KINETIC STUDY OF THE NITRATION OF SOME AROMATIC COMPOUNDS IN NITROBENZENE SOLUTION

Kh. A. Letchuk, B. V. Tronov

In the present work we studied the nitration of aromatic compounds in nitrobenzene solution. Nitrobenzene mixes with nitric acid even when the latter contains up to 20% water; we therefore had the opportunity to study the rate of nitration in a homogeneous medium.

Many works have been devoted to the study of the rate of nitration of aromatic compounds.

Hirshbach and Kessler [1] also determined the rate of action of nitric acid on benzene in nitrobenzene solution. The influence of the relative amounts of benzene, nitric acid, nitrobenzene, and water was ascertained.

Martinsen [2] used concentrated sulfuric acid as the solvent and measured the rate of nitration of compounds that are less readily attacked by nitric acid, chiefly those already containing the $\mathrm{NO_2}$ group.

Wibaut [3] investigated the nitration of certain aromatic hydrocarbons and halogen derivatives with a solution of nitric acid in acetic anhydride. The nitrating agent in this case is the mixed nitric–acetic anhydride, which is formed from nitric acid and acetic anhydride.

One of the authors of the present article carried out experiments on the nitration of hydrocarbons and other compounds of the aromatic [4] and aliphatic series [5] under Wibaut’s conditions, and also by the method of Hirshbach and Kessler, i.e., using nitrobenzene as the solvent [6]. In both works fuming nitric acid was used.

The present investigation was undertaken with the aim of studying the kinetics of the nitration of aromatic compounds in nitrobenzene solution by absolute nitric acid.

Experimental Part

For the investigation, chemically pure absolutely anhydrous nitric acid of specific gravity 1.5477 at 0°C was used.

Nitrobenzene and all aromatic compounds used were chemically pure, twice distilled, with physical constants corresponding to the tabulated data.

We determined the rate of nitration of the following aromatic compounds: benzene, toluene, chlorobenzene, bromobenzene, and iodobenzene at 20°C.

To avoid a rise in temperature when mixing nitrobenzene with nitric acid and, consequently, acceleration of the nitration process of nitrobenzene, we proceeded as follows. The required amount of nitrobenzene in a flask with a ground-glass stopper was frozen in a thermostat at 0°C, and then the required amount of nitric acid at a temperature of 0°C was added. Under the influence of the heat evolved, the nitrobenzene melted and a homogeneous mixture was obtained; the temperature rose only slightly, by 1–2°. After this, the resulting homogeneous mixture was transferred to a thermostat at 20°C, kept for 20 minutes, and then the calculated required amount of the aromatic compound at 20°C was added.

For all experiments a single nitrating mixture was prepared, consisting of 2 moles of nitrobenzene and 1 mole of nitric acid.

Earlier experiments on the study of the nitrobenzene—nitric acid system showed that the nitration process of nitrobenzene at this ratio of components proceeds very slowly and that during the course of the study no more than 5–6% of the acid reacts.

The aromatic compound was added in the amount calculated for one mole to the nitrating mixture, i.e., mixtures were studied that contained the compound to be nitrated, nitrobenzene, and nitric acid in the molecular ratio 1:2:1.

We followed the rate of nitration by the consumption of nitric acid. For this purpose, samples were withdrawn at specified time intervals and titrated with a solution of $\mathrm{Ba(OH)_2}$. The $\mathrm{Ba(OH)_2}$ solution was preferred to a $\mathrm{NaOH}$ solution, since with it it is easier to guarantee the absence of $\mathrm{CO_2}$.

Discussion of the Results

The data of the study are presented graphically in Fig. 1. As can be seen, the nitration process in all the systems considered proceeds rather slowly, whereas the addition of 100% nitric acid to the pure components causes such an energetic reaction that the mixture boils, and carrying out a kinetic study under such conditions is impossible.

Moreover, in our case nitration does not go to completion and, with the exception of toluene, ends when 36–43% of the acid has been consumed. In the system with toluene, approximately 60% of the acid taken is consumed.

Such slow and partial nitration must be explained by the fact that nitric acid is bound by the nitro group of nitrobenzene into complex compounds.

From the table, which gives the relative rates of nitration in the systems, with the rate

Fig. 1. Rate of nitration of benzene, toluene, iodobenzene, bromobenzene, and chlorobenzene.

Table

the rate of nitration of benzene, it is evident that the relative rates of nitration differ little in all systems. Toluene nitrates somewhat faster, then benzene, iodobenzene.

Chlorobenzene and bromobenzene at first nitrate at almost the same rate, and then the rate of nitration of chlorobenzene slows somewhat. The average relative rate for the aromatic compounds studied is as follows: \(C_6H_5CH_3\)—1.095, \(C_6H_6\)—1, \(C_6H_5J\)—0.93, \(C_6H_5Br\)—0.83, \(C_6H_5Cl\)—0.80.

Conclusions

1. The rate of nitration of benzene, toluene, chlorobenzene, bromobenzene, and iodobenzene by absolute nitric acid in the presence of excess nitrobenzene has been determined.

2. Owing to the complex addition of nitric acid to the nitro group, the reactivity of the acid with respect to the substances being nitrated is greatly reduced.

3. In addition to the general decrease in the rate of reaction with nitric acid, an equalization of the activity of all the compounds taken with respect to \(HNO_3\) was observed. According to the reaction rate under these conditions, they are arranged in the series: \(C_6H_5CH_3 > C_6H_6 > C_6H_5J > C_6H_5Br > C_6H_5Cl\), which corresponds to the data available in the literature; however, in the initial stages of the process the difference in rate is small.

LITERATURE

1. Gierschbach J., Kessler A. Zts. phys. Chem., 2, 676, 1888; Kessler A., Zts. phys. Chem., 2, 69, 1888.
2. Martinsen H. Zts. phys. Chem., 50, 385, 1904–1905; 59, 605, 1907.
3. Wibaut J. P., Cbl., 2, 653, 1915.
4. Tronov B. V., Kamai G. Kh., Kovalenko A. G. ZhRKhO, 60, 1013, 1928.
5. Tronov B. V., Korotkikh V. S. ZhRKhO, 60, 1014, 1928.
6. Tronov B. V., Ber I. Ya. ZhRKhO, 62, 2337, 1930.