The Chemistry of Olive Oil Michael N. Quigley Chevron Science Center, University of Pittsburgh, Pittsburgh. PA 15260 Olive oil is a substance that has been prized since the dawn of time. Used as an ingredient in the preparation of foods, cosmetics, and medications, its continuing presence today-if not immediately obvious-is all pervasive. Because of olive oil's widespread popularity, experiments based variously on isolation, synthesis, and analysis prove to be of interest to most students. With this in mind, a collection of organic chemistry experiments based on olive oil have been assembled here at Pittsburgh. Some of these experiments are described below. Composition and Classification Olive oil consists mostly of glycerol fatty acid esters. Because glycerol is a trihydric alcohol, it can react with one, two, or three monobasic fatty acid molecules, but only the triglycerides are naturally occurring. The most important constituent of olive oil is triolein-the glyceryl ester of oleic acid. In common with other triglycerides, triolein exists as three polymorphs. See Figure 1. Each polymorph has its own characteristic melting point, and in the case of triolein, these occur at 5, -12, and -32 'C.
18
(a) Meic Acd
I
OH
(b)Linoleic aca
Figure 2. Unsaturated acids present in olive oil: (a) cis-9-oo tadecenoic acid; (b) cis-9,cis-12-octadecadienoicacid; (c) 9,12.15octadecatrienoic acid.
Figure 1. Polymorphs of triolein: the major constituent of olive oil. In nature, molecules of oleic acid have the configuration shown in Figure 2(a). Other important saturated and unsaturated acids that form olive oil's major triglycerides are shown in Figure 2 and are listed in the table. Major Acids of Olive Oil Type of Acid Unsaturated Acids 1 cis double bond: 2 cis double bonds: 3 cis double bonds: Saturated Acids:
Component
Concentration Range (%)
Oleic acid Linoleic acid Linolenic acid
64-80 8-16
Palmitic acid Stearic acid 332
Journal of Chemical Education
1-2
7-1 4 2-4
Arecent literature survey through the pages of this Journal revealed a number of methods involving the hydrolysis (saponification)of the triglycerides to free fatty acids (13). The methods go on to describe esterification of the fatty acids to methyl derivatives prior to gas liquid chromatographic analysis. Alternative methods involve the determination of the triglycerides themselves using normal phase, high performance liquid chromatography ( I , 4 ) . More basic, but equally interesting, experiments have been reported. For instance, in the early 1930's, an experiment was described for the hydrogenation of oil (51, and more recently, a simple test for saturation was detailed (6). Oils and fats are commonly classified in two different ways. The saponification number refers to the number of milligrams of potassium hydroxide required to convert 1g of fat or oil to glycerol and soap. The iodine number refers to the number of grams of iodine required to saturate 100 g of oil or fat and is a quantitative measure of unsaturation. Classical procedures for either technique may be found in many organic chemistry textbooks (71,and an interesting coulometric determination of the iodine number recently has been reported (8). Extraction of Oleic Acid from Olive 011 The glyceryl esters in olive oil are hydrolyzed easily by reaction with alkali using either a reflux technique or a nonreacting organic heating medium such as triethylene glycol. The reaction is shown in Figure 3. The saturated acids are separated conveniently by filtration aRer treatment with a nonaqueous solvent such as diethyl ether or
acid wntains three. For both of these acids, the two significant end chains are of unequal length which means that they are less likely to become trapped in the center of the urea helices. See Figures 4(b) and 4(c), respectively. In practice, this difference is sufficient to separate the more likely trapped oleic acid from the less likely trapped linoleic and linolenic acids. The latter remain in solution, while oleic acid forms a urea inclusion complex. Separation of the Saturated Acids
Potassium obate (asoap) L
