Microdetermination of the Nitro and Nitroso Groups in Aromatic Compounds by Reduction with Iron(l1) in Acidic and Alkaline Media W. I. Awad,‘S. S. M. Hassan, and M. T. M. Zaki Microanalytical Research Laboratory, Department of Chemistry, Faculty of Science, Ain Shams University Cairo, Egypt
Nitro and nitroso groups in aromatic ComPoUnds are determined on the microscale by reduction with iron(ll) in acidic and alkaline media. Six and four equivalents of iron(ll] are consumed mole of the nitro and nitroso’groups, respectively, in both media. The reduction is performed with 10 mg/ml of iron(l1) ammonium sulfate in 10N hvdrochloric acid or 3N sodium hydroxide. Results aicurate to =t0.3% absolute are obtained with 28 aromatic compounds of different nature. NITROAND NITROSO GROUPS can be determined using many reductants; titanium(II1) ( I , 2 ) , chromium(I1) ( 3 ) , vanadium (11) ( 4 ) , tin(I1) ( 5 ) , hydriodic acid ( 6 ) , molybdenum(II1) (7), metals (8, 9), and alloys ( I O ) have been described. Colorimetric ( I I ) , polarographic ( I 2 ) , and coulometric (13) methods have been suggested also. However, titrimetric methods using titanium(II1) and chromium(I1) remain the most commonly recommended reagents. Six and four equivalents of these reductants are consumed per mole of.the nitro and nitroso compounds, respectively. However, both reagents are not selective for nitro and nitroso groups, since many other nitrogeneous and nonnitrogeneous groups can also be reduced. Iron(I1) though widely used for reduction of the nitrate and nitrite groups, has little application for reduction of nitro and nitroso groups in aromatic compounds (14). In the present work, iron(I1) in acidic and alkaline media is used as a new reducing agent for the quantitative titrimetric microdetermination of aromatic nitro and nitroso groups. The effect of iron(I1) concentration, acid and alkali concentration, substituent groups, and Ihe time of reduction are studied. Some Present address, Department of Chemistry, University College for Women, Ain Shams University, Heliopolis, Cairo, Egypt. (1) T. S. Ma and J. V. Earley, Microchim. Acta, 1, 129 (1959).
(2) I. M. Kolthoff and C. Robinson, Recl. Trac. Chim. Pays-Bas, 45, 169 (1926). (3) J. P. Tandon,Z. Anal. Chem., 167, 184 (1959). (4) P. C. Banerpee, J . Iizdian Chem. Soc., 19, 35 (1942). (5) S. W. Young and R. E. Swain, J . Amer. Chem. SOC.,19, 812 (1 897). (6) R. Aldrovandi and F. Lorenzi, Ann. Chim. (Rome), 42, 298 (1952). (7) M. V. Gapchenko, Zacod. Lab., 10, 245 (1941); Chem. Abstr., 35, 7312 (1941). (8) H. Mormann, J. Lamberto, and G. Fries, Z . Phys. Chem., 306, 42 (1956). (9) C. E. Vanderzee and W. F. Edgell, ANAL.CHEM.,22, 572 (1950). (10) M. M. Lobunets, Unic. Piat Kiec, Bull. Sci., Rec. Chim., 4, 23 (1939); Chem. Abstr., 35, 1356 (1941). (11) E. Sawicki, ANAL.CHEM.,24, 1204 (1952). (12) I. M. Kolthoff, and J. J. Lingane, “Polarography,” Vol. 11, 2nd ed., Interscience, New York, N.Y., 1952, pp 442, 746-764. (13) V. B. Ehlers and J. W. Sease, ANAL.CHEM., 31, 16 (1959). (14) N. D. Cheronis, and T. S . Ma, “Organic Functional Group Analysis by Micro and Semimicro Methods,” Interscience, John Wiley and Sons, New York, N.Y., 1964.
