Specific Spot Tests for Nitromethane and Nitroethane Based on the Nef Degradation FRITZ FEIGL and DAVID GOLDSTEIN laborat6rio da Produfao Mineral, Ministkrio da Agricultura, Rio de laneiro, Brazil Translated by RALPH E. OESPER, University of Cincinnati, Cincinnati, Ohio
b Alkali salts of the aci- form of nitromethane and nitroethane are decomposed by an excess of strong mineral acids with production of formaldehyde and acetaldehyde, respectively. Consequently, an alkaline solution of nitromethane yields the violet color characteristic of formaldehyde when treated with chromotropic acid suspended in concentrated sulfuric acid. The acetaldehyde produced by acid decomposition of an alkaline nitroethane can be detected through the blue color it imparts to a bright yellow solution of sodium nitroprusside containing piperidine (or morpholine). Within the technique of spot test analysis the attainable limits of identification are 2.5 y of nitromethane and 8 y of nitroethane. The tests are specific within the group of monoand polynitroparaffins.
W
an equivalent quantity of dilute mineral acid is added dropwise to an aqueous solution of a n alkali salt of a primary or secondary nitroparaffin, the nitroparaffin is regenerated almost quantitatively (1, 7 , 9). Consequently it is rather striking that Nef (9) found aliphatic aldehydes and ketones rather than the acid-stable nitroparaffins to be the chief products of the action of excess strong mineral acid on such solutions. This Nef degradation has been developed by Johnson andDegering (6) into a procedure for the preparation of aliphatic aldehydes and ketones from nitroparaffins (with yields around 80 to 85%). They formulate the reaction of sodium nitroparaffins with excess sulfuric acid as follows: 2RCH=N02Na 2 HZSO~ 2 RCHO NzO 2 NaHSO, HzO (1) 2 RZC=NO2Na 2 HzSO~ -* 2 RCOR NzO 2NaHSOd HzO (2) HEN
++ + +
-++
++
Even though Reactions 1 and 2 are merely net reactions, they should not be taken as indicating that the alkali salts enter into the reaction directly. A more correct interpretation is provided by the classic studies of Hantzsch and Veith (4) on the action of dilute
acids on the alkali salts of nitroparaffins. I n the case of primary nitroparaffins, the reaction RCH=NOz-
+H+
+
RCHzN02
(3)
is the result of the partial reactions RCH=N02-
+ H+
-C
RCH=NOzH
RCHzNOzH -+ RCHzNOz
(4) (5)
Reaction 4 proceeds instantaneously, whereas Reaction 5 is a time reaction and hence determines the speed of the regeneration of primary nitroparaffins from the solutions of their alkali salts. As analogous partial reactions are valid for the regeneration of secondary nitroparaffins from their alkali salts, it seems justifiable to assume that the Nef degradation rests on the decomposition, favored by the strong acids, of the initially produced aci-nitro compounds (nitronic acids) : 2 RCH=NOtH + 2 RCHO
+ NzO + Hz0
(6)
2 RzC=NOzH -C 2 RCOR
+ NzO + Hz0
(7)
According to Reaction 6 the Nef degradation yields formaldehyde from nitromethane and acetaldehyde from nitroethane. These aldehydes can be detected sensitively and specifically by the Eegriwe test (f?) with chromotropic acid suspended in concentrated sulfuric acid, and through the color reaction with sodium nitroprusside plus piperidine (6). As these tests can be accomplished by spot test analysis in the liquid or gas phase of the aldehyde (S), the Nef degradation has been made the basis of new tests for nitromethane and nitroethane. DETECTION
OF NITROMETHANE
Procedure. A drop of the ethanol solution of the test material is placed in a micro test tube and shaken with a drop of 2y0 sodium hydroxide solution. After several minutes, 3 or 4 drops of the reagent solution are added and the mixture is warmed in a water bath. A violet color indicates the presence of nitromethane, the shade depending on the amount a t hand. The reagent is a freshly prepared sus-
pension of pure chromotropic acid in concentrated sulfuric acid. Limit of identification is 2.5 y of nitromethane. This test will readily reveal the presence of nitromethane in commercial nitroethane. The waiting period in the procedure is important, because time is necessary for the conversion of primary nitroparaffins into their alkali salts, and vice versa. DETECTION OF NITROETHANE
Procedure. A micro test tube is used. The reagent consists of a freshly prepared mixture of equal volumes of 20% aqueous solution of piperidine and 5% aqueous solution of sodium nitroprusside. (The piperidine can be replaced by the cheaper morpholine.) Three drops of 1 to 1 sulfuric acid are combined with a drop of the alkaline methanolic test solution. Care must be taken that the upper walls of the tube are not moistened with the liquids. The mouth of the tube is covered with a piece of filter paper moistened with the reagent solution. The tube is then placed in a boiling water bath. Depending on the quantity of nitroethane present, a more or less intense blue stain appears on the yellow paper, almost a t once or within 1 or 2 minutes. Limit of identification is 8 y of nitroethane. Ethanol vapors yield a faint but definite blue color on contact with the reagent paper because the alcohol is partially oxidized to acetaldehyde by sodium nitroprusside. Therefore, if possible, methanol test solutions should be employed instead of ethanol when nitroethane is being determined, However, if not too dilute ethanolic solutions of nitroethane are presented for examination, the preliminary addition of a 2% aqueous solution of alkali hydroxide and subsequent dilution with an equal volume of water will prevent the oxidation of ethanol and hence avert a positive blank test. ACKNOWLEDGMENT
The Conselho Kacional de Pesquisas kindly provided funds for the support of this study. VOL. 29, NO. 10, OCTOBER 1957
1521
LITERATURE CITED
Bamberger, E., Rust, E., Ber. 35, 46 (1902). Eegriwe, E., 2. anal. Chem. 110, 22 (1937). Feigl, F., “Spot Tests in Organic
bnalysis,” 5th ed., pp. 331, 334, Elsevier, New York, 1956. Be?. 32, 607 (4) Hantasch, A., Veith, -4., (1899j. (5) Johnson, K., Degering, E. F., J . Org. Chern. 8 , 10 (1943). (6) Lewin, L., Ber. 32, 3388 (1899).
