GB2132197A - Process for the chlorination of aromatic compounds - Google Patents

Abstract

Process for preparing a parachlorinated aromatic compound which process comprises, in a first step, chlorinating an aromatic compound having an ortho-directing and para-directing substituent, the chlorination being performed in the liquid phase using ferric chloride and, in a second step, regenerating ferric chloride by chlorinating the ferrous chloride formed in the first step, wherein the two steps are carried out in the presence of the same solvent, said solvent having a melting point below 50 DEG C and a boiling point above 200 DEG C.

Description

SPECIFICATION Process for the chlorination of aromatic compounds The present invention relates to a process for the chlorination of aromatic compounds having an ortho- and para- directing substituent so as to produce para-chlorinated aromatic compounds.

Aromatic compounds chlorinated in the para position, such as parachlorotoluene, are known, for example, as chemical intermediates in the production of pesticides, dyes or pharmaceutical products. They may also be used directly without further transformation, e.g. paradichlorobenzene may be used as a deodorant.

The chlorination of aromatic compounds having an ortho-directing or para-directing substituent is usually effected in the liquid phase in the presence of ferric chloride as the chlorinating agent. This technique, which was described for the chlorination of toluene or monochlorobenzene in J. Gen. Chem. USSR 8, 1780, 1938 - C.A. 33 4037, 1939, or in J.A.C.S. 1954-5, 5491-94, has the disadvantage of yielding reaction products with relatively low para/ortho isomer ratios.

In order to increase the para/ortho ratio, it has been proposed (J. Org. Chem. 1961, 26, 214-17) that ferric chloride be combined with another metal chloride such as TiCI4 or AICI3.

Finally, according to German Offenlegungsschrift No. 2,230,369, aromatic compounds chlorinated in the para position and more particularly parachlorotoluene are prepared by chlorination of toluene with FeCI3, catalysed by AICI3, using trichlorobenzene as the reaction solvent. Then the ferrous chloride has to be separated off by filtration, washed scrupulously with carbon tetrachloride to ensure that no traces of trichlorobenzene remain, which would be a nuisance in the subsequent regeneration step as it would lead to the formation of heavy products. The regeneration of ferrous chloride is effected by suspending it in carbon tetrachloride and then subjecting it to chlorination.The ferric chloride formed then has to be filtered again, and then washed scrupulously with trichlorobenzene to eliminate the carbon tetrachloride which would be liable to react with the toluene in a Friedel-Crafts reaction catalysed by FeCI3.

This process has the disadvantage of having a complex cycle caused by the need to use two solvents which have to be removed and recovered on each occasion.

According to the invention, this disadvantage is overcome by effecting chlorination, in the liquid phase, of aromatic compounds having an ortho-directing and para-directing substituent, using ferric chloride in the presence of a single solvent in the chlorination step for the aromatic compound and in the step of regeneration of the ferrous chloride formed by chlorination.

Accordingly, the present invention provides a para-chlorinated aromatic compound which process comprises, in a first step, chlorinating an aromatic compound having an ortho-directing and para-directing substituent, the chlorination being performed in the liquid phase using ferric chloride and, in a second step, regenerating ferric chloride by chlorinating the ferrous chloride formed in the first step, wherein the two steps are carried out in the presence of the same solvent, said solvent having a melting point below 50"C and a boiling point above 200"C.

The process of the present invention is particularly useful for the chlorination of toluene and monochlorobenzene with a view to producing parachlorotoluene and paradichlorobenzene.

The solvent used, which must of course be inert to the reactants, is preferably a halogenated, chlorinated or fluorinated solvent, wih a melting point below 50"C and a boiling point above 200"C. As a guide, mention may be made of solvents such as fluorinated oils satisfying the above criteria; examples of suitable solvents are hexachlorobutadiene and hexachlorocyclopentadiene.

A solvent of this kind is particularly useful in the chlorination of toluene and monochlorobenzene since, apart from the resultant simplification of the two reaction steps, the fact that the solvent has a higher boiling point than the monochlorotoluenes and dichlorobenzenes makes it.

possible to effect separation of the reaction products without first having to distil off the solvent.

In this case, the products of the synthesis are recovered first by distillation, and the solvent which remains behind can be recycled directly numerous times without purification after regeneration of the ferric chloride in situ.

As an illustration, since this type of synthesis is known per se and is described in numerous publications and patents such as French Patent No. 2,144,709, in a first step the aromatic compound having an ortho-directing and para-directing substituent is reacted with ferric chloride in the presence of the solvent according to the invention. The chloroaryl compounds formed are recovered by fractional distillation, and any unreacted aromatic compound is recycled. In a second step, the ferrous chloride formed, dispersed in the solvent which is the distillation residue, is regenerated in situ by chlorination to form ferric chloride, and the final combination of solvent and ferric chloride is recycled. The flow chart shown in the accompanying drawing illustrates diagrammatically the complete reaction cycle in the case of the chlorination of toluene or monochlorobenzene.

The ferric chloride may, without any disadvantage and without affecting the reaction cycle, be combined with a known catalyst selected from metal chlorides, such as AICI3, TaCI5, TiCI4, MoCI4, HfCI4, WCI6 and GaCI3.

