BROMINATION OF BENZENE IN THE PRESENCE OF VARIOUS CATALYSTS

L. A. PERSHINA

Introduction

The importance of aromatic halogen derivatives

Aromatic halogen derivatives belong to the most important intermediate products of the organic chemical industry. Their importance has grown especially in recent times, when methods have been found for the comparatively easy and inexpensive conversion of these compounds into phenols and amines, from which many dyes, pharmaceuticals, plastics, etc. are subsequently produced.

Despite the more than half-century-long interest of industrial technology in obtaining halogen derivatives of the aromatic series, and despite the still greater age of the first research works on the study of halogenation [17], many even fundamental questions of the synthesis of halogen-substituted compounds by the direct action of halogens on aromatic hydrocarbons have not been sufficiently elucidated either theoretically or practically.

Methods of halogenation

The most common method of obtaining halogen derivatives of aromatic hydrocarbons consists in the direct action of halogens (chlorine and bromine) on hydrocarbons; in this case the nature of the products obtained depends very strongly on the reaction conditions.

Thus, in sunlight and in the absence of catalysts, the presence in the benzene nucleus of a system of conjugated double bonds leads to the possibility of an addition reaction; chlorine and bromine first dissolve in benzene and then slowly form addition products: hexachlorocyclohexane and hexabromocyclohexane.

N. N. Vorozhtsov [1] succeeded in proving experimentally that benzene, without the participation of catalysts, only adds chlorine. The reaction has a chain character and is similar to the reaction of hydrogen with chlorine.

Halogenation of benzene in the presence of catalysts leads to the formation of substitution products.

The number of catalysts proposed for the halogenation of aromatic compounds is very large. Thus, Beilstein [13] indicates that with prolonged action of chlorine on benzene in the presence of iodine and at elevated temperature, 1,2,4-trichlorobenzene, 1,2,4,5-tetrachlorobenzene, pentachlorobenzene, and hexachlorobenzene are obtained. With prolonged action of chlorine on benzene in the presence of aluminum chloride, tetra-, penta-, and hexachlorobenzene are formed; hexachlorobenzene is also obtained upon complete chlorination of benzene in the presence of antimony pentachloride and ferric chloride; chlorination of benzene in the presence of ...

amalgamated aluminum, thallium chloride, tin, molybdenum chloride; when benzene is heated with $\mathrm{SO_2Cl_2}$, chlorobenzene is obtained at $160^\circ$. With prolonged action of bromine on an equimolecular amount of benzene, bromobenzene is formed [14].

In Beilstein we find indications that bromination of benzene proceeds more rapidly in the presence of iodine, zinc chloride, aluminum chloride or amalgamated aluminum, ferric chloride, ferric bromide, aluminum bromide, phosphorus trichloride, phosphorus tribromide, antimony trichloride, antimony tribromide; under the action of sulfur bromide and nitric acid on benzene, bromobenzene is obtained. Gross and Cohen [15] discuss the influence of pyridine on the bromination of benzene.

Metals may serve as catalysts in the halogenation of aromatic compounds: iron [2], aluminum [1], and their oxides [1].

In patents there are indications of the use, as halogenation catalysts, of antimony, sulfur, phosphorus, and iodine.

While studying the chlorination of benzene in the presence of various amounts of ferric chloride, N. N. Vorozhtsov and I. S. Travkin [1] established that this reaction is a typical homogeneous-catalytic one, and that the catalysis is caused only by ferric chloride dissolved in the reaction mixture.

As for the mechanism of substitution reactions, there are many statements on this subject.

