DETECTION OF HIGH BOILING ESTERS

9. DETECTION OF HIGH BOILING ESTERS

a. Detection of Various Esters. 

Relatively odorless esters frequently are added to essential oils to increase the apparent ester content. Fortunately, most such esters are high boiling and permit of easy separation. The best general method for the detection of such added esters is to separate the acids and identify them. Detection of added esters of acetic and formic acid (by isolation and identification of the acids) is not practical since these acids usually occur as natural constituents of essential oils.

Procedure:¹⁹⁶ Saponify 10 cc. of the oil for 2 hr. with 20 cc. of 0.5 N alcoholic potassium hydroxide.¹⁹⁷ Add 25 cc. of water and evaporate off most of the alcohol.198 Wash out the unsaponified oil by shaking with 3 equal portions of ether. Theaqueous solution is then made distinctly acid with hydrochloric acid (1:3) and again shaken out with ether. The ethereal solution will now contain the relatively insoluble acids, such as benzoic, cinnamic, oleic, phthalic, and lauric acid.

Upon evaporation of the ether these may be recovered. The V- aqueous solution will contain the readily water-soluble acids, such as citric, oxalic, and tartaric acid. This solution should, therefore, be made just alkaline to phenolphthalein and an excess of saturated barium chloride solution added. After warming for about 10 min., a crystalline precipitate of the insoluble barium salts will be obtained from which the acids can be liberated and identified.

The chemists of Schimmel and Company¹⁹⁹ devised a method for the detection of esters of acids which are not readily volatile with steam e.g., succinates, citrates, oxalates, and the esters of the higher fatty acids.

Procedure: Determine the saponification number of the oil in the usual manner. Then add a few drops of 0.5 N alcoholic sodium hydroxide to the contents of the saponification flask and evaporate to dryness on a steam bath. Dissolve the residue in 5 cc. of water and add 2 cc. of dilute sulfuric acid (1:3). Distill off the volatile acids with steam, using the apparatus shown in Diagram 4.17. The distillation should be carried out at such a rate that a distillate of 250 cc. is collected in the receiver at the end of 30 min.; the volume of the liquid in the saponification flask should be kept at about 10 cc. with the aid of the small flame. Collect a further 100 cc. of distillate in a second receiver. Add a few drops of a 1% alcoholic phenolphthalein solution to each receiver and titrate the free acids with 0.5 N potassium hydroxide solution. The first 250 cc. contain most of the volatile acids; the next 100 cc. should require only 1 or 2 drops of the alkali. From the total amount of alkali required to neutralize the acids, acid number II is calculated. A large difference between the saponification number and acid number II indicates the presence of esters of acids only slightly volatile with steam.²⁰⁰

The presence of the high boiling glyceryl acetates is not revealed by either of the procedures described above, since the acid liberated is acetic acid, which is volatile with steam, and which occurs naturally in many oils (see "Detection pf Acetins” p. 338). 

 DIAGRAM 4.17. Apparatus for the detection of high boiling esters.

b. Detection of Phthalates. 

This method is based upon a preliminary saponification of the oil, followed by a separation of phthalic acid as the lead salt. The separation is not specific since certain acids other than phthalic (e.g., oxalic, citric, and phosphoric) give rise to insoluble lead salts. Therefore, it is important to regenerate the acid and determine its melting point.

Procedure: ²⁰¹ Introduce 2 g. of the oil in a 100 cc. saponification flask. Add 25 cc. of an alcoholic sodium hydroxide solution prepared by dissolving 1.25 g. of metallic sodium in 100 cc. of 95% alcohol.202 Saponify for 1 hr. Remove and permit the flask to cool to room temperature and then immerse it in an icesalt mixture. After standing for 30 min. filter off the precipitated sodium salts, using a well-cooled Biichner funnel. Wash these crystals with ice cold anhydrous alcohol. A precipitate at this point may be indicative of any number of organic acids (phthalic, salicylic, citric or tartaric). Transfer the salt to a 250 cc. beaker and dry in an oven at 105 for 2 hr. Cool and add 40 to 50 cc. of distilled water and 2 or 3 cc. of glacial acetic acid. Heat this solution to the boiling point and add 30 cc. of a 10% lead acetate solution. Upon thoroughly cooling in an ice bath, the lead salt of phthalic acid will precipitate out almost quantitatively. The lead salts of benzole acid, cinnamic acid, and salicylic acid are soluble and remain in the filtrate. Separate the lead salt of phthalic acid by filtration. Regenerate the phthalic acid with acid, recrystallize and determine the melting point. Phthalic acid melts at about 206^o,²⁰³

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196 Parry, "The Chemistry of Essential Oils," D. Van Nostrand Co., Inc., New York (1922), Vol. II, 321.

197 If the oil has a high ester number, a larger amount of alkali will be required.

198 Some chemists prefer to evafx>rate to dryness and thei* take up the residue in a small amount of water.

199 Ber. Schimmel & Co., October (1910), 43.

200 This procedure was originally proposed for the examination of bergamot oils; pure oils showed a difference between the saponification number and acid number II of not more than 7.

201 See Naves and Sabetay, "Phthalic Esters," Perfumery Essential Oil Record 29 (1938), 25

202 If the oil has a very high ester number, a larger amount of alkali will be required.

203 Phthalic anhydride may be formed; the anhydride melts at 131.