The following Examples illustrate the invention. The yields are given in percentages by weight.

EXAMPLE 1 700 g of hexachlorobutadiene, 2 mol of anhydrous ferric chloride and 30 9 of TiCI4, brought into contact in a reactor and heated to 80"C. At this same temperature, 1.3 mol of toluene are introduced in 30 minutes. The mixture is then kept at 90-110"C for about 2 hours or until the evolution of hydrochloric acid has ended. The mixture is degassed for 30 minutes by means of a current of nitrogen at a flow rate of 30 I/hour. The apparatus is then subjected to a vacuum of 30 mm of mercury so as to separate off, by distillation, the unreacted toluene and the monochlorotoluenes formed. The tiCI4 is collected with the toluene fraction and is recycled to the next reaction.

After distillation has ended, the vacuum is released and 1.25 mol of chlorine are introduced in 1 hour 1 5 minutes at a temperature of 110-125"C. A second cycle is started by introducing the recycled toluene and making it up to 1.3 mol with fresh toluene. After 7 cycles the results obtained are as follows: Yield of monochloro- Yield of monochloro- para + meta toluenes based on toluenes based on ortho the total iron toluene consumed 82 85 5.7 80 76 9 76 79 11 74 80 13 72 78 14 71 75 14.7 68 71 14.85 The precise assessment of the operations is obtained by weighing the fractions and analysing them by gas chromatography.

The average meta content of the monochlorotoluenes is 2% EXAMPLE 2 The same conditions are used as in Example 1, except that the TiCI4 is replaced by 5 g of aluminium chloride. The results after three cycles are as follows: Yield of monochloro- Yield of monochloro- para + meta toluenes based on toluenes based on ortho the total iron present toluene consumed 68.6 69 6.45 52.8 75 10 66 91 11.7 EXAMPLE 3 The same conditions are used in Example 1, except that 5 g of tantalum chloride (TaCI5) are used instead of TiCI4.The results after four cycles are as follows: Yield of monochloro- Yield of monochloro- para + meta toluenes based on toluenes based on ortho the total iron present toluene consumed 91 94 7.3 85 84 13.1 84.4 75.8 14.6 85.6 76.4 17.6 EXAMPLE 4 The same conditions are used as in Example 1, but the toluene is replaced by monochlorobenzene and the reaction is carried out at 100-125"C. The results obtained after six cycles are as follows: Yield of dichloro- Yield of dichloro- para + meta benzenes based on benzenes based on the total iron present the monochlorobenzene consumed 95 96 27.7 86 93.5 22.2 96 99 31.2 94 98.5 22.2 97 94 24.5 96 96.5 25.7 The composition of the dichlorobenzenes formed is in each case approximately o/m/p = 3.65/0.15/96.2.

EXAMPLE 5 50g of hexachlorocyclopentadiene, 0.2 mol of anhydrous ferric chloride and 10 9 of TiC14, are brought into contact in a reactor and heated at 120"C for 3 hours in the presence of 0.2 mol of toluene.

At the end of the reaction, the monochlorotoluenes are recovered by distillation to give a yield of 75 based on the FeCI3 introduced and with a para ortho ratio of 7.

After regeneration of the FeCI3 a new reaction cycle is started.

EXAMPLE 6 Example 5 is repeated, except that the hexachlorocyclopentadiene is replaced by a fluorinated oil with a boiling point of 256"C and a freezing point of 45"C (Voltalef 3 S).

At the end of the reaction, the monochlorotoluenes are recovered in a yield of 95 based on the FeCI3 introduced and with a para ortho ratio of 8.5.

After regeneration of FeCI3 a new reaction cycle is started.

Claims (10)

1. Process for preparing a para-chlorinated aromatic compound which process comprises, in a first step, chlorinating an aromatic compound having an ortho-directing and para-directing substituent, the chlorination being performed in the liquid phase using ferric chloride and, in a second step, regenerating ferric chloride by chlorinating the ferrous chloride formed in the first step, wherein the two steps are carried out in the presence of the same solvent, said solvent having a melting point below 50"C and a boiling point above 200"C.

2. Process as claimed in Claim 1, wherein the solvent is a halogenated compound.

3. Process as claimed in Claim 2, wherein the solvent is a fluorinated or chlorinated compound.

4. Process as claimed in Claim 3, wherein the solvent is hexachlorobutadiene.

5. Process as claimed in any one of the preceding Claims, wherein in the second step the ferric chloride is regenerated in situ in the solvent of the first step.

6. Process as claimed in any one of the preceding Claims, wherein parachlorotoluene is prepared from toluene.

7. Process as claimed in any one of Claims 1 to 5, wherein paradichlorobenzene is prepared from monochlorobenzene.

8. Process for preparing a para-chlorinated aromatic compound substantially as described in any one of the foregoing Examples.

9. Process for preparing a para-chlorinated aromatic compound substantially as hereinbefore described with reference to the accompanying drawing.

10. A para-chlorinated aromatic compound whenever prepared by a process as claimed in any one of the preceding claims.