Quantitative Determination of Methoxyl and Ethoxyl Groups LLOYD M. COOKE AND HAROLD HIBBERT, McGill University, iMontreal, Canada
T
HE determination of alkoxyl groups is based on their conversion into volatile alkyl iodides on treatment of an
dure and developing a simpler and more accurate method for the simultaneous determination of methoxyl and ethoxyl groups. The first difficulty, that of the complete removal of the crystalline tetramethyl salt, was overcome by washing out the absorption tubes with a small quantity of water followed by stock 97 per cent ethanol. As a result of this innovation, the necessity for using an absolute ethanol solution of trimethylamine was also avoided, 95 t o 97 per cent ethanol being found satisfactory. The addition of water necessitated the drying of the mixed quaternary ammonium iodides prior to the extraction of the ethyl salt, but this step presented no difficulty,
alkoxyl-containing compound with constant-boiling hydriodic acid. The direct determination of total alkoxyl, or of methoxy1 or ethoxyl alone, is accomplished very readily b y the Zeisel procedure (8) as modified b y Viebock ( 4 6 ) and b y Friedrich (2). In this procedure the alkyl iodide, after passing through a scrubber solution of cadmium sulfate and sodium thiosulfate (to remove hydriodic acid) is oxidized to iodate in a bromine-glacial acetic acid-potassium acetate solution (6) according to the following reactions:
+ +
Br2 R I IBr 2Bra
RIBr2 + 2H20
+
+ RBr
4
HIOs
++ IBr 5HBr
TABLE I. EFFICIENCY OF ABSORPTION OF METHYL IODIDE FROM VANILLIN
The resulting iodate is then determined iodometrically by conversion to free iodine, which is titrated with sodium thiosulfate. I n order to differentiate between methyl iodide and ethyl iodide, the difference in solubility of tetramethyl ammonium iodide and of trimethylethyl ammonium iodide in alcohol is employed.
(10- t o 15-mg. samples in 10% alcoholic trimethylamine) No. of Tetramethyl Salt OMe Absorption Absorption Extracted with (Theoretical Tubes Solution Absolute EtOH Value 20.4) 2 HOAc-Brl .... 20.3 2 HOAc-Brr 20.2 2 HOAc-Bra .... 20.2 2 (CHa)aN No 19.8 2 (CHahN No 20.0 Y e8 19.9 19.7 Yes 19.3 Yes 19.9 Yes 19.9 Yes NO 20.3 No 20.0 No 20.2 Yes 20.3 19.7 Yes 20.0 Yes Yes 20.2
....
In the method of Willstatter and Utzin er (6), the alkyl iodides are absorbed in a 10 per cent absolute afcoholic trimethylamine solution. Although the reaction between trimethylamine and methyl iodide is rapid (98 per cent complete in 1 hour), that with ethyl iodide is very much slower; in consequence, the absorption solution is allowed to stand for at least 24 hours before the trimethylethyl ammonium iodide is separated from the tetramethyl ammonium iodide by extraction of the mixture of iodides with absolute alcohol. Each of the iodides is then determined gravimetrically by conversion to silver iodide with aqueous silver nitrate. When Wilson ( 7 ) , however, showed that 100 cc. of absolute alcohol dissolve 0.040 gram of tetramethyl ammonium iodide at 25" C., Phillips and Goss (5') modified the Rillstatter procedure by applying a correction for the solubility of the tetramethyl salt in the alcohol. In the Phillips and Goss procedure 100- to 300-ing. samples of known mixtures of p-ethoxybenaoic acid and anisic acid were used. In order to avoid the necessity of correcting for the solubility of the tetramethyl salt in absolute ethanol, Wilson suggested the use of a saturated solution of tetramethyl ammonium iodide in this solvent as the trimethylethyl salt extractant. This modification has been incorporated in a tentative A. 0. A. C. method for the determination of methanol in distilled liquors (1).
The weakest point in the present methods for a combined methoxyl-ethoxyl determination lies in the fact that it is difficult to isolate the ethyl iodide quantitatively. Willstatter (6) showed that the formation of trimethylethyl ammonium iodide is 9 per cent complete in one hour, 77 per cent in 6 hours, and only 90 per cent in 24 hours. I n order to avoid this difficulty i t was found that the determination could be run in less time and with greater accuracy b y flrst determining total alkoxyl according to the standard Viebock-modified Zeisel procedure, then determining the methoxyl content alone according t o the modified Willstatter procedure described below. I n order to simplify the actual determination of the tetramethyl ammonium iodide the gravimetric method was replaced by a volumetric method, using the same reagents and procedure employed in the total alkoxyl determination according to Viebock (4,6).