nitrogeneous and nonnitrogeneous unsaturated functions are tested. Simple microprocedures with results accurate to *0.3% absolute are described. Advantages and limitations of the methods are reported. EXPERIMENTAL
Apparatus. An electrolytic reduction automatic microburet (15) is used for preparation, storage, and uses of titanium(II1) solution. Reagents. All reagents are of analytical grade unless otherwise specified. All the samples tested are pure. Standard titanium(II1) sulfate solution 0.04N was prepared, stored, and standardized according to reference (15). Procedure. DETERMINATION OF THE NITROAND NITROSO WITH IRON(II) IN ACIDICMEDIUM. GROUPSBY REDUCTION For the determination of the nitro group, accurately weigh 3-5 mg of the sample in the reaction flask. Dissolve the sample in acetone and add 20 ml of 11N hydrochloric acid. Sweep the air with carbon dioxide or nitrogen for 5 minutes a t the rate of 100 bubbles per minute, followed by adding 1.5-2 grams of iron(I1) ammonium sulfate. Boil for 15 minutes, cool, and add 1 ml of 10% ammonium thiocyanate solution. Titrate with titanium(II1) sulfate solution till the red color of the iron(II1) thiocyanate disappears. Carry a blank experiment. Calculate the percentage nitro group based on the consumption of 6 equivalents iron(I1) per mole of nitro group. For the determination of the nitroso group, the above procedure is followed using 15 ml of 11N hydrochloric acid, 1 gram of iron(I1) ammonium sulfate, and the time of boiling is only 7 minutes. Calculate the percentage nitroso group based on the consumption of 4 equivalents iron(I1) per mole of the nitroso group. BY DETERMINATION OF THE NITRO AND NITROSOGROUPS REDUCTIONWITH IRON(II) IN ALKALINEMEDIUM. Weigh 3-5 mg, of nitro or nitroso compound in the reaction flask. Dissolve the sample in ethanol and add 30 ml of 10 % sodium hydroxide solution. Sweep the air with carbon dioxide or nitrogen for 5 minutes at the rate of 100 bubbles per minute, followed by adding 1 ml of 1 N iron(I1) ammonium sulfate solution. Boil for 10 minutes, cool, acidify with concentrated hydrochloric acid (-20 ml), and add 1 ml of 10% ammonium thiocyanate solution. Titrate with titanium(II1) sulfate solution till the red color disappears. A blank experiment is carried out under the same experimental conditions. Calculate the percentage nitro and nitroso groups based on the consumption of 6 and 4 equivalents of iron(I1) per mole of the nitro and nitroso groups, respectively. RESULTS
Accuracy of the Method. The nitro group in organic compounds is satisfactorily analyzed by reduction with iron(I1) ammonium sulfate in strong hydrochloric acid medium. Reduction of mono-nitro aromatic compounds substituted with electron attracting groups shows an average recovery (15) W. I. Awad and S. S . M. Hassan, Talanta, 16, 1383 (1969). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
0
911
Table I. Microdetermination of the Nitro and Nitroso Groups in Some Aromatic Sample
Calcd
Nitro or nitroso group, Acid reduction Found Recovery
