Process for the preparation of 4-bromophthalic anhydride

A process for the production of 4-bromophthalic anhydride which comprises the reaction of 4-chlorotetrahydrophthalic anhydride with elemental bromine, in the presence of a catalytically effective amount of iron or an iron salt is disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to a process for the preparation of 4-bromophthalic 
anhydride by reacting 4-chloro tetrahydrophthalic anhydride with bromine 
in the presence of iron. 4-Bromophthalic anhydride is useful as an 
intermediate for the preparation of various commercial products including 
polymers, dyes and plasticizers. It is particularly useful in the 
preparation of dianhydride monomers such as oxydiphthalic dianhydride 
which may be polymerized with a suitable diamine to form a polyimide. 
4-Bromophthalic anhydride has been prepared by the reaction of phthalic 
anhydride, in aqueous alkali, with bromine, and subsequent acidification 
and dehydration (E. T. Sabourin, et al., J. Org. Chem., Vol. 48, 5137 
(1983)). Our process produces 4-bromophthalic anhydride in better yields 
than we have been able to achieve using the method reported by Sabourin, 
et al. 
4-Chloro tetrahydrophthalic anhydride may be prepared by the condensation 
of 2-chloro-1,3-butadiene with maleic anhydride. This reaction produces 
mostly 4-chlorotetrahydrophthalic anhydride with the double bond in the 4 
position. A small percentage of product has the double bond in the 3 
position. The presence of two isomers has no apparent effect upon the 
reactions which are the subject of this invention. 
The dehydrogenation of tetrahydrophthalic anhydrides to yield phthalic 
anhydrides has been observed. For example, Bergmann, J. Amer. Chem. Soc. 
64, 176 (1942) discloses the aromatization of tetrahydrophthalic anhydride 
products of Diels-Alder reactions. The author discloses that 
dehydrogenation occurred when the tetrahydrophthalic anhydride product is 
boiled in nitrobenzene. However, it is further disclosed that 
dehydrogenation does not occur when p-bromonitrobenzene, 
p-chloronitrobenzene, or m-dinitrobenzene in xylene is employed. 
U.S. Pat. No. 4,560,772 to Telschow discloses the reaction of 
4-methyltetrahydrophthalic anhydride with excess sulfur and a catalytic 
amount of zinc oxide and 2-mercaptobenzothiazole to produce 
4-methylphthalic anhydride and hydrogen sulfide. 
U.S. Pat. No. 4,560,773 to Telschow discloses a similar reaction between 
the electron rich 4-methyltetrahydrophthalic anhydride and bromine in the 
presence of a catalytic amount of an acid acceptor such as 
dimethylformamide or pyridine in the liquid phase. 
Skvarchenko et al., Obshchei Khimii, Vol. 30, No. 11. pp. 3535-3541 
disclose the aromatization of chloro-substituted tetrahydrophthalic 
anhydride by heating with phosphorus pentoxide. In the aromatization 
process described, however, decarboxylation also occurs with the formation 
of the corresponding chloro-substituted benzene compound. The preparation 
of various other tetrahydrophthalic acids and anhydrides and various 
methods for dehydrogenation and aromatization thereof are reviewed by 
Skvarchenko in Russian Chemical Review, No. 1963, pp. 571-589. 
A co-pending application, Ser. No. 07/405,606, discloses that 
4-chlorotetrahydrophthalic anhydride reacts with elemental bromine to form 
4-chlorophthalic anhydride. 4-Bromophthalic anhydride is sometimes found 
as a side product in low percentage, but is not economically recoverable. 
Iron catalyzes the bromination of aromatic molecules (ORGANIC CHEMISTRY, 
Louis F. Fieser and Mary Fieser, 3rd Edition, D. C. Heath and Company, 
Boston 1956, pages 644-645 and FUNDAMENTALS OF ORGANIC CHEMISTRY, 2nd 
Edition, T. W. Graham Solomons, John Wiley & Sons, New York 1986, pages 
445-446 and 455). For example, benzene reacts with bromine, in the 
presence of iron, to form bromobenzene. Chlorobenzene, under similar 
conditions, forms bromochlorobenzene. Halogens and carboxyl groups, when 
substituted on a benzene ring, tend to de-activate the ring to further 
substitution. (Solomons, p. 455). The presence of three de-activating 
groups on the ring would be expected to cause ring bromination to be slow. 
