Reduction of 2-Naphthol-Azoxylene WILLIAM SEAMAN, A. R. NORTOX, AND J. HUGONET Calco Chemical Division, American Cyanamid Company, Bound Brook, N. J.
2-N
APHTHOL-azoxylene, also known as Oil Red XO and F. D. and C. Red No. 32, can consist of a mixture of the isomers produced by coupling 2-naphthol with the diazonium compounds from any of the six possible isomers which may be present in commercial 0- and p-xylidine oil. For use as a food color, the dyestuff must conform to government specifications which limit the amount of m-xylidine present (l-amino-2,6-dimethylbenzene;l-amino-2,4-dimethylbenzene; and l-amino-3,5-dimethylbenzene), The most obvious way of analyzing for the presence of these isomers involves the reduction of the dyestuff in order to liberate the xylidine, followed by an examination of the xylidine so liberated. (The method used for the determination of m-xylidine in admixture with 0- and p-xylidines has been reported, 6.) rlny method of reduction which is to be used for this purpose must yield a quantity of xylidine that will approach the theoretical amount to be expected from the dyestuff; or else the method must indicate in some other manner that the isomeric composition of the xylidine obtained is the same as that which was combined in the dyestuff. Gsually the reduction of azo dyestuffs for the purpose of isolating and identifying the diazo component is carried out in aqueous medium with the aid of such reducing agents as zinc, stannous chloride, or sodium hydrosulfite. The 2-naphtholazoxylenes are insoluble in water. Hence, as would be expected, when a reduction is attempted in an aqueous medium, using the ordinary reducing agents mentioned, it is impossible to get adequate contact of the dyestuff with the reducing agent. Consequently, low yields of xylidine are obtained in any reasonable time of reduction. Rowe (4) has reported the use of a dispersing agent to keep azo dyestuffs from aggregating during reduction with alkaline sodium hydrosulfite solution. This may be one method of solving the problem, but it seemed preferable to run the reduction in actual solution, if possible. I n order to use the ordinary methods of reduction advantageously, a water-miscible solvent would have to be used. To make possible the use of such solvents as the aromatic hydrocarbons, in which the 2-naphthol-azoxylenes are most soluble, one could probably use methods involving reduction ivith hydrogen in the presence of a nickel catalyst, as has been reported b y Brochet ( 1 ) ; or tetralin in the presence of a palladium catalyst, as has been reported by Kotake and Mita ( 3 ) . For the purpose in view, i t was considered desirable to reduce the dyestuff b y the more usual agents, such as zinc and hydrochloric acid, thus necessitating the use of a water-soluble solvent. The solvent should, in addition to its solubility in water, not undergo reduction itself, and it should possess vapor pressure or other properties that would facilitate its separation from the liberated xylidine. 2-Xaphthol-azoxylene is fairly soluble in hot alcohol. Reduction was tried in this medium with zinc and hydrochloric acid and with stannous chloride and hydrochloric acid, but the addition of such water as was introduced with the acid resulted in precipitation of the dyestuff. The use of alcoholic solutions of stannous chloride with added hydrochloric acid was unsuccessful for the same reason. The problem was finally solved b y taking advantage of the excellent solvent properties of 1,4-dioxane, in which the dye dissolved readily. Stannous chloride and hydrochloric acid did not seem to act efficiently in the presence of dioxane. Heating with zinc and hydrochloric acid, however, resulted in
a rapid and complete reduction. The xylidine could be recovered with a yield of from 90 to 95 per cent of the theoretical. This could probably be increased by using a larger sample, but the time involved for the procedure would be greater. Judging by the rapidity and smoothness of the reduction, i t is likely that the 5 to 10 per cent difference from 100 per cent recovery is caused by manipulative losses rather than by incomplete reduction. However, if i t were due to incomplete reduction, it is unlikely t’hat,under the conditions of the reduction, this could cause any great change in the proportions of the various xylidine isomers from that which existed originally in the dyestuff. The dyestuff is in complete solution during the reduction, so that the speed of reduction of the isomers would not be influenced b y possible differences in rate of solution. Once in solution, i t is most likely, according to Conant and Pratt (g), that all the isomers would reduce a t similar rates. These authors, working with water-soluble azo dyes, showed that the structure of the aromatic nucleus which carries the hydroxyl group is the determining factor as regards the “apparent reduction potential” (the normal potential of the reversible system that will just cause appreciable reduction, 20 to 30 per cent in 30 minutes, under the conditions specified). All the compounds which they studied, which had the hydroxyl group on a naphthalene ring, fell in one class as regards the apparent reduction potential. All the 2-naphthol-azoxylenes would, therefore, be expected t o fall in one group. Consequently, if there were incomplete reduction, the ratio of the isomers to one another should not be altered, provided the dyestuff was completely dissolved during the reduction. The analysis for the isomeric composition of the recovered xylidine could accordingly be used to draw conclusions concerning the isomeric composition of the original dyestuff.
