Spot Test Procedures for Differentiation of Quinoline and lsoquinoline FRITZ FEIGL and VICENTE GENTIL Laborato'rio da Produqio Mineral, Ministdrio da Agricoltura, Rio de Janeiro, b a z i l Translated by
RALPH E. OESPER
University o f Cincinnati, Cincinnati, Ohio
2,3- and 3,4-Pj ridinedicarboxjlic acids (quinolinic and cinchonieronic acids) can be differentiated by the fact that onlj quinolinic acid forms an insoluble blue copper salt in weak mineral acid solution. Quinoline and isoquinoline can be distinguished from each other by reduction with zinc and hydrochloric acid. On subsequent oxidation the dihj droquinoline yields a brown precipitate when treated with potassium persulfate and copper sulfate, or a red precipitate when bromine water is added. The dihydroisoquinoline gives no reaction with persulfate and only a slight coloration with bromine water. Quinaldine behaves like quinoline, but its precipitation sensitivity is much less. Solutions of eight other derivatives of quinoline with substituents in the pyridine ring do not give these reactions after they have been reduced with zinc and hydrochloric acid.
N
0 PURELY chemical procedure is known for differentiating between quinoline (boiling point, 240' C.) and isoquinoline (boiling point, 239' C.). Differentiation of these isomeric bases has depended on the preparation of derivatives with characteristic melting points. The picrates (melting points, 203" and 222' C.) and methiodides (133" and 159" C.) have been recommended particularly ( 3 ) . Although the preparation of such derivatives and the determination of their melting points can be accomplished on the semimicro or micro scale, the necessary manipulations require not only special equipment but also more time, material, and experience than do spot tests based on color or precipitation reactions. The sensitive tests described here should have considerable interest and value, particularly in view of their importance n-ith respect to clarifying the structures of alkaloids and the like, whose degradation products may include quinoline and isoquinoline. These procedures were derived through consideration of the chemistry of specific, selective, and sensitive reactions ( 4 ) . Oxidation with permanganate in neutral or alkaline solution converts quinoline (I) into 2,3-pyridinedicarboxylic acid (1,a) and isoquinoline (11) into 3,4-pyridinedicarboxylic acid ( 1 1 , ~ ) (7).
I1
II,a
These products (quinolinic and cinchomeronic acid, respectively) might be expected to show a divergent behavior tovard copper ions because of the relative position of their coordinatable nitrogen atoms w-ith respect to the carboxyl group in the a- and &position. I n fact, when an acid solution of I, a, is treated with a solution of copper sulfate or acetate a blue color results, rhereas 11, a, gives no reaction. The blue color obviously is due to the formation of a chelated copper compound. In concentrated solutions, a precipitate appears: its composition conforms to C U ( C ~ H ~ O A.H20 ')~ (1). If the reaction of quinolinic acid xvith copper ions is conducted in dilute solution, the visible precipitate of the organic copper salt may not appear for several hours. However, the product can be brought doxn almost immediately by adding several drops of ether and warming gently. This device is of importance in spot test procedures to accelerate sluggish precipitations. The rapid precipitation is probably caused by the marked solubility of ether in water, which therefore increases the rate of formation of the nuclei and the rate of gromTh of water-insoluble compounds. It seems to be effective only when crystalline precipitates are involved. I t seemed logical to base a possible differentiation of quinoline and isoquinoline on their oxidation to the respective dicarboxylic acids and the divergent behavior of these products toward copper ions. This method can be followed for the identification of macro amounts of quinoline, but the sensit,ivity is inadequate for spot test purposes. The same difficulty was encountered when quinoline was oxidized with hydrogen peroxide in the presence of a copper salt in the hope of thus arriving directly a t the precipitation of the blue chelated product. Because the oxidation of quinoline to quinolinic acid with hydrogen peroxide and copper sulfate does not proceed rapidly enough for analytical purposes, attempts were made to reach this goal after hydrogenation of the quinoline. Koenigs (9) found that dihydroquinoline or its dimer is formed when quinoline is dissolved in glacial acetic acid and treated with metallic zinc. The reduction might be expected to proceed more rapidly in hydrochloric acid solution; with more rapid production of dihydroquinoline a more abundant yield of the oxidation product should be detectable by precipitation of the copper salt of quinolinic acid. This expectation was not fulfilled, although the following reaction, which was characteristic, took place. h drop of 5% qujnoline containing considerable hydrochloric acid was placed in a depression of a spot plate along with several granules of zinc. As soon as the evolution of hydrogen started, a drop of 3070 hydrogen peroxide and a drop of lY0copper sulfate solution were added. A violet color appeared almost a t once and quickly changed to red-brown; this was followed by a precipitate of the same color. Dilute solutions of quinoline yield light t o dark brown colorations. Isoquinoline solutions show the same effect but to a much smaller degree. However, when potassium persulfate was used as oxidant in place of the peroxide, iscquinoline gave no color, whereas with quinoline the color or precipitation reaction started after 1 or 2 minutes and reached its maximum in about 3 minutes. These observations served as the
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1310
ANALYTICAL CHEMISTRY
foundation for developing a new method for differentiating between quinoline and isoquinoline. .4nother and more rapid differentiation of the two isomeric bases is provided by their behavior toward bromine water after their solutions have been subjected to the action of zinc and hydrochloric acid. Even dilute solutions of quinoline give an immediate red precipitate, while isoquinoline yields a similar precipitate only when the concentration of its solution exceeds 10%. These colored precipitates disappear within a few minutes if they are treated with sulfosalicylic or sulfurous acid. -411 attempts to isolate pure specimens of the colored precipitate obtained from dihydroquinoline by the action of persulfate and copper sulfate, or bromine water, have failed. Lacking a reliable analysis, there is no real experimental basis for conjectures regarding the chemistry of these reactions. There is little doubt that persulfate oxidizes the dihydroquinoline (111) obtained from the quinoline. Karrer (8) has found that quinoline is hydrogenated ortho to the nitrogen atom. Serious consideration must be given t o the formation of the quinoidal oxidation product (IV) shown in the following series of reactions.