1. According to one view, the catalyst performs a special function: transfer of halogen, owing to a change in the valence of the element bound to it. The halogen is first fixed by the catalyst, combining with it to form a product that can transfer its halogen to the aromatic hydrocarbon. This interaction may be expressed by the scheme:

\[ 2\mathrm{Fe} + 3\mathrm{Br}_2 \longrightarrow 2\mathrm{FeBr}_3, \]

\[ \mathrm{C}_6\mathrm{H}_6 + 2\mathrm{FeBr}_3 \longrightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{Br} + 2\mathrm{FeBr}_2, \]

\[ 2\mathrm{FeBr}_2 + \mathrm{Br}_2 \longrightarrow 2\mathrm{FeBr}_3,\ \text{etc.} \]

However, aluminum is one of the best catalysts, although it is always trivalent.

1. A hypothesis enjoys some recognition according to which the ease of splitting off the halogen is of chief importance, and the substitution process proceeds through the formation of organometallic compounds:

\[ \mathrm{C}_6\mathrm{H}_6 + \mathrm{AlBr}_3 \longrightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{AlBr}_2 + \mathrm{HBr}, \]

\[ \mathrm{C}_6\mathrm{H}_5\mathrm{AlBr}_2 + \mathrm{Br}_2 \longrightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{Br} + \mathrm{AlBr}_3. \]

In fact, all halides that are catalysts in substitution reactions readily give up halogen. But this explanation also encounters a contradiction. Thus, silicon tetrachloride gives up chlorine even more readily than $\mathrm{AlCl}_3$ or $\mathrm{FeCl}_3$, but it has no catalytic action at all (see below).

1. In catalytic action, the capacity for complex formation may be important.

In all halides that catalyze the substitution reaction, the coordination number of the central atom exceeds its valence. Thus, for aluminum the coordination number is six, in some cases four. Iron has a coordination number also equal to six. The coordination number of antimony, tin, etc., is the same.

Silicon tetrachloride, although the silicon atom sometimes has a coordination number of six, does not itself form complexes and is not a catalyst in substitution reactions. Hence the mechanism of the process of substitution of hydrogen in benzene may be represented as follows: the catalyst attracts-

...binds by its central atom to one of the carbon atoms of the benzene ring; as a result, the bond of carbon with hydrogen is loosened, and the latter is readily split off under the action of the halogen (Fig. 1).

Experimental data lead to the conclusion that the principal factor determining the catalytic action of halides is their ability to form complexes.

In this case the catalyst must be a positive complex-forming agent, an electron acceptor, i.e., act as a positive atomic nucleus that is attracted to the negative carbon of the benzene ring [7]. The attraction may also be insufficiently strong to form a stable complex that could be isolated. N. D. Zelinsky [4] points to the formation of complex compounds of aluminum bromide with aromatic hydrocarbons.

Compounds of benzene with sulfur trichloride and tribromide, etc., are known.

Fig. 1

Purpose of the Work

The purpose of the present work is to study the bromination reaction of benzene in the presence of as large a number as possible of different catalysts.

Catalysis at the present time is of enormous importance in all branches of the chemical industry. Meanwhile, the selection of catalysts and of the conditions for carrying out catalytic processes has until now been chiefly empirical in character. The most important task in this field is the development of a general theory of catalysis that would make it possible to predict in advance in what direction and at what rate a reaction will proceed in the presence of a given catalyst and under given external conditions. The elucidation of general regularities is especially difficult in organic chemistry, where almost always several directions of a chemical process are possible. Here it is necessary to carry out ...

a vast research effort, which will undoubtedly yield great practical benefits.

Experimental Part

Selection of Catalysts

As catalysts we used the following metals and nonmetals: silver, copper, magnesium, zinc, cadmium, mercury, aluminum, tin, lead, phosphorus, arsenic, antimony, bismuth, chromium, molybdenum, tungsten, sulfur, selenium, tellurium, manganese, iodine, iron, nickel, cobalt.

Of the prepared halides, the following were taken: calcium chloride, silicon tetrachloride, hydrogen bromide [6,9,18], ferric bromide, aluminum bromide, iodine tribromide, antimony tribromide, arsenic bromide, zinc bromide, sulfur bromide, phosphorus pentabromide, copper bromate.