Reduction of 2-Naphthol-Azoxylene and Recovery of Xylidine Ten grams of the 2-naphthol-azoxylene were dissolved in 100 ml. of dioxane in a 2-liter, 2-necked flask equipped with a reflux condenser and a dropping funnel. Twenty grams of zinc dust were added and the mixture was brought to a boil. While boiling, concentrated hydrochloric acid was added at a slow rate, with occasional shaking, until no further lightening of the yellow color was caused by further addition of acid, and boiling was continued for a few minutes. (About 40 ml. of hydrochloric acid were used.) The mixture Tvas cooled, diluted with 100 ml. of water, and made strongly alkaline to phenolphthalein paper with an excess of 40 per cent sodium hydroxide solution. I t was then distilled with steam. At first a clear distillate consisting mainly of dioxane and water distilled over and then a turbid distillate containing the xylidine and an alkali-soluble substance. A small portion of the distillate was collected in a test tube from time to time, a few drops of 20 per cent sodium hydroxide solution were added, and the tube was cooled in ice. The removal of xylidine was considered complete when t h e test liquid was clear by transmitted light. A slight turbidity which could be observed against a dark background was disregarded. Usually a total volume of less than 1liter of distillate v-as collected. The distillate \vas made strongly acid with considerable excess of sulfuric acid (benzopurpurine 4B indicator paper), and again distilled with steam until the absence of striations in the upper part of the condenser indicated a sufficient removal of the dioxane. The residual liquor was made alkaline with sodium hydroxide solution and the xylidine extracted with one 100-ml. portion and three 50-ml. portions of ether. A further extraction with 50 ml. of ether should leave no more than a few milligrams of oil after evaporation of the ether; otherwise the extraction should be continued. The combined ether extracts were washed with small 464
-4NALYTICAL EDITION
AUGUST 15, 1940
portions of water untll the washings were no longer alkaline to phenolphthalein paper. A 250-ml. beaker was heated on a steam bath in direct contact with the steam, Tviped with a cloth, allowed to cool in the open, then weighed. The beaker was heated directly with steam and the ether extract added in several portions, each portion being allowed to concentrate to a small volume before the next was added. By this rapid evaporation, creeping of the 011 up to the edge of the beaker with consequent losses was minimized.
Analysis and Yields of Recovered Xylidine The recovered xylidine was analyzed for total amine content by diazotization with excess 0.1 N sodium nitrite solution in a stoppered flask, followed by treatment with a measured excess of 0.1 N sodium sulfanilate solution and a back-titration with 0.1 N sodium nitrite solution. The recovered xylidine u-ill have a diazotization value close to 100 per cent (about 98 to 100 per cent) xylidine if the method has been carried out properly. Yields of xylidine (calculated as 100 per cent by diazotization) will be about 90 to 95 per cent of the theoretically calculated value.
465
Summary The dyestuff 2-naphthol-azoxylene (Oil Red XO or F. D. and C. Red No. 32) may be reduced smoothly in dioxane solution b y means of zinc and hydrochloric acid for the purpose of determining the proportion of combined m-xylidine in the dyestuff by means of the analysis of the recovered xylidine. With a 10-gram sample of dyestuff, yields of from 90 to 95 per cent of t'hetheoretically expected xylidine may be recovered.
Literature Cited (1) Brochet, Bull. SOC. i n d . Mulhouse, 88, 703 (1922). (2) Conant and P r a t t , J . rlm. Chem. Soc., 48,2468 (1926). Japan, 59,634 (1938). (3) Kotake a n d Mita, J . Chem. SOC. J . SOC.Dyers Cotourisfs, 52,205 (1936). (4) R o a e , F. &I., (5) Seaman, Norton, and Mason, IKD. ENQ.CHEM.,-4nal. Ed., 12,345 (1940).
Wijs Iodine Method J. W. XIcCUTCHEON, Lightfoot Schultz Co., Hoboken, N. J.