Alkaline reduction Found Recovery
Mono nitro compounds
p-Nitrobenzoic acid
27.54
3-Nitro phthalic acid
21.80
rn-Nitro ethyl benzoate
23.59
p-Nitroaniline
33.33
2-Chloro-5-nitro aniline
26.67
2-Chloro-Cnitrophenylacetic acid
21.35
p-Br omo nitrobenzene
22.77
p-Nitroacetophenone
27.88
rn-Nitrda-pheny1)cinnamic acid
17.10
3-Nitro-Chydroxy phenyl arsonic acid
17.49
p-Toluene- 3-nitro arsonic acid
17.62
p-Nitro phenyl hydrazine
30.07
1-Nitronaphthalene
26.59
3-Nitrodiphenyl
23.12
27.3 27.4 27.4 21.7 21.7 21.7 23.3 23.4 23.4 32.9 33.0 33.1 26.6 26.6 26.6 21 .o 21.1 21.2 22.6 22.7 22.7 27.7 27.7 27.8 16.9 16.9 17.0 17.5 17.5 17.6 17.5 17.6 17.7 30.0 30.0 30.0 22.0 22.2 23.7 18.7 19.2 19.9
of 99.3%; the average absolute error being =k00.27& Diand poly-nitro aromatic compounds show a n average recovery of 9 9 . 4 z , the average absolute error being *0.4z. However, nitro hydrocarbons and nitro compounds substituted with electron repelling groups are not reduced quantitatively. The average recovery is 76.5 %; the average absolute error is 8.9 % (Table I). Reduction of nitro compounds in alkaline medium is successful for nitro hydrocarbons and mono nitro aromatic compounds substituted with both electron attracting and electron repelling groups. The average recovery is 99.9% and the mean absolute error is +O.l Z.p-Nitro phenyl hydrazine is the only nitro compound that is not reduced quantitatively. The average absolute error is - 12 % and the average recovery is 60%:. Di- and poly-nitro aromatic compounds show a n average recovery of 75.2% and the average absolute error is - 12.9 (Table I). Nitroso compounds are satisfactorily analyzed with iron(I1) in acidic and alkaline media, the average recovery being 99.6 and 99.4 %, respectively (Table I). 912
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
99.1 99.5 99.5 99.5 99.5 99.5 98.8 99.2 99.2 98.7 99.0 99.3 99.7 99.7 99.7 98.4 98.8 99.3 99.3 99.7 99.7 99.4 99.4 99.7 98.8 98.8 99.4 100.0 100.0 100.6 99.3 100.0 100.5 99.8 99.8 99.8 82.7 83.5 89.1 80.9 83.0 86.1
27.3 27.4 27.4 21.7 21.7 21.8 23.5 23.5 23.6 33.2
99.2 99.5 99.5 99.5 99.5 100.0 99.6 99.6 100.0 99.6 99.9 99.9 99.7 99.7 100.1 99.8 99.8 100.2 99.3 99.7 99.7 99.4 99.7 100.1 99.4 100.0 100.0 100.0 100.0 100.0 100.5 100.5 100.5 58.5 60.2 61.5 99.3 99.7 99.7 99.9 99.9 99.9
33.3
33.3 26.6 26.6 26.7 21.2 21.2 21.3 22.6 22.7 22.7 27.7 27.8 27.9 17.0 17.1 17.1 17.5 17.5 17.5 17.7 17.7 17.7 17.6 18.1 18.5 26.4 26.5 26.5 23.1 23.1 23.1 DISCUSSION
Reduction with Iron(1I) in Acidic Medium. Iron(I1) ammonium sulfate quantitatively reduces the aromatic nitro and nitroso groups in strong hydrochloric acid medium in accordance with the equations: W
+
4Fe2+
D
N
+
4Hf &NH,
H
,
+
6Fe3+
+
2H20
--t
+
4Fe3+
+
H20
W
Six and four equivalents of iron(I1) are consumed per mole of the nitro and nitroso groups, respectively. The effects of iron(I1) concentration, acid concentration, time of boiling, and substituent groups o n the reduction of the nitro and nitroso groups are studied.