In fact, we have found that under the reaction conditions used for the 
aromatization of 4-chlorotetrahydrophthalic anhydride, 4-chlorophthalic 
anhydride does not react with bromine in the presence of iron. 
SUMMARY OF THE INVENTION 
It has now been found that surprisingly, a mixture of 4-chlorophthalic 
anhydride and 4-bromophthalic anhydride is produced by the reaction of 
4-chlorotetrahydrophthalic anhydride with elemental bromine in the 
presence of iron. 
DETAILED DESCRIPTION OF THE INVENTION 
Surprisingly, 4-chlorotetrahydrophthalic anhydride reacts with bromine, in 
the presence of iron, to form a mixture of 4-chlorophthalic anhydride and 
the 4-bromophthalic anhydride. 4-Chlorophthalic anhydride does not react 
with bromine under these reaction conditions. Accordingly, the 
4-bromophthalic anhydride produced in the reaction does not come from the 
conversion of 4-chlorophthalic anhydride. 
The process of this invention is carried out in the liquid phase, in the 
presence of a solvent, at atmospheric pressure or under applied or 
autogenous pressure at temperatures ranging from about 100.degree. to 
about 200.degree. Celsius and preferably about 110.degree. to about 
150.degree. Celsius. Solvents that may be employed are preferably 
substantially non-reactive with bromine as well as with the starting 
material and products of the reaction. In addition, the solvents should 
dissolve both the starting material and the products of the reaction. The 
boiling point of the solvent is not a factor in conducting the reaction 
since the reactions may be conducted under pressure to achieve the 
temperature necessary for the reaction. Typical of the solvents that may 
be employed are aromatic solvents such as nitrobenzene, monohalobenzenes, 
and dihalobenzenes and aliphatic solvents such as chloroform, carbon 
tetrachloride, or chlorinated lower alkanes, including monochloro and 
polychloro alkanes, or polychlorinated lower alkenes. The most preferred 
solvent is chlorobenzene. 
The iron catalyst may be added to the reaction mixture in the form of an 
iron salt, such as ferric chloride, ferric bromide, or as elemental iron. 
Iron salts, other than halides also function in this reaction because the 
presence of elemental bromine and traces of HBr from the dehydrogenation 
reaction are able to convert any salt of iron into a catalytically active 
form. Typically, 1 mole % (based upon starting material) of iron or an 
iron compound is a catalytically effective amount. However, it has been 
found that amounts of iron or an iron salt on the order of 0.04 mole have 
some catalytic effect. The effect of the iron in this reaction is as a 
catalyst and iron need not be added to the reaction provided that traces 
of iron from some other source are present. If the reactants or solvent 
contain trace impurities of iron, the catalytic effect of the iron in 
forming 4-bromophthalic anhydride will be observed. 
This reaction produces a mixture of 4-bromophthalic anhydride and 
4-chlorophthalic anhydride. For many uses, the mixture of the two 
anhydrides may be used without further purification. Where pure 
4-bromophthalic anhydride is desired, it is possible to separate the 
4-bromophthalic anhydride from the 4-chlorophthalic anhydride by careful 
distillation. 
During the reaction it is preferred to condense the exiting vapors at a 
temperature sufficient to condense bromine, but allow HBr to escape (to be 
recovered by scrubbers for other uses). 
The following specific examples are provided to further illustrate this 
invention and the manner in which it may be carried out. It will be 
understood, however, that the specific details given in the examples have 
been chosen for purposes of illustration and are not to be construed as a 
limitation on the invention. In the examples, unless otherwise indicated, 
all parts and percentages are by weight and all temperatures are in 
degrees Celsius.

