Halogenation of Benzoic Acid and Monohalogen Derivatives of Benzene in the Presence of a Sulfuric–Nitric Acid Mixture

A. N. Novikov

(Presented by Prof. B. V. Tronov, Doctor of Chemical Sciences)

It is known that certain aromatic compounds can be halogenated with free halogens (chlorine, bromine, iodine) in the presence of nitric or sulfuric acid of various concentrations, as well as mixtures of these acids. In this case, depending on the reactivity of the starting material and the reaction conditions, halogen- or nitro compounds, or a mixture with predominance of one or another product, are formed.

A review of the extensive literature on this question [1, 2, 3, 4, 5] has convinced us that the situation with the elucidation of the mechanism of this type of reaction is far from satisfactory.

Chemists have explained the halogenating action of acids by their oxidizing properties, which increase with increasing concentration and especially when the acids are used together. Therefore, many investigations were carried out in the presence of fuming nitric and sulfuric acids and their mixtures. As a result, the reactions led not only to halogenation and nitration, but also to more profound changes in the organic molecules, with the consequence that, as a rule, the yields of the reaction products were insignificant.

We, however, having chosen for halogenation a sulfuric–nitric acid mixture (nitrating mixture) composed of sulfuric acid ($d$ 1.84) and nitric acid ($d$ 1.4), proceeded from the consideration that acids, acting as catalysts, accelerate halogenation reactions by activating the organic compound and the halogen. The organic compound is activated by complex formation with the acid; the halogen, under the action of the same acids, passes into a strongly active cation.

The interaction of the complex with the halogen cation causes the halogenation reaction.

The present work is a continuation of the investigations begun to clarify the influence of a sulfuric–nitric acid mixture (as a catalyst) on the halogenation reactions of aromatic substances [6, 7, 8, 9].

Subjected to halogenation were benzaldehyde, benzoic acid, fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene.

The results of the work performed make it possible, first of all, to obtain p-iodofluorobenzene, p-iodobromobenzene, p-bromochlorobenzene, and p-dibromobenzene by halogenation of the corresponding compounds with iodine or bromine in the presence of a nitrating mixture. According to the literature, these compounds can be prepared by diazotization of halogen-substituted ani-

aniline with subsequent replacement of the diazonium group by halogen [10]; halogenation in the presence of aluminum chloride, ferric chloride [11], a mixture of fuming sulfuric and nitric acids [12], and silver salts [13].

In the bromination of benzaldehyde in the presence of a nitrating mixture, under the most varied conditions (temperature, concentration of the reacting substances, time), benzoyl bromide was always obtained (instead of the expected m-bromobenzaldehyde), as were benzoic and m-bromobenzoic acids. Iodination of benzaldehyde under exactly the same conditions leads to the formation of m-iodobenzaldehyde [7].

In the products obtained in chlorination reactions (in contrast to bromination and iodination reactions), nitrogen and chlorine were detected qualitatively. Consequently, the substance was chlorinated and nitrated. From the products obtained by chlorination in the presence of a sulfuric–nitric acid mixture, it was not possible to isolate a noticeable amount of a pure chloro derivative. The quantitative chlorine content in the crude product (from 3 to 6%) indicated an insignificant degree of chlorination.

Comparing the results obtained under the same conditions (Table 1) for chlorination, bromination, and iodination reactions, it may be said that iodination under our conditions proceeds more readily than bromination and chlorination.

Table 1

We explain this by the fact that the halogenating agents are halogen cations, formed by the action of the nitrating mixture on elemental chlorine, bromine, and iodine. Chlorine is a more electronegative element; formation of the chlorine cation should be more difficult than that of iodine or bromine, and therefore chlorination, relative to bromination and especially iodination, proceeds considerably more slowly.

The second consequence following from our experimental data is the conclusion that the nature of the halogen affects the rate of the iodination and bromination reactions. The halogens, in decreasing order of their effect on increasing the rate, are arranged in the series: F, I, Br, Cl (Table 2).

Table 2

These synthetic data differ somewhat from the results of kinetic studies carried out by foreign chemists Robertson, Maron, and Suezlund [14] in 1953 on the chlorination rate of these same substances. They obtained the following relative chlorination rates: for \(C_6H_5F\)—1.2; \(C_6H_5I\)—0.18; \(C_6H_5Cl\)—0.11; \(C_6H_5Br\)—0.08.