Compounds by Reduction with Iron(I1) in Acidic and Alkaline Media
Sample
Calcd
m-Nitrophenol
33.09
p-Nitrophenyl acetic acid
25.41
1-Nitroanisole
30.07
p-Nitrotoluene
33.58
Dinitro compounds 2,5-Dinitrobenzoic acid
43.40
m-Dinitrobenzene
54.76
Poly-nitro compounds Trinitrotoluene
(TNT)
2,4,6-Trinitrobenzoic acid
60.79 53.70
Nitroso compounds 1-Nitroso-2-naphthol Nitroso-R salt
17.34 7.96
2-Nitroso- 1-naphthol-4-sulfonic acid (sodium salt)
10.91
p-Nitrosodimethylaniline hydrochloride
16.09
Nitroso benzene 28.04 Research mono nitroso sample (CioHzoN70Cl)
10.38
Effect of Iron(I1) Concentration. Nitro aromatic compounds are stable toward reduction with dilute iron(I1) solutions of concentration up t o 10 mg of iron(I1) ammonium sulfate/milliliter. Reduction of micro amounts of p-nitrobenzoic acid with various concentrations of iron(I1) ammonium sulfate shows that solutions of more than 60 mg/ml (Le., ~ 0 . 1 5 N )are needed for quantitative reduction (cf., Figure 1). Six, 12, and 18 equivalents of iron(I1) are consumed per mole of mono-, di-, and tri-nitro compounds, respectively, indicating that the reduction proceeds toward the formation of the corresponding amino compounds. Iron(I1) ammonium sulfate solution of 30 mg/ml (i-e., -0.08N) is suitable for the quantitative reduction of aromatic nitroso compounds. The end reduction product of this function is the corresponding amine (i.e., 4 equivalents reduction reaction). Effect of Acid Concentration. Reduction of representative nitro compound such as p-nitrobenzoic acid with a solution of iron(I1) ammonium sulfate (60 mglml) is conducted in various concentrations of hydrochloric acid. The results obtained
Nitro or nitroso group, Acid reduction Found Recovery 68.3 22.6 73.4 24.3 86.7 28.7 90.9 23.1 91.3 23.2 93.3 23.7 48.9 14.7 52.5 15.8 57.5 17.3 30.7 10.3 33.1 11.1 33.4 11.2
Alkaline reduction Found Recovery 99.7 33.0 100.0 33.1 100.3 33.2 99.6 25.3 100.0 25.4 100.4 25.5 99.8 30.0 99.8 30.0 100.1 30.1 33.5 99.8 33.6 100.1 100.4 33.7
43.3 43.3 43.4 54.5 54.6 54.7
99.8 99.8 100.0 99.5 99.7 99.9
34.9 35.1 36.9 43.9 44.9 45.4
80.4 80.9 85.0 80.2 82.0 82.9
60.1 60.1 60.2 53.5 53.5 53.6
98.9 98.9 99.0 99.6 99.6 99.8
40.8 41.4 42.0 41.2 42.0 42.2
67.1 68.1 69.1 76.1 78.2 78.6
17.3 17.3 17.3 7.9 7.9 8.0 10.8 10.8 10.9 16.0 16.1 16.1 27.8 27.9 27.9 10.2 10.3 10.3
99.8 99.8 99.8 99.3 99.3 100.5 99.0 99.0 99.9 99.4 100.1 100.1 99.1 99.5 99.5 98.3 99.2 99.2
17.2 17.3 17.3 7.9 7.9 8.0 10.8 10.8 10.8 16.0 16.0 16.1 27.8 26.8 27.9 10.2 10.2 10.2
99.2 99.8 99.8 99.3 99.3 100.5 99.0 99.0 99.0 99.4 99.4 100.1 99.1 99.1 99.5 98.3 98.3 98.3
'
(cf. Figure 2) show that as the acid concentration increases, the degree of reduction increases. The least acid concentration required for quantitative reduction of the nitro group is 8N. However, 5N hydrochloric acid is suitable for the quantitative reduction of the nitroso group. Effect of Boiling Time. Reduction of nitro and nitroso groups with iron(I1) proceeds by boiling the reaction mixture for a few minutes. The effect of boiling time on the reduction of p-nitrobenzoic acid shows that quantitative reduction is obtained within 15 minutes. As the time of boiling diminishes, the reduction recovery decreases (cf. Figure 3). However, 8 minutes are sufficient for quantitative reduction of the nitroso group in aromatic compounds. Effect of Substituent Groups. In the nitro compounds, the presence of substituents other than the nitro group in the benzene nucleus increases or decreases the rate of reduction of the nitro group depending upon whether the substituent is electron attracting or repelling. Since the nitro-nitrogen atom is positively charged, electron attracting groups(e.g., +
COOH, C1, Br, COOR, COR, NOz,NH3, A s 0 (OH)z) intenANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
0
913
0
20 40 I r o n ( E ) ammonium
60 80 100 s u l f a t e mg/ml.