Finally, it was of interest to study the action of certain benzene derivatives, in particular nitro compounds, which are distinguished by a great capacity for complex formation, and iodobenzene, in which iodine readily increases its valence. Of the nitro compounds we took nitrobenzene, dinitrobenzene, and picric acid. The catalytic action of acetic acid, water, and pyridine was also investigated.

Preparation and Purification of the Reagents Used

Benzene was purified from thiophene with sulfuric acid, washed with water, dried with calcium chloride and sodium, and twice distilled. Bromine was not purified. Hydrogen bromide was obtained from Br₂, P and water [3] and benzene was saturated with it. The concentration of hydrogen bromide was determined by titration, and the solution was diluted with benzene to the desired concentration.

Of the metals and nonmetals used as catalysts for the bromination reaction, some were analyzed for the content of impurities interfering with the investigation. Iron was tested for arsenic content; zinc—for iron; copper—for iron and lead content; bismuth—for iron, copper, and zinc; aluminum—for zinc content [11]. The results of the analyses were negative. All metals and nonmetals used as catalysts were taken in finely divided form.

The experimental part of our work consists of two sections: in the first section, a reconnaissance study was carried out of the catalytic action of 24 metals and nonmetals, 4 prepared bromides, and 8 organic compounds. In some cases the reaction products were investigated. In the second section we investigated the action of 9 prepared bromides.

Bromination of Benzene in the Presence of Simple Substances and Organic Compounds

In all experiments bromine was taken in a ratio to benzene of 1:50. Catalysts were taken in an amount of 0.01 mole per mole of bromine.

For a large series of experiments, a single mixture of benzene with bromine was prepared and poured into separate flasks, to which the catalysts were added. In each series a control experiment was set up (without catalysts).

The solutions stood in the dark at room temperature. After definite intervals of time, 5 ml samples were taken, in which the pro—

the unreacted bromine was titrated with hyposulfite (after addition of KI) [5].

We carried out four series of experiments on the bromination of benzene.

1st series. Bromination of benzene without catalysts

A mixture of 16.9864 g of bromine and 462.3 ml of benzene, placed in a volumetric flask with a ground-glass stopper, was put in a dark place for a prolonged time until the reaction was complete. Under our conditions the reaction was fully completed in 22 months.

Table 1

Bromination of benzene over a prolonged time

2nd series. Bromination of benzene in the presence of hydrogen bromide

We tested the effect of hydrogen bromide at concentrations of 0.1 N and 0.05 N.

The ground-glass stoppers of volumetric flasks containing the mixture of bromine and benzene saturated with hydrogen bromide were sealed with paraffin. In parallel, for comparison, bromination of benzene without catalysts was set up. The results of determining the amount (in %) of unreacted bromine at various time intervals are given in Table 2.

Table 2

Bromination of benzene in the presence of hydrogen bromide

Series 3. Bromination of benzene in the presence of acetic acid and water

The reaction in the presence of acetic acid proceeded in 28 months, and with water—in 30 months.

Table 3

Bromination of benzene in the presence of acetic acid and water

Series 4. Bromination of benzene in the presence of various catalysts

The catalysts were taken at a concentration of 0.01 m relative to bromine. The results of the experiments are given in Tables 4 and 5.

As is evident from the experiments presented, zinc proved to be a good catalyst in the bromination of benzene. We investigated the action of zinc at different concentrations and different temperatures, as well as the action of prepared zinc bromide in an amount of 0.01 m relative to bromine.

The results of determining the amount of bromine that had reacted (in %) are given in Tables 6 and 7.

In all cases where the reaction proceeded to completion, we investigated the products of this reaction.

The benzene layer was worked up in the usual manner. When washing the reaction mixture, we used a 10% solution of caustic alkali. The washing was carried out successively with water, alkali, and water.