Experimental Part\(^1\)

The experiments were carried out in a three-necked flask equipped with a condenser, dropping funnel, and stopper. With mechanical stirring, the reaction mixture was heated in a sulfuric-acid bath warmed by an electric hot plate connected to the mains through a thermostat. In iodination, nitric acid was added gradually to the mixture of starting materials.

In bromination reactions, bromine dissolved in acetic acid containing nitric acid was added to the flask from a dropping funnel with a tube passing into the solution of the substance being halogenated.

Chlorine for chlorination was obtained from hydrochloric acid and potassium permanganate. After purification, the chlorine was passed through a rheometer to measure the amount entering per unit time into the solution of the substance being halogenated. Fluorobenzene was prepared from aniline through the diazonium borofluoride according to Schiemann.

Iodination of Fluorobenzene

Quantity of starting materials: fluorobenzene 4.7 ml (0.05 mole), iodine 6.35 g (0.025 mole), sulfuric acid (\(d\) 1.84) 5.6 ml (0.1 mole), nitric acid (\(d\) 1.4) 2.7 ml, glacial acetic acid 15 ml, carbon tetrachloride 4 ml.

Nitric acid was added over 15 minutes. The temperature of the experiment was 92–93°. Time: 1 h. After cooling, the mixture was diluted with water, the aqueous acid layer was separated, the oil was washed with water and dissolved in ether. The ethereal solution was treated with sodium sulfite solution, washed with water, dried over calcium chloride, and the ether was distilled off. After distillation, \(p\)-iodofluorobenzene with b.p. 183–185° was obtained in an amount of 6.6 g (yield 60%).

The qualitative test for nitrogen was negative.

Iodination of bromobenzene proceeded under the conditions specified for fluorobenzene. After cooling of the reaction mixture, water was added, the precipitate of \(p\)-iodobromobenzene was filtered off, washed with water, and recrystallized from ethanol. 6.65 g of \(p\)-iodobromobenzene with m.p. 91–92° was obtained (yield 45%).

The synthesis can be conducted at 120–125° for 40 minutes.

Bromination of Chlorobenzene, Bromobenzene, and Iodobenzene

Quantity of starting materials: halobenzene 10 ml, bromine 4 ml, sulfuric acid 5.6 ml (\(d\) 1.84), nitric acid (\(d\) 1.4) 4.0 ml, glacial acetic acid 10 ml. Temperature of the syntheses: 120–125°; time: 30 minutes. The solid reaction products were filtered off, washed with water, and crystallized from ethanol. The following were obtained: \(p\)-bromochlorobenzene with m.p. 66–67°, 4.4 g (30%); \(p\)-dibromobenzene with m.p. 87°, 7.3 g (35%); \(p\)-bromoiodobenzene with m.p. 91–92°, 10.9 g (45%).

\(^1\) Students D. V. Dyagilev, A. D. Kozina, and G. M. Bystrushkina took part in the experiment.

Chlorination, Bromination, and Iodination of Benzoic Acid

For the experiments, 6.1 g (0.05 mole) of benzoic acid was taken; halogen (iodine 6.35 g, bromine 1.3 ml, chlorine 22.4 l), sulfuric acid ($d$ 1.84) 5.6 ml, nitric acid ($d$ 1.4) 3.5 ml, glacial acetic acid 15 ml, and carbon tetrachloride 4 ml. The temperature of the syntheses was 85°, and the time was 4 hours. In the products of iodination and bromination nitrogen is absent; in the product of chlorination the reactions for nitrogen and chlorine are positive. Yield: $m$-iodobenzoic acid 47.0% (m.p. 185–187°); $m$-bromobenzoic acid 42.0% (m.p. 151–153°).

According to quantitative determination of chlorine in the chlorination product, no more than 6% of the chloro derivative was obtained.

Conclusions

1. Bromo- and iodo-derivatives of monohalogen-substituted benzenes were obtained by direct halogenation in the presence of a sulfuric–nitric acid mixture composed of nitric ($d$ 1.4) and sulfuric ($d$ 1.84) acids.

2. It has been shown that chlorination in the presence of a sulfuric–nitric acid mixture does not give satisfactory results.

3. It has been established that, in monosubstituted benzenes, the halogens, in decreasing order of their influence on the rate of iodination and bromination reactions, are arranged in the series: F, I, Br, Cl.

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