Figure 1. Reduction of p-nitrobenzoic acid with different concentrations of iron(I1) ammonium sulfate in acidic media
100
2
'
-
80
C 0
0
Figure 3. Reduction of p-nitrobenzoic acid with iron(I1) [60 mg of iron(I1) ammonium sulfate/ml] in 8N hydrochloric acid
The substituent groups in the nitroso compounds have little effect on either the rate or the degree of reduction. Reduction with Iron(I1) in Alkaline Medium. Reduction in alkaline media is possible with iron(I1) whereas other reductants such as titanium(III), chromium(II), vanadium(II), and tin(I1) are utilized only in acidic media. Nitro aromatic compounds, however, are quantitatively reduced with strong iron(I1) solutions in alkaline medium. Six equivalents of iron(I1) are consumed per mole of the nitro group :
U
.-
Time ( m i d
60
1
'CI
u
&
D
LO
N
O
,
+
GFe(OH),
+
4H,O
+
e, +
20
Nitroso aromatic compounds are also quantitatively reduced under these conditions with the consumption of 4 equivalents of iron(I1) per mole of the nitroso group according t o the equation:
0 Hydrochloric acid
cohcen trations
Figure 2. Reduction of p-nitrobenzoic acid with iron(I1) [60 mg of iron(I1) ammonium sulfate/ml] in different concentrations of hydrochloric acid sify this positivity and facilitate the reduction. On the other hand, electron repelling groups (e.g., OH, OCH3, CHJ decrease the positive charge on the nitrogen atom and consequently decrease its affinity toward reduction.
914
GFe(OH),
* ANALYTICAL CHEMISTRY, VOL. 44,
NO. 6, MAY 1972
O
N
0
+
4Fe(OH),
+
3H20 &NH, W
---t
+
4Fe(OH),
Effect of Iron(I1) Concentration, Alkali Concentration, and Boiling Time. High concentration of iron(I1) ( 4 0 moles iron(I1) hydroxide/mole of the nitro or nitroso group) is needed for quantitative reduction. In general, quantitative and rapid reduction of the nitro and nitroso groups with low blank value proceeds within 10 minutes with 20 mg of iron(I1) ammonium sulfate/milliliter in 2.5N sodium hydroxide solution, The increase of alkali concentration is accompanied by an increase in the blank value. Effects of Substituent Groups. Reduction with iron(I1) in alkaline medium, is successful with nitro hydrocarbons and all types of mono nitro and nitroso compounds whether substituted with electron repelling or electron attracting groups. In contrast, di- and poly-nitro compounds are not quanti-
tatively reduced under these conditions, probably due to the fact that aromatic poly nitro compounds are saponified after brief warming with alkalies giving variable amounts of alkali nitrite (16): Ar-N02
+ NaOH
+
Ar-0 Na
+ NaN02 + H20
Sodium nitrite is not quantitatively reduced under the present experimental conditions, since alkaline reduction of nitrates and nitrites with iron(I1) are quantitative only in 28% sodium hydroxide (17) and lower alkali concentration requires silver or copper catalyst (18). Interferences. The use of iron(I1) in acidic and alkaline media for reduction of nitro and nitroso groups has the advantage over titanium(III), since very little interference is (16) F. Feigl, “Spot Tests in Organic Analysis,” Elsevier, Amsterdam, 1966, p 299. (17) S . Carsley, J . Phhys. Chem., 34, 186 (1930). (18) Z. Szabo and L,. Bartha, Nature, 166, 309 (1950); Anal. Chim. Acra, 5 , 33 (1951).
observed with other nitrogen functions. For example, whereas 8 equivalents of titanium(II1) are required per mole of p-nitrophenyl hydrazine (19) (due to reduction of the hydrazine group along with the nitro group), only 6 equivalents of iron(I1) are needed in acidic medium. The activation influence of one nitro group upon the hydrazine function is not sufficient to permit reduction with iron(I1). Some reducible nonnitrogeneous groups that are occasionally present in organic compounds as ketonic, aldehydic, carboxylic, sulfonic ( e . g . , toluene sulfonic acid), arsonic, phosphonic ( e . g . ,benzene phosphonic acid) as well as ethylenic and acetylenic ( e . g . , diphenyl acetylene) bonds are not reduced with iron(I1) in either acidic or alkaline media. RECEIVED for review June 7, 1971. Accepted November 24, 1971. (19) J. V. Earley, and T. S. Ma, Microchim. Acta, 1960,685.