In the experiments on bromination of benzene without catalysts, with acetic acid, and with water, two fractions were collected: the first fraction, 80–90°, and the second—everything that had a boiling point above 90°. The second fraction was distilled with steam. Bromobenzene, in the form of heavy drops, was collected at the bottom of the receiver. The residue, according to the literature data, is the product of addition of bromine to benzene—hexabromocyclohexane; upon heating it, hydrogen bromide was evolved vigorously.

The reaction product of bromination of benzene in the presence of the catalysts zinc, iron, bismuth, tellurium, and mercury is bromobenzene.

The results of the study are given in Table 8.

Table 4

Bromination of benzene in the presence of various catalysts
(in %)

Table 5

Bromination of benzene in the presence of various catalysts
(in %)

Table 6

Bromination of benzene in the presence of various amounts of zinc

Table 7

Table 8

Use of Ready-Made Bromides in the Bromination of Benzene

In this part of the work we set ourselves the aim of investigating the relative activity of certain bromides used as catalysts in the bromination reaction of benzene.

As a result of a number of preliminary experiments, we came to the conclusion that the following conditions would be the most favorable.

1. The molecular ratio of bromine to benzene is equal to 1:2, since at greater dilution the reaction rate will be lowered, while at the ratio \( \mathrm{C_6H_6 : Br_2 = 1 : 1} \) the yield of bromination products will decrease as a result of large losses of bromine.

An experiment carried out on the bromination of benzene with zinc bromide at \( \mathrm{C_6H_6 : Br_2 = 10 : 1} \), \( t^\circ = 45^\circ \), the amount of \( \mathrm{ZnBr_2} = 0.01 \) mole relative to bromine; bromination time—10 h, gives only 0.42 g of bromobenzene, which amounts to ~5%.

At a molecular ratio of benzene to bromine equal to 1.5:1, under the same conditions (\( t^\circ = 45^\circ \), time 10 h, 0.01 mole, amount of \( \mathrm{ZnBr_2} \)), the yield of bromobenzene is 34.7%, and at the ratio \( \mathrm{C_6H_6 : Br_2 = 2 : 1} \)—47.1%.

1. In the preceding experiments the action of catalysts was studied at 0.01 mole relative to bromine, which is the most favorable amount, at least for the best of the catalysts—zinc, where (\( t^\circ = 25^\circ \)) a fourfold decrease (0.0025 mole) sharply lowered the reaction rate; at 0.005 mole the reaction rate is the same as at 0.01 mole.

We chose a catalyst concentration of 0.01 mole relative to bromine, taking zinc bromide as a guide, which acts somewhat more weakly than Zn powder.

1. The time during which the reaction was carried out was chosen by us as 10 h on the basis of preliminary experiments. Bromination of benzene for 4 h, \( t^\circ = 45^\circ \), at a molecular ratio of bromine to benzene of 1:2, in the presence of zinc bromide, taken in an amount of 0.01 mole relative to bromine (0.45 g), gives 31.3% bromobenzene (9.83 g).

The same experiment, carried out for 10 h, other conditions being equal, gives 47.1%.

In the preceding experiments we did not maintain a constant temperature. The experiments were carried out with temperature fluctuations of the order of 12°, which was noticeably reflected in the reaction rate. This omission is mitigated by the fact that entire series of experiments were set up under identical conditions, and therefore their relative reaction rates could be compared. In addition, these experiments may be regarded as preliminary, in order to investigate a large number of catalysts.

In this series of experiments we adopted a constant temperature. The temperature at which all the experiments were carried out was 45°. Constancy of the temperature to within 0.2° was maintained in a thermostat by means of a mercury-toluene thermoregulator.

We studied the bromination of benzene under the conditions described above (\(t^\circ = 45^\circ\), time—10 h, \(\mathrm{C_6H_6 : Br_2 = 2 : 1}\), amount of catalyst \(0.01\ m\)) in the presence of nine bromides: \(\mathrm{AlBr_3}\), \(\mathrm{FeBr_3}\), \(\mathrm{ZnBr_2}\), \(\mathrm{CuBr_2}\), \(\mathrm{S_2Br_2}\), \(\mathrm{AsBr_3}\), \(\mathrm{SbBr_3}\), \(\mathrm{IBr_3}\), \(\mathrm{PBr_5}\).

