Process for preparing polyfluoroaromatic compounds

Details Process for preparing polyfluoroaromatic compounds Process for preparing polyfluoroaromatic compounds

2000-10-18

2001-07-24

Siegel, Alan (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Halogen containing

Reexamination Certificate

active

06265627

ABSTRACT:

TECHNICAL FIELD
The present invention relates to the organofluorine chemistry and more particularly to a process for preparing polyfluoroorganic compounds which are used as heat carriers, dielectrics, solvents and stock materials for the synthesis of drugs, pesticides, oils, lubricants, etc, (Industrial Organofluorine Products. A Handbook. Leningrad, Khimiya, 1990, pp. 329, 333 (in Russian)).
BACKGROUND ART
Known in the art is a process for preparing fluoroaromatic compounds by reacting a chloroaromatic compound with potassium fluoride in an aprotic polar solvent (sulfolane) at a temperature of 210° C., as a catalyst use being made of isoalkylpyridinium salts of the formula
wherein R
1
and R
2
are monovalent or diva ent radicals selected independently from substituted on unsubstituted hydrocarbon radicals having from 1 to 13 carbon atoms or divalent alkylene radicals capable of forming a cycle containing from 4 to 8 carbon atoms, R
3
is a residue of the formula CH
2
C(R
5
)HR
4
, R
4
and R
5
=alkyl C
1
-C
8
, X=F, Cl, Br (U.S. Pat. No. 4,927,980, C07C 79/12, publ. May 22, 1990). The preferred catalyst is N-(2-ethylhexyl)-4-(N′,N′-dimethylamino)pyridinium chloride.
In the description of the process no data can be found on the possibility of preparing highly fluorinated compounds, such as hexafluorobenzene and pentaflorobenzene; examples are presented, illustrating the substitution of only one halogen atom in the aromatic compound. From 4-chloronitrobenzene a mixture is obtained which comprises 48% of 4-fluoronitrobenzene and 45% of 4-chloronitrobenzene. Conversion of 4-chloronitrobenzene is 45%.
The process is disadvantageous in a low yield of target products and using a solvent which must be separated from the reaction products and regenerated.
A process is known for preparing polyfluoroaromatic compounds of the formula C
6
Cl
n
Br
m
F
p
(n and m=0-6, p=0-5, n+m+p=6) by reacting a haloaromatic compound with an alkali metal fluoride in the presence of an aminophosphonium catalyst, preferably tetrakis(dialkylamino)phosphonium halides, which is used in an amount of 3-6 mol. % of the molar quantity of the starting haloaromatic compound (U.S. Pat. No. 5,624,827 C07C 25/13, publ. Oct. 20, 1998). The process is carried out in a reactor at a pressure of 5-7 kg/cm
2
without a catalyst at a temperature of 150-350° C. The total molar yield of hexafluorobenzene and pentafluorobenzene is 70-86% with the hexachlorobenzene conversion of 80-95%.
It is also proposed to carry out the reaction in the liquid phase, for instance, in an aprotic polar solvent or in haloaromatic compounds—dichlorotetrafluorobenzene, trifluorotrichlorobenzene and others, which are in the liquid state at the reaction temperature. In these particular embodiments of the invention with the use of solvents particular examples of preparing fluoroaromatic compounds are not presented.
The process is disadvantageous in the use of a reactor under a pressure, in obtaining a mixture of polyfluorobenzenes instead of one target product, as well as in using catalysts whose synthesis is technologically labor-intensive.
The most relevant technical solution is a process for preparing hexafluoro- and pentafluorochlorobenzenes, which consists in treating hexafluorobenzene with potassium fluoride in sulfolane in the presence of a catalyst, 18-crown-6-macrocyclic polyether. 0.59 mole of the catalyst is used per mole of hexafluorobenzene. The reaction is carried out at a temperature of 180-200° C. for 24 hours. The resulting mixture contains 23-24% of C
6
F
6
, 44-45% of C
6
F
5
Cl and 30-31% of C
6
F
4
Cl
2
. The total yield of hexafluorobenzene and chloropentafluorobenzene as calculated for the charged hexafluorobenzene is 45% (RF Inventor's Certificate No. 676864, C07C
25/13,
publ. Apr. 30, 1994).