~~
~~
Separation of High-Boiling Petroleum Distillates Using Gradient Elution Through Dual-Packed (Silica Gel-Alumina Gel) Adsorption Columns D. E. Hirsch, R. L. Hopkins, H. J. Coleman, F. 0. Cotton, and C. J. Thompson Bartlesaille Energy Research Center, Bureau of Mines, US.Department of the Interior, Bartlesville, Okla. 74003 The Bureau of Mines has developed a chromatographic separation procedure using a single dual-packed adsorption column containing silica gel and alumina that will separate high-boiling petroleum distillates into four concentrates: saturates, monoaromatics, diaromatics, and polyaromatics-polar. Spectral, adsorption, and radiotracer data indicate that the concentrates produced are predominantly as labeled. Subdividing the aromatics into major aromatic types greatly simplifies subsequent separation and characterization studies. EARLYLIQUID-SOLID chromatographic separation methods were generally applicable only to simple binary mixtures or blends ( I ) . Subsequent progress was made on methods for the complete separation of aromatics from mixtures containing paraffins, naphthenes, and aromatics ( 2 , 3). Silica gel and, to some lesser extent, Alcoa alumina grades F-1, F-20, and H - 4 1 were the first and most commonly used adsorbents for separating a wide variety of samples (4-9). (1) B. J. Mair and J. D. White, J . Res. Nat. Bur. Stand., 15, 5 1 (1935). (2) B. J. Mair and A . F. Forziati, ibid., 32, 165 (1944). (3) M. R. Lipkin, W. A. Hoffecker, C. C. Martin, and R. E. Ledley, ANAL.CHEM., 20, 130 (1948). (4) A. E. Hirschler and S . Amon, Znd. Eng. Chem., 39, 1585 (1947). (5) C. Karr, Jr., W. D. Weatherford, and R. G. Capell, ANAL. CHEM., 26, 252 (1954). (6) H. E. Lumpkin and B. H. Johnson, ibid., p 1719. (7) C. M. McKinney and R. L. Hopkins, ibid., p 1460. (8) C. J. Thompson, H. J. Coleman, H. T. Rall, and H. M. Smith, ibid., 27, 175 (1955). (9) H. J. Coleman, C. J. Thompson, R. L. Hopkins, and H. T. Rall, J . Chromatogr., 20, 24.0 (1965).
The techniques of applying these adsorbents to petroleum samples usually consisted of separating the oil into an aromatic and a nonaromatic portion by adsorption on silica gel followed by rechromatographing the aromatic portion through alumina (6, IO). Both steps utilized elution chromatography with solvents of differing polarity. Tenney and Sturgis ( I ] ) , working with pure hydrocarbons and various adsorbents, established numerous guidelines for compound separability by elution chromatography. L. R. Snyder (12-14) later suggested the use of partially deactivated gel to achieve some degree of adsorption isotherm linearity, and several useful separation schemes resulted from his comprehensive work along these lines. However, the use of deactivated gel not only reduces sample adsorptive capacity of the gel by 10 to 100 times (13, but it also crowds retention volumes together close enough to reduce separability of certain compounds or compound types. In this research a liquid-solid chromatographic procedure was developed to separate high-boiling petroleum fractions into four relatively distinct compound types-namely, saturates, monoaromatics, diaromatics, and polyaromaticspolar. A monoaromatic is defined here as any compound which has only one aromatic ring regardless of the number of saturated rings. A diaromatic is a compound having two (10) E. M. Charlet, K. P. Lanneau, and F. B. Johnson, ANAL. Chem., 26, 861 (1954). (11) H. M. Tenney and F. E. Sturgis, ibid., p 946. (12) L. R. Snyder, J. Chromatogr., 5,430 (1961). (13) Zbid., 6, 21 (1961). 33, 1527 (1961). (14) L. R. Snyder, ANAL.CHEM., (15) Zbid., 39, 698 (1967). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
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