EXAMPLE 1 
37.3 grams of 4-chlorotetrahydrophthalic anhydride were added to 5.6 grams 
of chlorobenzene. The mixture was heated to 150.degree..degree.C. 0.36 
grams of ferric chloride were added. The temperature was maintained at 
150.degree. C. while 63.9 grams of bromine were added over a nine hour 
period. The reaction mixture was then heated for four more hours at 
165.degree. C. The mixture was analyzed by a gas chromatographic method in 
which the area corresponding to each product was compared to the total 
area. The results indicated that the reaction mixture contained 58.5% 
4-chlorophthalic anhydride and 31.8% 4-bromophthalic anhydride. 
EXAMPLE 2 
37.3 grams of 4-chlorotetrahydrophthalic anhydride were dissolved in 5.6 
grams of chlorobenzene and heated to 110.degree. C. 0.11 grams of iron 
powder were added. The temperature of the reaction mixture was maintained 
at 110.degree. C. while 44.8 grams of bromine were added dropwise over a 
five hour period. The reaction mixture was then warmed to 135.degree. C. 
and 19.2 grams of bromine were added. The reaction mixture was heated to 
165.degree. C. for 31/2 hours. The mixture was analyzed by a gas which 
involved comparison of the peak areas corresponding to various compounds 
to the total peak area. The results indicated that 29.3% of 
4-chlorophthalic anhydride and 41.6% of 4-bromophthalic anhydride had been 
produced. 
EXAMPLE 3 
A mixture of 559.5g (3.0 moles) of 4-chlorotetrahydrophthalic anhydride and 
84.0 g of monochlorobenzene was heated and maintained at 105.degree. C. 
while 720.0 g (4.5 moles) of bromine were added over a three hour period 
at which time a sample of the reaction mixture was analyzed by gas 
chromatography and found to contain 47% (g.c. area %) 4-chlorophthalic 
anhydride. 
The reaction mixture was heated to 135.degree. C. and maintained thereat 
for 3 hours while 240.0 g (1.5 moles) of bromine were added slowly, then 
heated to 165.degree.-170.degree. C. over a 20-minute period. A sample was 
analyzed and found to contain 79% 4-chlorophthalic anhydride. The 
temperature was maintained at about 165.degree.-170.degree. C. while 30g 
(0.1875 mole) bromine were added over a 35-minute period. Temperature was 
maintained for an additional 5 hours. Final analysis by gas chromatography 
indicated (in area %) 94.7% 4-chlorophthalic anhydride, 2.1% bromophthalic 
anhydride and no detectable 4-chlorotetrahydrophthalic anhydride starting 
material. 
EXAMPLE 4 
36.5 grams of 4-chlorophthalic anhydride were dissolved in 5.5 grams of 
chlorobenzene. The mixture was heated to 110.degree. C. 0.029 grams of 
iron powder was added and 10 grams of bromine were added over a three hour 
period. The reaction temperature was increased to 135.degree. C. and 
another 5 grams of bromine were added. The reaction mixture was then 
heated to 165.degree. C. and another 6.2 grams of bromine were added. The 
mixture was analyzed by a gas chromatographic method and it was found that 
the 4-chlorophthalic anhydride had not reacted to form 4-bromophthalic 
anhydride. 
EXAMPLE 5 
18.65 grams of 4-chlorotetrahydrophthalic anhydride were dissolved in 2.8 
grams chlorobenzene and heated to 110.degree. C. 0.0026 grams iron powder 
were added. The temperature of the reaction mixture was maintained at 
110.degree. C. while 22.4 grams of bromine were added over a 2 hour 
period. The reaction temperature was then increased to 130.degree. C. and 
an additional 9.6 grams of bromine were added over a 11/2 hour period. The 
reaction temperature was then increased to 165.degree.-170.degree. C. for 
4 hours and the resulting mixture contained 76.7% 4-chlorophthalic 
anhydride and 7.8% 4-bromophthalic anhydride (GC area %).