Figure 3. Hydrolysis of triolein to potassium oleate. acetone. Saturated compounds have a low solubility in these solvents and precipitate quickly. Oleic acid is separated readily fmm the other unsaturated acids contained in the filtrate by treatment with a urea-containing solution. Hydrogen bonding and steric hindrance between adjacent urea molecules cause them to form helical structures with a central cylindrical channel. Hydrocarbons with an unbranched length of at least six carbon atoms are unable to enter freely and leave the channel. As a result, they become trapped, forming ureainclusion complexes. Higher unsaturated acids, with at least one unbranched six-member chain, form such iuclusion complexes; therefore, a good method exists for separation of long chain acids from short chain acids. The three major unsaturated acids present in hydrolyzed olive oiloleic, linoleic, and linolenic acids-are all angled a t the point of double bond occurrence. See Figure 2. This is a common feature of cis-unsaturated acids (9).As a result, none of these acids can fit entirely into one urea channel. In the case of oleic acid, the C1-C9 chain fits into one channel and the C 1 0 4 1 8 chain fits into another. See Figure 4(a). Linoleic acid contains two double bonds, and linolenic
Weigh 10 g olive oil into a 125-mL flask and add 20 mL triethylene glycol and 2 g potassium hydroxide. Heat the mixture to 160 'C for 10 min to hydrolyze the glyceryl esters. Allow to cool, before adding 50 mL distilled water and 10 mL concentrated hydrochloric acid solution. Extract most of the unsaturated acids contained in the resulting emulsion three times with 10-mL portions diethyl ether each time. Reduce the water content in the combined ether extract by washing with saturated sodium chloride solution and by addition of anhydrous sodium sulfate. Follow with filtration. Evaporate the resulting solution to constant volume by heatingunder vacuum, over a steam bath. Addition of 75 mL acetone followed by cooling in a n icdacetone bath to -15 'C, crystallizes all of the remaining saturated acids present. Remove the crystals by vacuum filtration and evaporate the filtrate to constant volume. Isolation of Oleic Acid
Prepare a solution of 10 g urea in 50 mL methanol and add the whole volume to the filtrate obtained above. Cool the solution in an icdacetone bath in order to crystallize the urea inclusion complex of oleic acid. Separate the crystals using vacuum filtration (leaving the other unsaturated acids in the filtrate). Transfer the crystals to a 250mL separating flask and add 50 mL of distilled water. Extract three times with 20 mL diethyl ether each time. Evaporate the ether over a steam bath to obtain pure oleic acid. Derivatization of Oleic Acid
Positively identify the crystals as oleic acid by performing a derivatization procedure that allows a melting point determination of the final product. First, the crystals obtained from part 03)are converted to an acid chloride. In a fume hood, place 0.5 g crystals in a 50-mL round-bottomed distillation flask and cautiously add 2.5 mL thionyl chloride. Attach a condenser to the flask, plug it with cotton wool, and reflux the solution for 30 min over medium heat. Oleic acid chloride will form accordine to the reaction shown in Figure 5(a).
-
C,,H,COOH
Obic acid
(b) Linoleic aca
I
+
SOCI,
-
C,,H,COCI
+
SO,
+
HCI
Olek acid chloride chloride (a)Formation of oleic acid chloride (oleoylchloride) Thionyl
FHa
Olek acld chloride
I NH,
pToiuidine
FH" I
N 0 / \ N H C-(C,,H,)
(c) Linolenic aca
Figure 4. Abilities of unsaturated acids to form urea inclust on complexes are depenaent on cnatn length. (Spiral conformattonof Lrea mo ecu es aepcted as a I ne forslmp icily).
(b) Formation of olek acld ptoluiiide
Figure 5. Steps in derivatization of oleic acid.
Volume 69 Number 4 April 1992
333
Remove the flask from the source of heat, allow to cool, and then add 50 mL diethyl ether followed by a solution of p-toluidine in 5 mL of a 10%wlv solution of ether. A thick, cream-colored precipitate will form if the reaction shown in Figure 5(b) is followed. Add excess dilute hydrochloric acid to remove any unreacted p-toluidine. Evaporate the ether over a steam bath and re-crystallizethe product from ethanol. Ayield of between 0.5-0.7 g is to be expected. The purep-toluidide derivative of oleic acid has a melting point of 42 'C. Soap Manufacture Soaps usually are made by hydrolyzing fats or oils with an alkaline solution of a sodium or potassium salt. A search through past issues of the J o u r a l failed to locate s i m ~ l eexoeriments on soao manufacture. although a recent article provided details of an interesting way to catalvze soao-forming reactions making them faster and more &licien