PROCEDURE
I11
IV (3)
Another possibility is the formation of a quinoidlike compound intermediate between I11 and IV. I n view of the decomposition by sulfosalicylic or sulfurous acid of the precipitate produced by bromine from acidic solutions of 111, the assumption of the production of a polybromide has some probability. A S shown in Equation 3, dihydroquinoline functions as a hydrogen donor. It was s h o m (6) that organic compounds which are reductants in alkaline solution can be detected by a color test that was originally recommended by Bose (2) for the detection of reducing sugars. The reagent for this test is an alkaline, alcohol solution of o-dinitrobenzene; it turns violet when warmed with hydrogen donors because of the formation of quinoidal onitrosonitrobenzene (IO). If the quinoline solution is reduced Rith zinc and hydrochloric acid and then made basic, it gives a violet color with o-dinitrobenzene. The same result was obtained with a reduced solution of isoquinoline; demonstrating that this base also yields a reduction product which functions as a hydrogen donor. Koenigs (9) also proved that, because dihydroquinoline yields a nitrosamine, it contains an NHgroup. This group is also formed when isoquinoline is hydrogenated, as demonstrated by its positive response to the color reaction for secondary amines with sodium nitroprusside and acetaldehyde ( 5 ) . Accordingly, it is highly probable that isoquinoline is converted to dihydroisoquinoline by zinc and hydrochloric acid, and that the differentiating reactions, employing persulfate or bromine, rest on the facts that in dihydroquinoline the reduction potential and the ability to form a polybromide are distinctly more marked than in the isomeric dihgdroisoquinoline.
With Persulfate. A drop of the test solution containing considerable hydrochloric acid is placed in a depression of a spot plate and 4 or 5 granules of 10-mesh zinc are added. After 1 or 2 minutes, the unused zinc is removed with the aid of a thin glass rod. A drop of 1% copper sulfate is added and then about 20 mg. of solid potassium persulfate. The system is agitated by blowing on the surface. Depending on the quantity of quinoline present, a red-brown precipitate or brown to yellow color appears within 1 to 2 minutes. The color reaches it maximum intensity after 3 to 4 minutes The limit of identification is 20 y of quinoline. Isoquinoline solutions, no matter what their concentration, show no color change Then subjected to this procedure. With Bromine Water. The reduction (hydrogenation) is performed as just described, and the reduced solution is then transferred to filter paper by means of a pipet. The spot is held over strong bromine water for about 30 seconds. The moist spot turns red immediately, or pink when smaller amounts of quinoline are present. When the paper is dried in an oven a t 110’ C., the color becomes paler but does not disappear entirely. The full color is restored if the spot is again exposed to bromine vapors. The limit of identification is 2.5 y of quinoline. Isoquinoline solutions of any concentration leave no more than a light yellow stain on the filter paper after being carried through this procedure. Although the procedures just described function best when quinoline and isoquinoline are not both present, a definite test for quinoline is obtained from the persulfate oxidation of 1 drop of a mixture of 50 y of quinoline and 2000 y of isoquinoline The behavior of derivatives of quinoline was tested with 10% solutions of: quinoline methiodide and ethiodide; quinaldine; acridine ; 6-nitroquinoline ; cincophen ; m-bromoquinoline nitrate; 4-hydroxy-7-chloroquinoline; 4-hydroxy-7-chloroquinoline-3-carboxylic acid; and p-naphthoquinoline. Only the methiodide, ethiodide, and quinaldine behaved analogously to quinoline, but the reaction with persulfate or bromine was much R-eaker n-ith quinaldine than x i t h the parent base. The liniit of identification was 50 y of quinaldine using either procedure. Consequently, thc tests described in this paper appear to be selective ACKNOWLEDGMENT
The authors gratefully acknowledge the suppor: of this study by the Conselho Nacionnl de Pesquisas. LITERATURE CITED
Boeseken, J., Rec. trav. chim. 12, 253 (1893:. (2) Bose, P. K., 2. anal. Chem. 87, 110 (1932). (3) Cheronis, K.D., Entrikin, J. E.. “Semimicro Qditative Organic
(1)
Analysis,” p. 410, Crowell, Kew York, 1947. (4) Feigl, Fritz, “Chemistry of Specific, Selective. and Sensitive Reactions,” Academic Press, Kew York, 1949: “Spot Tests,” 4th ed., chap. I, Elsevier. New York, 1954; ; ~ N A L . CHEST. 27, 1315 (1955). ( 5 ) Feigl, Fritz, Anger, V., Mikrochim. Acta 1, 138 (1937). (6) Feigl, Fritz, Voka?, L., Ibid., 1955, 101. (7) Karrer, P., “Organic Chemistry,” 4th English *,3., pp. 812, 817, Nordeman, Kew York, 1950. (8) Karrer, P., University of Zurich, Zurich, Swi-zeriand, private communication t o senior author. (9) Koenigs, W., Ber. 14, 98 (1888). (IO) Kuhn, R., Weygand, F., I b i d . , 69, 1969 (1936;. RECEIVED for revien hIarch 5 , 1956.
.4ccepted Aprii 1 2 , 1956.