Of these, only aluminum bromide was prepared by us, according to the Gustavson method described in Kuryakin’s manual [6].

Bromine was added dropwise to a flask heated to 100°. The bromine vapors formed pass over aluminum shavings placed in an inclined glass tube. Aluminum bromide was collected in a retort, from which \(\mathrm{AlBr_3}\) was distilled at \(t^\circ = 260^\circ\). The \(\mathrm{AlBr_3}\) was stored in ampoules. The remaining bromides were ready-made dry substances, sealed in ampoules, of “pure for analysis” grade.

All experiments were carried out in a flask with a ground-in reflux condenser and a mercury seal with a stirrer, filled with benzene.

Work-up of the Reaction Products

After the lapse of time (10 h), the reaction mixture was subjected to the treatment recommended in Houben’s manual: the unreacted bromine was neutralized with 10% alkali. It was then washed several times with distilled water. The reaction mixture, dried over calcium chloride, was twice fractionated; the second time a dephlegmator was used.

In the first distillation, 3 fractions were collected: fraction 1 at 80–83°; fraction 2 at 83–95°; fraction 3 at 140–160°.

Fractions 2 and 3 were subjected to a second fractionation with a dephlegmator, as a result of which the reaction mixture was separated into 2 fractions: fraction 1, \(t^\circ = 80\text{–}83^\circ\) (benzene); 2—155–157° (bromobenzene).

The residue in the flask, in all experiments not exceeding 1.1 g, is a mixture of \(o\)- and \(p\)-dibromobenzenes. Separation of the isomers was carried out by means of a clay plate. The still-hot mixture of products was poured onto the plate; liquid \(o\)-dibromobenzene was absorbed by the pores of the plate, while crystalline \(p\)-dibromobenzene remained fairly pure on the surface. The percent yield of bromination products was calculated on bromine.

The bromobenzene obtained from all experiments was distilled once more with a dephlegmator.

Amount of bromobenzene = 30 ml,
fraction 1 up to 156° = 2 ml,
fraction 2 at 156° = 28 ml,
\(\mathrm{C_6H_5Br}\)% = 96.6.

In the process of working up and fractionating the reaction mixture, losses are inevitable. To determine the percentage of losses during work-up and fractionation, we carried out the following experiment: 10 ml of benzene with \(t^\circ_{\mathrm{b.p.}} = 80.5^\circ\) and 20 ml of bromobenzene with \(t^\circ_{\mathrm{b.p.}} = 156^\circ\) were mixed. The mixture was treated with alkali (10%) and several times with water. The mixture, dried over calcium chloride, was twice—

... were distilled, the first time simply from a Würtz flask, the second time using a dephlegmator. As a result, the mixture was distilled into 2 fractions:

1st fraction, 80–83°—benzene, 7.5 ml,
2nd fraction, 155–157°—bromobenzene, 18 ml.

As a result of the treatment and distillation of the mixture of benzene and bromobenzene, we lost 2 ml of bromobenzene out of the 20 ml taken, which amounts to about 10%.

Experiment 1. Bromination of benzene in the presence of aluminum bromide.
Starting substances taken:

benzene . . . . . . . . . . 35 ml,
bromine . . . . . . . . . . 10.1 ml,
aluminum bromide . . . 0.53 g.

Reaction conditions: \(t^\circ = 45^\circ\); time—10 h.
Bromination products obtained:
bromobenzene—18.65 g with b.p. \(t^\circ = 155\text{–}157^\circ\), which amounts to 56.2%.

No dibromo derivatives were detected.

Experiment 2. Bromination of benzene in the presence of ferric bromide.
Starting substances taken:

benzene . . . . . . . . . . 35 ml,
bromine . . . . . . . . . . 10.1 ml,
ferric bromide . . . . . 0.59 g.

