Direction and Rate of Toluene Bromination in the Presence of Various Catalysts
Abstract Generated abstract
This study examines how various additives affect both the rate and direction of toluene bromination under comparable conditions. Bromine consumption was followed iodometrically for 19 catalysts, and reaction products were analyzed chemically to estimate the proportions of nuclear bromination products and benzyl bromide. Zinc, iron, tellurium, bismuth, cadmium, and iodine chiefly promoted substitution in the aromatic ring, while water and dinitrobenzene favored side-chain bromination. The results also indicate that weaker catalysts, and reactions without catalyst, can yield substantial benzyl bromide, especially under conditions with a large excess of toluene.
Full Text
Direction and Rate of Toluene Bromination in the Presence of Various Catalysts
L. A. Pershina
The direction of the halogenation reaction of benzene homologues depends on the conditions under which it proceeds.
A new development in this area is the use of catalysts for introducing a halogen into the aliphatic side chain [2].
In the literature it is recommended that pyridine and dibenzylamine be used in chlorination of the side chain of ethylbenzene [1]. Phosphorus chlorides and bromides also accelerate substitution in the side chain.
The aim of our work was to determine the influence of various additives on the direction and rate of the bromination reaction of toluene; some of them, according to literature data or preliminary considerations, were expected to promote bromination of the benzene nucleus, while others were expected to promote bromination of the side chain.
As catalysts accelerating bromination of the benzene ring, we used the following: copper, magnesium, zinc, cadmium, mercury, aluminum, tin, lead, antimony, bismuth, tellurium, sulfur, iodine, iron, C₆H₅I, HBr; as catalysts for bromination of the side chain: water, acetic acid, dinitrobenzene.
Experimental Part
The experimental part of our work consists of 2 sections:
- In the first section, a study was carried out of the relative activity of 19 catalysts in the bromination of toluene.
- In the second section, we studied the products of the bromination reaction of toluene.
Determination of the Relative Activity of the Catalysts
In all experiments, bromine was taken in relation to toluene as 1:50. The catalysts were taken in an amount of 0.01 mole per molecule of bromine. The experiments were carried out at \(t = 18—20^\circ\). The reaction rate was monitored by iodometric determination of unreacted bromine [3]. The results of the experiments are given in Table 1.
The simple substances taken as catalysts, which, of course, reacted in the form of bromides, as well as some organic substances, are arranged in the following order of decreasing reaction rate in the bromination of toluene: iron \(>\) zinc \(>\) bismuth \(>\) mercury \(>\) tellurium \(>\) cadmium \(>\) tin \(>\) aluminum \(>\) iodobenzene \(>\) hydrogen bromide \(>\) antimony \(>\) lead \(>\) copper \(>\) sulfur \(>\) without catalyst \(>\) dinitrobenzene, water.
Bromination of toluene in the presence of catalysts promoting substitution in the benzene nucleus proceeds by the same mechanism as in the case of bromination of benzene [3].
Table 1
BROMINATION OF TOLUENE IN THE PRESENCE OF CATALYSTS
(in percent)
| No. | Duration of experiments | Cu | Mg | Zn | Cd | Hg | Al | Sn | Pb | Sb | Bi | Te | S | I | Fe | H₂O | C₆H₄(NO₂)₂ | C₆H₅I | HBr | Without catalyst |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 30 minutes | 0% | 0 | 2.9 | 1.8 | 1.7 | 7.1 | 0.9 | 0.6 | 5.7 | 0 | 0 | 0 | 0 | 2.9 | 0 | 0 | 0 | 0 | 0 |
| 2 | 1 hour | — | 2.9 | 32 | 5.8 | 8 | — | — | — | — | 14.3 | 1.2 | 2.3 | 1.7 | 32 | 0 | 0 | 0 | 0 | 0 |
| 3 | 2 hours | 9.2 | — | 79 | — | — | 17.7 | 22.9 | 13.2 | 15.5 | 72 | 10.9 | — | — | — | 0 | 1.8 | 1.8 | 18.0 | 0 |
| 4 | 3 hours | — | — | 70.5 | — | — | — | — | — | — | 90.3 | 40.6 | — | — | 90.3 | — | — | — | — | — |
| 5 | 5 hours | 26.3 | 40.3 | 80.6 | 46 | 76.6 | 27.5 | 33.8 | 28 | 18.9 | — | 68.6 | 24.6 | 42.3 | 100 | 8 | 5.8 | 8.6 | 21.2 | 2.9 |
| 6 | 9 hours | — | — | 100 | 63.8 | 100 | 29.2 | — | — | — | 100 | 98.9 | — | — | 8 | — | 9.6 | — | — | |
| 7 | 1 day | 26.3 | 61.2 | — | 89.2 | 42.9 | 55.5 | 40 | 46.3 | 100 | 45.8 | 77.2 | 14.8 | 14.9 | 25.8 | 33.2 | 14.9 | |||
| 8 | 3 days | 45.8 | 62.9 | 98.9 | 66.9 | 94.9 | 52.3 | 74.9 | 61.2 | 88 | 25.8 | 26.9 | 31 | 44.5 | 25.8 | |||||
| 9 | 4 days | 60.6 | 65.5 | 100 | 86.9 | 100 | 57.2 | 82.9 | 67.6 | 88.8 | 42.9 | 42.9 | 61.8 | 61.8 | 42.9 | |||||
| 10 | 7 days | 91.5 | 67.6 | 97.2 | 84.6 | 90.3 | 81.2 | 88.8 | 64.6 | 61.8 | 98 | 97.2 | 61.6 | |||||||
| 11 | 10 days | 94.3 | 91.5 | 100 | 94.3 | 96.6 | 91.5 | 88.8 | 87.2 | 81.8 | 100 | 100 | 82.2 | |||||||
| 12 | 11 days | 98.3 | 100 | 100 | 100 | 97.2 | 88.8 | 88.6 | 87.2 | 84.6 | ||||||||||
| 13 | 12 days | 100 | 100 | 89.2 | 90.0 | 89.2 | ||||||||||||||
| 14 | 13 days | 100 | 100 | 100 |
Study of the Reaction Products
The reaction mixture was treated in the following sequence: with water, 10% alkali, water to neutral reaction, dried with calcium chloride, then subjected to distillation with a dephlegmator; the following fractions were collected:
I fraction: \(t = 110—111^\circ\)—toluene
II fraction: \(t = 180—200^\circ\).
