Complexes of Aminobenzoic Acids and Their Salts with Dinitrobenzenes
Abstract Generated abstract
This study examines complex formation between ortho-, meta-, and para-aminobenzoic acids, their sodium salts, and the three isomeric dinitrobenzenes in acetone solution. Using photoelectric colorimetry, the authors compare color deepening and optical-density maxima across 18 systems to infer the composition and relative strength of the complexes, with several products also isolated in crystalline form. The results indicate interaction through complex bonding involving nitro and amino nitrogen atoms, with most maxima corresponding approximately to 1:1 molecular ratios, though some are shifted toward excess dinitrobenzene. Sodium salts show stronger complexing ability than the free acids, and para-dinitrobenzene exhibits the greatest tendency to form complexes among the dinitrobenzene isomers.
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COMPLEXES OF AMINOBENZOIC ACIDS AND THEIR SALTS WITH DINITROBENZENES
B. V. TRONOV, N. V. SYRNEVA
Amino acids have two functional groups of opposite electrochemical character. This should make them capable of complex formation with a wide variety of substances. In complex formation with nitro compounds, which likewise possess reactivity of a mixed type, amino acids might function both as electron donors, through the nitrogen atom, and as electron acceptors, through the carboxyl hydrogen. Salts of amino acids with alkali metals should be only electron donors, and stronger ones than the free acids, owing to the presence of a negative charge on the oxygen atoms.
The question of the predominance of one or another type of activity in amino acids can most simply be resolved from the change in color in such systems as amino acid—nitro compound. Complex formation of amines with nitro compounds, through addition of the unshared electron pair of amino nitrogen to the positively charged nitrogen of the nitro group, leads to a sharp deepening of color, which is not observed in the reactions of nitro compounds with acids.
In the present work we took ortho-, meta-, and para-aminobenzoic acids, their sodium salts, and all three isomeric dinitrobenzenes.
The amino acids were purified by crystallization from water. The salts were prepared from alcoholic solutions of the acids by adding an alcoholic solution of caustic soda, with a slight excess of acid, which was washed out with alcohol. Meta-dinitrobenzene was purified by crystallization from alcohol; ortho- and para-dinitrobenzenes were synthesized by diazotization from nitroaniline borofluorides and purified by sublimation. All 18 systems were studied colorimetrically in acetone solution. The total concentration of dissolved substances was \(1/800\) gram in 25 ml. Measurements of optical density were carried out with a photoelectric colorimeter, model FEK-M, in a cuvette of length 50 mm with a blue light filter. Colorimetry of each system was performed three times, and the mean values were taken. The experiments were conducted at a temperature of 19°.
The results of the measurements are presented graphically (Figs. 1, 2, 3) and in the table, where the maxima of optical density and the molecular ratios of the components (acid : dinitrobenzene) corresponding to these maxima are given.
Fig. 1. Optical density of the systems “\(m\)-dinitrobenzene + acid”: 1—\(p\)-aminobenzoic acid; 2—\(o\)-aminobenzoic acid; 3—\(m\)-aminobenzoic acid; and “\(m\)-dinitrobenzene + salt of the acids”: 4—\(p\)-aminobenzoic acid; 5—\(o\)-aminobenzoic acid; 6—\(m\)-aminobenzoic acid.
Abscissa: mol. % \(m\)-dinitrobenzene. Ordinate: optical density.
Fig. 2. Optical density of the systems “\(p\)-dinitrobenzene + acid”: 1—\(p\)-aminobenzoic acid; 2—\(o\)-aminobenzoic acid; 3—\(m\)-aminobenzoic acid; and “\(p\)-dinitrobenzene + salt of the acids”: 4—\(p\)-aminobenzoic acid; 5—\(o\)-aminobenzoic acid; 6—\(m\)-aminobenzoic acid.
Abscissa: mol. % \(p\)-dinitrobenzene. Ordinate: optical density.
Fig. 3. Optical density of the systems “\(o\)-dinitrobenzene + acid”: 1—\(p\)-aminobenzoic acid; 2—\(o\)-aminobenzoic acid; 3—\(m\)-aminobenzoic acid; and “\(o\)-dinitrobenzene + salt of the acids”: 4—\(p\)-aminobenzoic acid; 5—\(o\)-aminobenzoic acid; 6—\(m\)-aminobenzoic acid.
Abscissa: mol. % \(o\)-dinitrobenzene. Ordinate: optical density.
By evaporation of the solutions the following complexes were obtained in crystalline form:
Ortho-aminobenzoic acid + meta-dinitrobenzene, bright yellow in color. Melting point 140.5–141°. After decomposition with alkali, dinitrobenzene was isolated in amounts of 52.9 and 51.8%, which is close to the molecular ratio 1:1 (dinitrobenzene content 55.07%).
Table
| No. | Aminobenzoic acids and their salts | o-dinitrobenzene: color of solution | o-dinitrobenzene: optical-density maximum | o-dinitrobenzene: mol. % dinitrobenzene | m-dinitrobenzene: color of solution | m-dinitrobenzene: optical-density maximum | m-dinitrobenzene: mol. % dinitrobenzene | p-dinitrobenzene: color of solution | p-dinitrobenzene: optical-density maximum | p-dinitrobenzene: mol. % dinitrobenzene |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | o-aminobenzoic acid | pale yellow | 0,069 | 50 | pale yellow | 0,071 | 50 | yellow | 0,156 | 60 |
| 2 | Sodium salt of o-aminobenzoic acid | yellow | 0,13 | 70 | yellow | 0,133 | 50 | bright yellow | 0,254 | 60 |
| 3 | m-aminobenzoic acid | pale yellow | 0,078 | 70 | pale yellow | 0,068 | 60 | yellow | 0,150 | 50 |
| 4 | Sodium salt of m-aminobenzoic acid | yellow | 0,119 | 70 | yellow | 0,106 | 50 | bright yellow | 0,228 | 50 |
| 5 | p-aminobenzoic acid | pale yellow | 0,06 | 60 | pale yellow | 0,068 | 50 | yellow | 0,159 | 50 |
| 6 | Sodium salt of p-aminobenzoic acid | yellow | 0,124 | 80 | yellow | 0,102 | 50 | bright yellow | 0,238 | 60 |
Ortho-aminobenzoic acid + para-dinitrobenzene, yellow in color.
Para-aminobenzoic acid + meta-dinitrobenzene, orange-red in color. Melting point 151–153°.
Para-aminobenzoic acid + para-dinitrobenzene, red in color.
Conclusions
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In all the systems studied, a deepening of color is observed, which indicates chemical interaction with the formation of a complex bond between the nitrogen atoms of the nitro group and the amino group.
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The maximum values of optical density in most systems occur at the molecular composition 1:1, sometimes shifted toward the dinitrobenzenes.
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All the amino acids taken give close values of the maximum optical density. The same was observed for their sodium salts, but the salts proved to be considerably stronger complexing agents than the free acids. This must be explained by the increase in electron density at the anion as compared with the neutral molecule.
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Among the dinitrobenzenes, the greatest ability for complex formation was found in the para isomer. It is considerably superior to the mutually similar ortho- and meta-dinitrobenzenes.