The process is disadvantageous in the use of a costly catalyst, in the process being periodic, and in obtaining a mixture of polyfluorobenzenes with a low yield of target products.
DISCLOSURE OF THE INVENTION
An object of the invention is to increase the yield of target products and to simplify the technological process.
Said object is accomplished by heating halogen-containing aromatic compounds with alkali metal fluorides in the presence of a catalyst, N,N′,N″-hexa-substituted guanidinium halide, in the medium of products of incomplete fluorination of the starting halogen-containing compound with simultaneous withdrawal of target products by fractionation or distillation.
As the catalyst use is made of N,N′,N″-hexa-substituted guanidinium halide of the formula
[(RR
1
N)
2
CNR
2
R
3
]
+
X
−
wherein R—R
3
=alkyl C
1
-C
7
, cycloalkyl C
5
-C
8
, aralkyl C
7
-C
12
; R and R
1
and/or R
2
and R
3
together with the nitrogen atom can form a heterocyclic residue, possibly containing one more heteroatom—oxygen or nitrogen; X=Cl, F, Br, I. Said guanidinium derivatives are described, for instance, in DE 2718275 C07C
129/12.
PREFERRED EMBODIMENTS OF THE INVENTION
It is preferable to use hexaethylguanidium chloride.
The amount of the catalyst in the reaction is 1-5% by weight of the starting mixture of aromatic compounds. A decrease in the quantity of the catalyst leads to a reduction in the yield of target products, whereas an increase in its quantity over 5% does not produce an essential effect on the process.
The reaction is carried out in the medium of products of incomplete fluorination of starting halogen-containing compounds which are initially charged into a reactor in an amount of 20-50% by weight of the starting mixture of haloaromatic compounds. Henceforth these compounds are constantly present in the process of fluorination as reaction products, and during the in-reactor fractionation they return by gravity into the reactor. This enables one to control the reaction temperature and to make the process continuous.
The synthesis is effected in a reactor provided with a stirrer, the reactor being simultaneously the still of a fractionating column. The process of fluorination is carried out with simultaneous fractionation and withdrawal of the target product found in the reaction zone. When obtaining fluorochloroaromatic compounds whose boiling point is higher than 160° C., withdrawal of the target products is achieved by continuous distillation from the bottom part of the column.
As starting substances for the reaction it is possible to use any aromatic compound which has at least one substitutable halogen atom, except fluorine, in the aromatic ring, e.g., hexachlorobenzene, pentachlorobenzonitrile, trifluoromethyl pentachlorobenzene, etc.
The process of fluorination is carried out with an alkali or alkaline-earth metal fluoride, preferably potassium fluoride in a 1.2-1.4-fold excess from the stoichiometry.
In accordance with the present process there have been prepared, in particular, hexafluorobenzene with the yield of 88.4%, octafluorotoluene with the yield of 92.3%, and other polyfluoroaromatic compounds with a yield not lower than 89%.
The products were analyzed by GLC and NMR-spectroscopy techniques.
The developed process makes it possible to simplify the process technology by excluding the step of purifying the reaction products from the solvent and of regenerating the solvent itself, as well as provides an opportunity for obtaining individual target products (instead of a mixture of polyfluorobenzenes) with a high yield.
The distinctive features of the proposed process are as follows: carrying out the process in the presence of said catalyst in the medium of products of incomplete fluorination of the starting halogen-containing compound with simultaneous continuous withdrawal of target products.
This combination of distinctive features has not been revealed in other technical solutions.


REFERENCES:
patent: 4684734 (1987-08-01), Kaieda et al.
patent: 4927980 (1990-05-01), Cantrell
patent: 5824827 (1998-10-01), Bildinova et al.
patent: 2718275 (1978-11-01), None
patent: 0781747 (1997-07-01), None
patent: 2084

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