SEPARATION OF PRIMARY, SECONDARY, AND TERTIARY AMINES BY CHROMATOGRAPHIC ADSORPTION ANALYSIS NYDIA GOETZ-IUTHY Stanford University, Stanford, California
Tm separation
of mixtures of primary, secondary, and tertiary amines has long been a classic problem in organic chemistry. The usual methods such as frattional distillation, recrystalli~ation, and Hinsberg separation (I) often break down when applied to the more unusually substituted amioes and especially where very small quantities are involved as may often be the case in research problems. As a result of our effort to synthesiw %amino- and Zalkylaminoquinolines in the search for potential antimalarials, we were confronted with the problem of separating equilibrium mixtures of 2-amino- and % methylaminoquinoline; 2-amino- and 2-dimethylaminoquinoliie; 2-methylamino- and 2-dimethylaminoquinoline. These compounds do not exhibit the usual amine behavior (they do not diasotiee under ordinary conditions) because of an amine-imine type tautomerism and amidme type resonance (2). I n any event macro methods did not lend themselves to a clean-cut separation of products where the total quantity of material was 0.2 to 0.3 g. Since the three compounds were all white crystalline substances with similar solubilities in most organic solvents (%methylaminoquinoline and 2-dimethylaminoquinoline both melt a t about 70°C., while 2-aminqquinoline melts a t 129°C.) the problem of separating any two of the cornpounds was further accentuated. The methods of chromatographic adsorption analysis (3, 4) offered a sensitive method for such separation. At first, known mixtures of the compounds were adsorbed from petroleum ether on activated alumina' and developed on a column with 5% acetone-petroleum ether solution. A series of fractious were collected since no visible color bands were formed on the column. After evaporation of the solvent and identification of the fractions, it was shown that the compounds could be eluted in the order that might be predicted for them: 2-dimethylaminoquinolme was more readily desorbed than 2-methylaminoquinoline, followed by 2-aminoquinoline which having the most acidic hydrogens for bondmg with the aluminum oxide was held the most tenaciously. The latter could be removed only by the addition of 1 % ethyl alcohol to the acetone-petroleum ether solution. A fluorescence test using a source of very short filtered ultraviolet rays was negative. However, later when a Activated Alumina, Grade F-20, mesh 80-200, activity 2-3 on the Brockman Soale can be obtained from the Aluminum Ore Company, Esst St. Louis, Illinois.
mercury arc lamp (GE, BH-4, 100 watt) was used, the compounds were able to adsorb some of the longer wave lengths which were recrnitted as a brilliant blue-toviolet fluorescence. This simplified the separation procedure to one of almost mechanical simplicity. The column and arrangement for suction filtration was taken to a darkroom and a petroleum ether solution containing approximately 6 mg. each of pure 2-amin0-~ %methylamino-, and %dimethylaminoquinolinez was adsorbed on a 48-cm. Pyrex column filled with 32 cm. of activated alumina. Development was made as before with 5 per cent acetone-petroleum ether solution and the separation into three fluorescent bands was followed using the mercury arc lamp as a source of ultraviolet W t . An attempt was made to photograph the column, but since no data were readily available on taking pictures Osing ultraviolet as the only source of illumination it was necessary to use a number of different exposures. One camera (35-mm. type) was loaded with fast black and white film and set on a tripod using the widest stop opening. Another camera (reflex-type) was set to take color pictures using Ansco daylight film, but in this ease an ultraviolet filter was used. Pictures were made as the bands progressed down the column; an eight-second exposure (F-3) gave the best results on black and white film, while a sixteen-second exposure (F-4.5)gave an acceptable color picture, which, however, could have been improved by approximately twice as much time. Although the room was absolutely dark except for the fluorescence on the column the sensitivity of the black and white film (no filter) gave an almost complete picture of the room, due to reflectedultraviolet rays. The bands themselves were, therefore, not the most spectacular part of the picture, but appeared as inched in" poPtions on the column. In those regions ultraviolet light was being absorbed rather than reflected and reCmittedasfluorescence. After each baud had been collected (at the rate of 40 to 50 drops Per minute) and the solvent evaporated, the residue was weighed and identified by its melting point, mixed melting point with known samples, and by formation of a ,picrate derivative whose melting point was compared with the known. Thus i t was found that each component of the mixture had been separated quantitatively. *For methods of preparing pure samples of these compounds see reference (81, page 176.
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treating Zdimethylaminoquinoline with lithium methylamide in ether solution and inert atmosphere, followed by separation of the products of the reaction by adsorp tion and elution on a column of activated alumina. It was shown further that as little as 5 mg. of 2-methylaminoquinoline could be separated from 200 mg. of Zdimethylaminoquinolme, a task that would have been impossible by ordinary fractional crystallization. I n another example, 1.7 mg. of 2aminoquinoline w a s separated from 18.2 mg. of 2-methylaminoquinoline. Additional proof of the identity of the compounds was made b y B A taking the adsorption Blask and White EnlargamenU from a Color Film curves in the ultraviolet Ansao Color Expaaure: 16 sso. F-4.6. A: "Pinched partiora''show two fluorescent bands developed on with the Beckrnan specthe column. B: Later, after aaalcing with solvent, three band* developed. From top t o bottom: 1st band. trophotometer and com2-aminoquinoline; 2nd band. 2-methulaminoquinoline: 3rd band. 2-dimethylaminoquinoliii. narine with the curves The experiences and results obtained from chromato- established for the pure known co£s. I t was observed that the heat of adsorption was congraphic adsorption analysis on known compounds was now applied to unknowns-equilibrium mixtures ob- siderable. The progress of a hand could also be foltained from a study of the reaction of 2-methylamino- lowed by touching the column; where the compound quinoline in the presence of sorlium-potassium amide in was being desorbed, the temperature gradient was conliquid ammonia. I t had been shown by this writer siderable. The writer often used this as a guide in the that both 2-arnino- and 2-methylaminoquinoline would absence of the mercury lamp. result (5). Although fractional recrystallization and LITERATURE CITED observation between crossed nicols of a polarizing micro(1)SHRINER, R.L., AND R. C. FUSON,"TheSystematic Identificsscope (study of the optical properties) had indicated tion of Organic Compounds," 2nd ed., John Wiley and Sons, New York, 1940, p. 48. th&e products, it remained for the quantitative separaF. W., Chem. Rev.,35, 135 (1944). tion of the two compounds by chromatographic adsorp- (2) BERRSTROM, (3) ZECHMEIRTER, I,., AND L., CHOLNOKY, "Principles and Praetion and elution to prove conclusively that the 2-amino tice of Chromatography," Chapman and Hall, Ltd., London, 1941. group would exchange for the Zn~ethylaminogroup in (4) STRAIN, H. H., Thromatographic Adsorption Analysis," the quinoline nucleus. Interscienoe Publishers, Inc., New York, 1945. Similarly, the exchange of the 2-methylamino group (5) LUTHY,N. G.,Ph.D. Thesis, Stanford University, Stanford, ('slifornia, 1948. for the 2-dimethylamino group was demonstrated by