Reaction conditions: \(t^\circ = 45^\circ\); time—10 h.
Bromination products obtained:
bromobenzene—23.51 g with b.p. \(t^\circ = 155\text{–}157^\circ\), which amounts to 74.88%;
\(o\)-dibromobenzene—0.73 g, or 3.0%,
\(p\)-dibromobenzene—0.18 g, or 0.76%.

Experiment 3. Bromination of benzene in the presence of antimony tribromide.
Starting substances taken:

benzene . . . . . . . . . . 35 ml,
bromine . . . . . . . . . . 10.1 ml,
antimony tribromide . . 0.728 g.

Conditions: \(t^\circ = 45^\circ\); time—10 h.
Bromination products obtained:

bromobenzene—4.94 g, or 15.7%.

No dibromo derivatives were detected.

Experiment 4. Bromination of benzene in the presence of cupric bromide.
Starting substances taken:

benzene . . . . . . . . . . 35 ml,
bromine . . . . . . . . . . 10 ml,
cupric bromide . . . . . 0.44 g.

Conditions: \(t^\circ = 45^\circ\); time—10 h.
Bromination products obtained:
bromobenzene—3.67 g, which amounts to 11.6%. No dibromo derivatives were detected.

Experiment 5. Bromination of benzene in the presence of sulfur bromide.
Starting materials taken:

The reaction conditions were the same as in the preceding experiments.
Bromination products obtained:

bromobenzene — 3.64 g, which is 11.5%.

Experiment 6. Bromination of benzene in the presence of tribromide.
Starting materials taken:

The reaction conditions were the same.
Bromination products obtained:
bromobenzene 9.29 g or 29.5%. Dibromobenzenes were not detected.

Experiment 7. Bromination of benzene in the presence of arsenic bromide.
Starting materials taken:

The reaction was carried out under the previous conditions: \(t^\circ = 45^\circ\); time — 10 h.
Bromobenzene obtained — 3.98 g — 12.6%.
No dibromo derivatives were detected.

Experiment 8. Bromination of benzene in the presence of phosphorus pentabromide.
Starting materials taken:

The reaction was carried out under the above-described conditions.
Bromobenzene obtained 2.2 g or 7.2%.
No dibromination products were detected.

Experiment 9. Bromination of benzene in the presence of zinc bromide.
Starting materials taken:

Reaction conditions:

\[ t^\circ = 45^\circ;\ \text{time — 10 h.} \]

Bromination products obtained:
bromobenzene with b.p. \(155\text{–}157^\circ\) — 14.8 g, which is 47.1%;

\(p\)-dibromobenzene with m.p. \(=\) 0.12 g, which is 0.5%;
\(o\)-dibromobenzene 0.53 g or 2.2%.

Experiment 10. Bromination of benzene without catalysts.
Initial substances taken:

benzene . . . . . . . . . . 35 ml,
bromine . . . . . . . . . . 10.1 ml.

Conditions: \(t^\circ = 45^\circ\); time — 10 h.
Obtained bromobenzene — 2.27 g, or 7.2%.

Discussion of the Results

The ready-made bromides used as catalysts for the bromination reaction of benzene are arranged in decreasing order of activity as follows:

\[ \mathrm{FeBr_3 > AlBr_3 > ZnBr_2 > IBr_3 > SbBr_3 > AsBr_3 > CuBr_2 > S_2Br_2 > PBr_5 >} \]

without catalyst.