( 7 ) Meyer, V., Ibid., 28, 202 (1895). (8) Michael, A , , J . prakt. Chem. 37, 507
(1888): (9) Nef, J. L’., Ann. 280, 263 (1894).
RECEIVED for review December 31, 1956. Accepted May 31, 1957.
Spot Tests for Nitromethane, Monochloro(bromo)acetic Acid, Dimet hy I S uIfate, Iodomet ha ne, and Methylsulfuric Acid FRITZ FEIGL and DAVID GOLDSTEIN laborat6rio da Produqtio Mineral, Ministirio da Agriculfura, Rio de Ianeiro, Brazil Translated by RALPH E. OESPER, Universify of Cincinnafi, Cincinnati, Ohio
b Nitromethane
condenses with 1,2naphthoquinone-4-sulfonic acid in alkaline solution to yield a blue-violet water-soluble p-quinoidal compound. The color reaction is specific for nitromethane within the group of nitroparaffins. The limit of detection is 0.6 y of nitromethane when the test is conducted as a spot test with calcium oxide as the OH- donor. Spot tests for monochloro(bromo)acetic acid, dimethyl sulfate, iodomethane, and methyl sulfuric acid can b e based on the fact that these compounds yield nitromethane on treatment with an aqueous solution of alkali nitrite. This product reacts directly in the reaction mixture with the sulfonic acid to give the characteristic blue-violet color, The limits of detection are: 5 y of monochloroacetic acid, 10 y of monobromoacetic acid, 50 y of dimethyl sulfate, 80 y of iodomethane, and 100 y of methylsulfuric acid.
I
studies of the reaction of 1.2-naphthoquinone-4-sulfonic acid (Ehrlich-Herter reagent) with organic compounds of rarious kinds, Sachs and Crareri (?) isolated. in addition to many other products, a yellow crystalline product of nitromethane. They observed that it dissolves in sodium hydroxide to give a blue color. The following successive reactions occur: N THEIR
O=O=CHNO,
+ NaOH
+
/--\
L.J
ONa I
This color reaction was mentioned incidentally by Turba, Haul, and Uhlen (10) in a paper dealing with the colorimetric determination of nitroparaffins. It has recently been made the basis of a specific colorimetric determination of nitromethane by Jones and Riddick (4). Seemingly none of these workers knew of the previous work by Sachs and Craveri. The color reaction with this reagent can serve for the sensitive spot test detection of nitromethane. By applying familiar methods of producing nitromethane from monochloroacetic acid, dimethylsulfate, iodomethane, or methylsulfuric acid, it has proved possible to tTork out spot tests for these compounds. \Then these tests are used, it should be noted-as was first pointed out by Ehrlich and Herter (1)-that 1,2-naphthoquinone-4-sulfonic acid is a sensitive reagent for compounds containing mobile XHL and CH2- groups. As such, it has also been recommended for spot test procedures ( 2 ) .
0
I/
+ 2 + 2XaOH
DETECTION OF NITROMETHANE
O = ~ S 0 3 ~ a
0 H 3 N 0
+
ONa
+ Na~S03+
2H20
1522
ANALYTICAL CHEMISTRY
Reactions 1 and 2, which are essential to the color test, can be accomplished in simple fashion and without increase in the reaction volume by using calcium carbonate as the alkalizing agent. The reagent solution is a 0.5% aqueous solution of 1,2-naphthoquinone-4-sulfonic acid (sodium salt).
Procedure. The test is conducted i n a micro test tube. One drop of t h e alcoholic test solution is treated with a drop of t h e reagent solution and then several milligrams of calcium oxide are added. T h e mixture is shaken. Depending on the quantity of nitromethane present, a more or less intense blue or violet color appears. Limit of detection is 0.6 y of nitromethane. The procedure can be used to verify the presence of nitromethane in commercial nitroethane. DETECTION OF MONOCHLOROACETIC ACID
The classic method of preparing nitromethane rests on the reaction (8J 9 ) :
++
CH2C1COOH NaN02 + SaCl CH2r\’02COOH+. CHSSOz
+ COS
Accordingly, the presence of monochloroacetic acid can be shown indirectly through detection of the resulting nitromethane. Ordinarily, the nitromethane is recovered from the reaction mixture by steam distillation; the yield is around 35% (5). However, if the reaction is conducted in the presence of calcium oxide (to bind the nitromethane produced) a very satisfactory test for monochloroacetic acid results when the Ehrlich-Herter reagent is added. llonobromoacetic acid reacts analogously. It is highly probable that monoiodoacetic acid will react in the same manner. Procedure. A micro test tube is used. One drop of the aqueous test solution is treated with a n excess of calcium oxide and several milligrams of solid sodium nitrite. The mixture is kept in hot water for 2 to 3 minutes, then a drop or two of the reagent solution is added. The intensity of the re-