There are several disadvantages in Willstfitter's (6) procedure for the determination of methoxyl and ethoxyl group3 even after the above modifications are included. In the first place, during the hydrolytic stage of the process, the tetramethyl ammonium iodide separates in the absorption tubes in t h e form of minute crystals which tend to adhere firmly to the side of the tubes or flasks. Such crystals must be carefully separated with a rubber policeman or fine feather; the removal of those crystals adhering t o the inside of the smallbore delivery tube is particularly difficult. Secondly, in order to isolate the ethyl derivative i t is necessary to allow t h e absorption solution to stand for at least 24 hours. Even after this period, it is generally found that the quantity of trimethylethyl ammonium iodide isolated is somewhat less than the theoretical value. Finally, t h e yield of the two iodides is determined gravimetrically, either directly as the quaternary ammonium iodide (1) or as silver iodide. The present investigation was carried out with the idea of eliminating the tedious, time-consuming steps in this proce-
Procedure METHOXYLDETERMINATIOX. A sample of 10 to 20 mg. (having from 10 to 40 per cent alkoxyl calculated as methoxyl) in a tin-foil microcontainer is added t o the reaction vessel of the Zeisel apparatus containing the standard reaction mixture of constant-boiling hydriodic acid (5 cc.), phenol (2.5 grams), and a small piece of Nichrome wire. The procedure for this phase of the determination duplicates that of the standard ViebockSchwappach except for the substitution of a total of 15 CC. of a 10 per cent ethanol solution of trimethylamine as the absorption solution in place of the acetic acid-potassium acetate solution of bromine, and the use of three test tubes as receivers (the 15 cc. of absorption solution being divided into 6-cc., 5-cc., and 4-cc. portions, respectively). 24
ANALYTICAL EDITION
January 15, 1943
At the end of a reflux period of 50 minutes the liquid contents of the absorption tubes are transferred to a 50-cc. Erlenmeyer flask with ethanol (95 t o 100 per cent). The crystalline residues left in the first and second absorption tubes are then washed with 2-cc. portions of water to dissolve the crystals of tetramethyl ammonium iodide adhering to the glass, then with two subsequent portions of ethanol. The combined absorption solutions plus washings are allowed to stand for a t least 3 hours at room temperature. A small boiling chip of porous plate is then added to the flask and the contents are evaporated almost to dryness on a steam bath, or by use of a hot plate (if carefully watched to prevent complete evaporation with resulting decomposition of the quaternary iodides). Complete drying is effected by leaving for 12 hours in a vacuum desiccator containing calcium chloride. The trimethylethyl ammonium iodide is then extracted as follows: A 3-cc. portion of a filtered, saturated solution of tetramethyl ammonium iodide in absolute ethanol is added to the flask by means of a medicine dropper pipet. (If absolute ethanol alone is used as extractant, a correction of 0.2 cc. of thiosulfate for 9 cc. of ethanol extractant must be added to the quantity of thiosulfate used in the titration.) After the mixture has been shaken gently for about 1 minute, the crystalline product is allon-ed to settle on one side of the tilted flask and the supernatant liquor is transferred to a glass-nail suction filter by means of a second pipet. This treatment is repeated twice. After the final decantation the transfer pipet is dried free from ethanol in a slow current of air, and the traces of tetramethyl compound are dissolved in distilled water. The aqueous solution is placed in a second flask, which also contains the air-dried filter paper. The residual contents of the original Erlenmeyer are freed from traces of ethanol in a vacuum desiccator. After 1 to 2 hours of this vacuum treatment, the aqueous contents of the second flask are transferred to the original flask, and the aqueous solution of tetramethyl ammonium iodide is then ready for volumetric determination. The aqueous solution of the quaternary ammonium iodide is treated with 10 cc. of a saturated glacial acetic acid solution of potassium acetate containing 8 to 10 drops of bromine for 2 to 3 minutes, followed by the addition of 5 cc. of 25 per cent aqueous sodium acetate. The bromine is reduced with 10 drops of formic acid (80 to 90 per cent) and the resulting clear solution then treated with 5 cc. of 10 per cent aqueous potassium iodide solution and 5 cc. of 10 per cent sulfuric acid. The liberated iodine is titrated with standard (0.05 N ) thiosulfate, using starch solution as an internal indicator. TABLE 11. METHOXYL-ETHOXYL AXALYSES Compound Vanillin Vanillin Vanillin p-Ethoxybenzoic acid p-Ethoxybenzoic acid 3-Ethoxy-4-methoxybensoic acid 3-Ethoxy-4-methoxybenzoic acid 3-Ethoxy-4-methoxybenzoic acid 3-Ethoxy-4-methoxybenzoic acid 3-Ethoxy-4-methoxybenmoic acid 3-Ethoxy-4-methoxybenzoic ocid 3-Ethoxy-4-methoxybenzoic acid 3-Ethoxv-4-n~ethoxybenzoicacid a-Ethoxypropiovanillone (I) Me0 H
Found OMe OEt 20.2 0.1 20.1 0.2 20.3 0.0 0.0 27.0 0.0 27.2 15,s 23.0 15.7 22.9 15.9 23.1 15.8 23.0 16.0 23.2 15.9 23.1 16.0 23.2 15.7 22.9 13.7 20.2 13.6 20.3 13.7 20.2
Calculated OMe OEt 20.4
0.0 -
0.0 -
27.2
-
15.8
23.0
-
-
-
-
13.8
20.1
-
25
Table I1 includes the results obtained by this procedure with certain reference compounds.