Table 9

We have accepted the mechanism of the substitution reaction of the hydrogens of the benzene ring by halogen atoms proposed by B. V. Tronov: the catalyst is attracted by its central atom to one of the carbon atoms of the benzene ring; as a result, the carbon–hydrogen bond is loosened, and the latter is readily split off under the action of bromine.

\[ \begin{aligned} &\text{[benzene ring complex]} + \overset{+}{\mathrm{AlBr_3}}{}^{-} \;\rightleftarrows\; \text{[benzene ring complex bound to } \overset{+}{\mathrm{AlBr_3}}{}^{-}\text{]} \\ \\ &\text{[benzene ring complex bound to } \overset{+}{\mathrm{AlBr_3}}{}^{-}\text{]} + \mathrm{Br_2} \longrightarrow \text{[phenyl–Br–AlBr}_3\text{ complex]} + \mathrm{HBr} \\ \\ &\text{[phenyl–Br–AlBr}_3\text{ complex]} \longrightarrow \text{[bromobenzene–AlBr}_3\text{ complex]} \end{aligned} \]

The above-described mechanism is applicable to a reaction occurring only in a homogeneous medium. Consequently, the bromides used in the bromination reaction of benzene must dissolve in benzene.

Ferric bromide, aluminum bromide, iodine tribromide, antimony tribromide, arsenic bromide, sulfur bromide, and phosphorus pentabromide dissolve well in benzene.

Zinc and copper bromides do not dissolve in benzene. The solubility of zinc and copper bromides was checked experimentally. A weighed portion of zinc bromide and cupric bromide was thoroughly shaken from time to time for 12 hours, after which the benzene was distilled off. The benzene distilled off to dryness; no residue was detected either from \(ZnBr_2\) or from \(CuBr_2\).

In addition, we brominated benzene that had been shaken with \(ZnBr_2\). Bromination was carried out under the previous conditions: \(t^\circ = 45^\circ\); time—10 hr.; \(C_6H_6 : Br_2 = 2 : 1\). A yield of 2.2 g of bromobenzene was obtained, which amounts to 70%.

The yield of bromobenzene is the same as in the bromination of benzene without a catalyst.

Consequently, when zinc and copper bromides are used as bromination catalysts, heterogeneous catalysis takes place. This, apparently, should also explain a certain discrepancy in the series of relative activity of simple substances and of the corresponding bromides.

The relative activity of simple substances in the bromination of benzene is as follows:

iron \(>\) zinc \(>\) bismuth \(>\) mercury \(>\) tellurium \(>\) copper \(>\) cadmium \(>\) lead \(>\) magnesium \(>\) molybdenum \(>\) iodine \(>\) iodobenzene \(>\) cobalt \(>\) manganese \(>\) aluminum \(>\) antimony \(>\) tin \(>\) nickel \(>\) arsenic \(>\) phosphorus \(>\) sulfur \(>\) selenium \(>\) tungsten \(>\) calcium chloride \(>\) without catalysts, silver, chromium, silicon tetrachloride \(>\) acetic acid \(>\) nitrobenzene \(>\) water \(>\) dinitrobenzene \(>\) picric acid.

The prepared bromides are arranged in order of decreasing activity as follows:

\[ FeBr_3 > AlBr_3 > ZnBr_2 > IBr_3 > SbBr_3 > AsBr_3 > CuBr_2 > S_2Br_2 > PBr_5. \]

Zinc bromide is less active than zinc; cupric bromide is considerably weaker than copper.

The different rate of formation of bromides is also significant when simple substances are used as catalysts. Aluminum, zinc, manganese, sulfur, selenium, tellurium, phosphorus, arsenic, antimony, bismuth, magnesium, mercury, cadmium, and copper combine with bromine fairly readily at ordinary temperature. Iron, cobalt, and nickel react with bromine with greater difficulty. Molybdenum and chromium interact with bromine only on heating. Tungsten does not react with bromine according to some literature sources and does react on heating according to others.

There are indications in the literature of heterogeneous catalysis in the bromination of aromatic compounds. Thus, Gay Duceller et al. [16] recommend using finely powdered manganese in the bromination of benzene, but not its bromide.

N. N. Vorozhtsov recommends aluminum oxide as a catalyst in the chlorination of benzene. Catalysis occurs not in solution, but in a heterogeneous system.

LITERATURE

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