A
LTHOUGH it has been shown repeatedly that the Wijs iodine value can be relied upon to give consistent results and values lying very close to those of the other available methods, the iodine value in general is lower than the theoretical unsaturation, as measured by the hydrogen value (3, 6). How much this value is lower in the case of the Wijs method has never been determined. as far as the author is aware, although in experiments on linoleic and linolenic acids, extending over some years, values never greater than 98.8 per cent of theory were obtained, whether the determination was made on the acid or the derived ester. Since linolenic acid, prepared through its hexabromide, is much more difficult to debrominate than linoleic, it was considered unlikely that the conbtant ratio of iodine value to unsaturation could be due to a coincidence or to the presence of residual traces of bromine which could not be identified by analysis. T o investigate this problem further, it was decided to prepare an unsaturated acid b y some means other than debromination. Elaidic acid J%-aschosen, since its high melting point allows a much better separation from linoleic acid than is possible with oleic. TABLE I.
COXSTANTS O F
1 159.5
ETHYL ELAID.4TE
2 171
7.5
4
185
200
12 212
16 221
51.3 1.4384
57.6 1,4359
Refractive Indices Temperature C. Refractive inhex
25.0 1,4486
29.2 1.4470
31.5 1,4461
40.0 1.4428
Specific Gravity 15.5'/4'
C.
Apparent True
25. 0°/40 C. 0.8704 0.8706
Apparent True
Various constants, together with boiling point data a t reduced pressures using equipment previously described (4), are given in Table I. Wijs iodine values, by the method of the American Oil Chemists' Society ( I ) , are given in Table 11, together with data on two highly purified unsaturated esters prepared by t'he debromination process (4). TABLE
11.
WIJS I O D I X E V A L U E S
Ethyl linoleate Methyl linolenate (from hexabromide of m. p. 181.9') Ethyl elaidate Elaidic acid
Boiling Point Data Pressure, mm.o Temperature, C. (corr.)
the elaidic was separated from the unchanged oleic acid by repeated crystallizations at -20" C. from a 20 per cent acetone solution. The elaidic acid (50.1 grams) was then converted into the lithium salt and recrystallized three times from 80 per cent alcohol, reconverted into the acid (20.0 grams of melting point 43.5" C.), esterified, and triple-distilled under a pressure of 0.5 mm. of mercury. The melting point on a small sample of the saponified ester was 43.6" C., which was not changed on crystallizing the derived lithium salt five times from 80 per cent alcohol solution. Solid acids by the method of Bertram ( 2 ) were less than 0.01 per cent.
0.8635 0.8636
Four hundred grams of pure olive oil were saponified in the usual manner, and the resulting 361.1 grams of fatty acids separated into liquid and solid acids by the Twitchell method, with the exception that the entire solid acid fraction from the first precipitation was discarded. The resulting 191.8 grams of liquid acids were fractionally chilled in a 50 per cent acetone solution, by volume, at -15' C. The filtrates from the first four crystallizations were combined and the fatty acids precipitated by di!uting with hot water. The yield was 147.8 grams, of iodine value 110.30. The acids Twre then elaidinized in the usual way with nitric acid and sodium nitrite, the mixture was washed thoroughly with hot water to remove traces of nitric acid, and
Found
Theoretical
A of Theory
162.49 162.29
164.7
98.6
257.33 80.85 88.82
260.57 81.81 89.93
98.8 98.8 98.8
The Wijs iodine value, as prescribed in the official methods of the American Oil Chemists' Society, gives results on the unsaturated acids and esters of the oleic series that lie very close to 98.8 per cent of the theoretical unsaturation. The reliability of the method is somewhat greater than is generally supposed, although corrections should be applied when the iodine value is to be used as a measure of the purity of a compound.
Literature Cited (1) Am. Oil Chem. SOC.."Official a n d Tentative Methods", p . 31,1939. (2) Bertram, S. H., Chem. m'eekblad, 24, 226 (1927). (3) Kok, IT. J. C. de, Waterman, €I. I., a n d Westen, H. A. Van, J . SOC.Chem. I n d . , 5 5 , 225-ST (1936). (4) McCutcheon, J. W., Can. J . Research, B16, 158-75 (1938). (5) Waterman, H. I., Bertram, S. H., a n d Westen, H. A. Van, J . SOC.Chem. Ind., 48, 50-1T (1929). CONTRIBUTION from the Department of Chemistry, University of Toronto, Toronto, Canada.