Literature Cited (1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 5th ed., p. 177 (1940). (2) Friedrich, 2. phusiol. Chem., 163, 141 (1927). (3) Phillips and Goss, J . Assoc. Oficial A g r . Chem., 20, 292 (1937). (4) Viebock and Brecher, Ber., 63,3207 (1930) ( 5 ) Viebock and Schwappach, Ibid., 63, 2818 (1930). (6) Killstatter and Uteinger, Ann., 382,148 (1911). (7) Kilson, J . Assoc. Oficial A g r . Chem., 18,477 (1935). (8) Zeisel, Monatsh., 6, 989 (1885); 7, 406 (1886).
Recovery of Solvents Used in the Chemical Determination of Thiamine MORTON PADER, Food Research Laboratories, Inc., Long Island City, N. Y.
HERE routine thiamine determinations are run by either the thiochrome method (1) or, the colorimetric procedure ( 2 ) i t is practical, and at t h e present time advisable, to recover the solvents, isobutanol in the former case and xylene in the latter. The distillation procedure generally used is tedious and requires attention, precautions must be observed to minimize fire hazards, and fluorescent materials in the isobutanol are not always coinpletely removed. The author has obtained isobutanol with blank readings much lower than those usually obtained b y distillation in an all-glass soill by treating waste solvent, containing thiochrome and other fluorescent materials, with activated vegetable charcoal (Darco-G-BO, obtained from the Darco Corporation, 60 East 42nd St., New York, N. Y.). The isobutanol recovered in this manner has been used repeatedly in determining thiamine in a large variety of biological materials. Technical grade isobutanol, which usually contains fluorescent substances, may also be treated in this manner. The recovered isobutanol has proved to be as satisfactory in every respect as t h e solvent redistilled in all-glass apparatus; it fails to give rise t o fluorescent substances on standing or after treatment with alkaline potassiclm ferricyanide or with alkali alone, according to the procedure for the estimation of thiamine.
-
The isobutanol, ,previously dried in the course of the assay, is recovered by shaking 100 cc. of solvent with 1.5 grams of activated charcoal for 15 to 30 minutes. This ratio of adsorbent to solvent is optimal, but equally good results are obtained by using an excess of charcoal-about 5 grams per 100 cc.-and shaking for only a few minutes. The charcoal suspension is then clarified by filtration. The filtrate, free from any fine particles of adsorbent, is used without further treatment.
The methoxyl and ethoxyl contents are calculated according to t h e following equations:
In the Jfelnick-Field method for the determination of thiamine ( 2 ) the red color of the xylene solution is subjected to colorimetric evaluation. Here also, the solvent may be decolorized by the addition of activated charcoal, filtered, and used again.
-
-
(normality of thiosulfate) (zOhle = (cc. of thiosulfate)6 XX (sample weight) alkoxy1 as OMe) - (% Ohle) 31
X 31
] x 45
The red dye complex in xylene appears to be adsorbed to the charcoal even more readily than the thiochrome from isobutanol solution. About 1 gram of charcoal is added to 100 cc. of the pigmented xylene solution and the suspension shaken for 5 minutes; this yields a colorless filtrate.
Discussion of Results Table I indicates that all the methyl iodide is collected when three receivers are used and that the yield of tetramethyl ammonium iodide ib not affected by the ethanol extractioii.
(1) (2j
Literature Cilecl Hennessv. D. J.. IXD.ESG. CHEM..A N . ~ L .ED., . 13.. 216 (1941) Ifelnick: D., a n d Field, H., J r . , j . B i d . Chem., 127, j b 5 , 515, 531 (1939).