The second fraction represents the product of bromination of toluene: para- and ortho-bromotoluenes and benzyl bromide. Since the quantities of these products were small (\(\sim 2\) g), we could not simply distill them apart, but determined them chemically. For this purpose we used the procedure described in Gattermann’s manual [4].
When filtering the liquid with the precipitate, we used glass filters No. 3 and 4, on which the precipitate of silver bromide was dried at \(100^\circ\) for 5 hours. Separation of the ortho and para isomers of the bromotoluenes was not carried out.
The results of the study of the products of toluene bromination are given in Table 2.
Table 2
Products of toluene bromination
| No. | Catalyst | Bromotoluenes (percent) | Benzyl bromide (percent) |
|---|---|---|---|
| 1 | Zinc | 97.0 | 3.0 |
| 2 | Iron | 95.9 | 4.1 |
| 3 | Tellurium | 95.8 | 4.2 |
| 4 | Bismuth | 95.4 | 4.6 |
| 5 | Cadmium | 95.0 | 5 |
| 6 | Iodine | 93.4 | 6.6 |
| 7 | Tin | 86.0 | 14.0 |
| 8 | Copper | 85.0 | 15.0 |
| 9 | Antimony | 84.0 | 16.0 |
| 10 | Aluminum | 67.0 | 33.0 |
| 11 | Lead | 56.4 | 43.6 |
| 12 | Mercury | 56.2 | 43.8 |
| 13 | Iodobenzene | 54.4 | 45.6 |
| 14 | Hydrogen bromide | 49.0 | 51.0 |
| 15 | Without catalyst | 44.0 | 56.0 |
| 16 | Magnesium | 43.0 | 57.0 |
| 17 | Dinitrobenzene | 25.0 | 75.0 |
| 18 | Water | Traces | 100.0 |
Discussion of the Results
Bromination in the presence of zinc proceeded almost entirely in the benzene nucleus. Strong catalysts for substitution in the nucleus are iron, tellurium, bismuth, cadmium, and iodine.
Tin, copper, antimony, lead, and mercury acted more weakly, i.e., elements with variable valence. These elements give bromides of two types:
\[ \mathrm{SnBr_2 \ and \ SnBr_4;} \qquad \mathrm{CuBr \ and \ CuBr_2;} \]
\[ \mathrm{SbBr_3 \ and \ SbBr_5;} \qquad \mathrm{Hg_2Br_2 \ and \ HgBr_2.} \]
As an example, let us consider the action of mercury bromides on the toluene bromination reaction.
The presence of \(\mathrm{Hg_2Br_2}\) catalyzes the substitution reaction of the hydrogens of the benzene nucleus according to the scheme:
\[ \begin{aligned} &\text{toluene} + \mathrm{Hg_2Br_2} \longrightarrow \text{addition complex of toluene with } \mathrm{Hg_2Br_2},\\ &\text{addition complex} + \mathrm{Br_2} \longrightarrow \text{bromotoluene complex with } \mathrm{Hg_2Br_2} + \mathrm{HBr},\\ &\text{bromotoluene complex with } \mathrm{Hg_2Br_2} \longrightarrow \text{bromotoluene} + \mathrm{Hg_2Br_2}. \end{aligned} \]
In the presence of \(\mathrm{HgBr_2}\).
\[ \mathrm{C_6H_5CH_3 + HgBr_2 \longrightarrow C_6H_5{-}CH_2 + HgBr + HBr.} \]
\[ (2\mathrm{HgBr} \longrightarrow \mathrm{Hg_2Br_2}) \]
\[ \mathrm{C_6H_5CH_2 + HgBr_2 \longrightarrow C_6H_5CH_2Br + HgBr} \]
\[ \mathrm{C_6H_5CH_3 + Br_2 \longrightarrow C_6H_5CH_2Br + Br} \]
\[ \mathrm{C_6H_5CH_3 + Br \longrightarrow C_6H_5CH_2 + HBr} \quad \text{etc.} \]
Dinitrobenzene and water promoted substitution of the hydrogens in the side chain of toluene.
Bromination of toluene without catalysts gives 56% of the product of substitution of hydrogens in the side chain.
In the presence of aluminum, iodobenzene, hydrogen bromide, and magnesium, a considerable amount of benzyl bromide was obtained.
It should be noted that the greater the excess of the substance being brominated that has a side chain, in our experiments with toluene, the more products are obtained from substitution of the hydrogens of the side chain. And since the catalysts listed above are weak catalysts, even without their participation the reaction of substitution of the hydrogens of the side chain by bromine can proceed to a significant extent.
LITERATURE
- Smith, A. P. 2034962, chem. Abst. 30, 2985, 1936.
- Erdmann, Ann. 272, 150, 1893.
- Perlina L. A. Bromination of benzene in the presence of various catalysts. Izv. TPI, vol. 71, 1952.
- Gatterman. Practical Work in Organic Chemistry. Goskhimizdat, 1948.