

  1. Shinowara and Brown, J. Am. Chem. Soc. 60 2734 1938.

Preparation of Alpha and Beta Eleostearic Acid

Procedure:
Tung oil (25gr) is saponified with100 ml of a 10% solution of potassium hydroxide in alcohol by boiling under reflux for 1 hour. The condenser is then repaced with a 3 holed rubber stopper, providing an entry tube for carbon dioxide, a syphon tube, and a dropping funnel. The flask is filled with carbon dioxide and the soaps are decomposed by adding 200ml of hot 1M sulfuric acid in small portions. The aqueous layer is siphoned off under carbon dioxide pressure. The fatty acids are then agitated with 250ml of boiling water and the aqueous layer is removed as before. Washing (at least 6 times) is repeated until sulfate is absent in the washings. The stopper is then removed and water at O is added to solidify the fatty acids. The flask is immersed in an ice bath for 30 minutes. The frozen nodules are filtered off on a Buchner funnel immersed in a dry ice bath. They are washed with very cold water and sucked dry. The nodules are transferred to a beaker filled with carbon dioxide and 4.5 gr of 90% ethanol per gr of eleostearic acid is added to dissolve them by heating to 35° C, the solution is stored at 10° C overnight and filtered under a carbon dioxide atmosphere. The crystals are washed 4 times with 5ml of 90% ethanol at room temperature. They are again recrystallized from 90% ethanol (4.5ml/gr) but the crystals are obtained by storing only 2 hr at 10° C. The third crystallization is conducted by chilling the solution 30 minutes in cracked ice. The final product melts at 48° C.

Notes:

  1. When handling alpha or beta eleostearic acid in the free state, an atmosphere of carbon dioxides must be maintained at all times.

Preparation of Beta Eleostearic Acid

Procedure:
Tung oil butter is obtained by heating tung oil wit a trace of sulfur for 1 hour the procedure for alpha eleostearic acid is followed with the following changes:

  1. Saponify with 200 ml of 10% alcoholic alkali by refluxing very gently for 1.5 hrs.
  2. Acidify with 300cc 1 M sulfuric acid.
  3. Use 2 gr of 90% ethanol/gr for recrystallizations.
  4. Let stand for 3 hours instead of 10.
  5. Use 30 minute periods in cracked ice for 2nd and 3rd crystallizations.

The product melts at 70°-71° C.

Reference:

  1. Thompson and Thompson, J. Am. Chem. Soc. 56 898 1934.

Preparation of trans-9, trans-11-octadecadienoic acid

Procedure:
Ricinoleic acid (500Gr) was elaidinized by mixing with 200ml of nitric acid (sp gr 1.2) and 15 gr of potassium nitrite dissolved in 200 cc water. The mixture was heated 10 minutes at about 50° C and then chilled rapidly. The ricinoleic acid was distilled at 240°-250° C at 15-30 mm from a large flask (Note2) because of much foaming. The distillate deposited a solid mass which was recrystallized from liquors. MP 54° C.

Notes:

  1. The procedure is that of Smit. It appears that it could be rendered more convenient by carrying out the elaidinization and dehydration upon methyl ricinoleate followed by saponification to yield the free acids.
  2. This step effects dehydration. The addition of a catalyst such as potassium bisulfate should be helpful although the reference does not call for this.

References:

  1. Smit Rec, Trav. Chim. 49 539 1930.
  2. Mangod, Monatsh 15 307 1894.

Preparation of trans-10, trans-12-octadecadienoic acid

Procedure:
Dehydrated castor oil was isomerized with alkali and the isolated acids were chilled at 3° C for 48 hrs. Repeated filtration and pressing on a suction funnel yielded a dry cake weighing 20% or more of the original total fatty acids. Two recrystallizations from petroleum ether gave snow white crystals. MP 55°-56° C.

Reference:

  1. von Mikusch, J. Am. Chem. Soc. 64 1580 1942.

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