Process for the preparation of halophthalic anhydrides

Halophthalic anhydrides are prepared by the liquid phase reaction of a brominating agent with halogen substituted hexa- or tetra-hydrophthalic anhydrides.

BACKGROUND OF THE INVENTION 
This invention relates to a process for the preparation of halophthalic 
anhydrides by dehydrogenation of halogen substituted saturated or 
partially saturated phthalo compounds, such as halogen, substituted 
tetrahydro or hexahydro phthalic anhydrides. Halophthalic anhydrides are 
useful chemical intermediates for the synthesis of various commercial 
products, including polymers, dyes and plasticizers. 
The increasing importance of high performance polyimides has led to an 
increased interest in halophthalic anhydrides. The latter are particularly 
useful as intermediates for the preparation of dianhydride monomers, such 
as oxydiphthalic anhydride which may be co-polymerized with a suitable 
diamine to form a condensation polyimide. The preparation of dianhydride 
monomers for the high performance polymer industry requires halophthalic 
anhydrides of very high purity, since the presence of even what normally 
would be considered as minor amounts of impurities would degrade the 
polymer product and perhaps render the product unsuitable for certain 
uses. 
Halophthalic anhydrides may be prepared by the reaction of bromine with 
halo-substituted saturated or partially saturated phthalic anhydrides, 
such as halotetrahydrophthalic anhydride or gem-dihalohexahydrophthalic 
anhydride, at temperatures in excess of 200.degree. Celsius. However, this 
approach has been found to result in relatively low yields and is in 
general, uneconomical. 
Various other methods for the preparation of phthalic anhydrides by the 
dehydrogenation of saturated or partially saturated cyclic anhydrides are 
known in the chemical literature. 
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. Moreover, 
it has been found that when the dihalohexahydrophthalic anhydrides are 
dehydrogenated in nitrobenzene, a portion of the nitrobenzene is reduced 
to aniline. The aniline reacts with the anhydride group of either the 
starting material or product to form imides and thus lower the yield of 
desired product. 
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. Nos. 4,560,773 and 4,55,405 to Telschow disclose the preparation 
of substituted phthalic anhydrides by reaction of bromine with an alkyl or 
aryl-substituted tetrahydrophthalic anhydride, especially 
4-methyltetrahydrophthalic anhydride, in the presence of an acid acceptor, 
such as pyridine or dimethylformamide. In the working examples, U.S. Pat. 
No. 4,560,773 discloses yields of 62-80% and purity of only 90-95% even 
after vacuum distillation. According to the patentee, the yield and purity 
of the desired end product would be even lower if the reaction were 
carried out in the absence of an acid acceptor. 
U.S. Pat. No. 4,517,372 to Tang, disclose a process for the preparation of 
4-fluorophthalic anhydride by dehydrogenation of gem-, difluoro- or 
gem-chloro-fluoro- hexahydrophthalic anhydrides in the presence of a 
dehydrogenation catalyst, such as palladium. 
U.S. Pat. No. 4,709,056 to Cotter, Lin, and Pawlak discloses the 
preparation of 4,4-difluorohexahydrophthalic anhydride and 
4-chloro-4-fluorohexahydrophthalic anhydrides by reaction of hydrogen 
fluorides with 4-chlorotetrahydrophthalic anhydride. 
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. 
Co-pending application Ser. No. 07/393.449. which is a C-I-P of Ser. No. 
160,033 and Ser. No. 160,034, is directed to the preparation of 
halophthalic anhydrides by the reaction of chlorine with 
halotetrahydrophthalic anhydride or gem-dihalohexahydrophthalic anhydride 
at temperatures of 200.degree. Celsius and higher. 
Although the chemical literature discloses a variety of methods for the 
preparation of substituted phthalic anhydrides, it will be appreciated 
that a need continues to exist for a more economical and efficient 
dehydrogenation process, suitable for the preparation of high purity 
halophthalic anhydrides. 
SUMMARY OF THE INVENTION 
It has now been found that halogen substituted phthalic anhydrides of the 
formula 
##STR1## 
or intermediates thereof, wherein each X is independently F--, Cl--, Br--, 
or I--, and n is 1 or 2, may be prepared efficiently and in high yield and 
purity by the liquid phase reaction of a brominating agent, at 
temperatures below 230.degree. Celsius, with a halogen substituted hexa-, 
or tetra-, hydrophthalo reactant of the formula 
##STR2## 
wherein Q is monohalo and is the same as X or is gem-dihalo, wherein at 
least one halogen is the same as X, and n is the same number as in formula 
I, and Y and Z are CN, COBr, COCl, or COF; or Y and Z when taken together 
may comprise an anhydride group. When Q is monohalo, each monohalo is 
directly attached to a double bond carbon and when Q is gem-dihalo, the 
gem-dihalo is directly attached to a non-double bond carbon. When Y and Z 
are CN, COBr, COCl, or COF, the product of the bromine reaction may, in a 
known manner, be hydrolyzed to the dicarboxylic acid which, in turn is 
dehydrated to form the anhydride of formula I. 
DETAILED DESCRIPTION OF THE INVENTION 
The starting reactants for the process of this invention, as represented by 
structural formula (II), above, are saturated and partially saturated 
halo-ortho-phthalo- hexa-, or tetra-hydroaromatic compounds including 
halotetrahydrophthalic anhydrides such as those of the formulae 
##STR3## 
and the like, and gem-dihalohexahydrophthalic anhydrides such as those of 
the formula 
##STR4## 
and the like, wherein Hal represents halogen; and the corresponding 
halotetrahydro- and gem-dihalohexahydro-ortho-phthalonitriles and 
ortho-phthaloyl dihalides. The preferred reactants are the saturated and 
partially saturated phthalic anhydrides. 
The process of this invention comprises the reaction of a brominating agent 
with a halogen substituted cyclohexane anhydride, cyclohexene anhydride or 
cyclohexadiene anhydride. The preferred brominating agent, based on 
process efficiency and economic considerations, is elemental bromine. 
Other brominating agents which may be employed include, for example, 
N-bromosuccinimide and bromine chloride. The brominating agent is 
preferably employed in at least stoichiometric amounts, that is two moles 
of brominating agent per mole of anhydride reactant, and most preferably 
in an amount of up to about 10 percent excess of that stoichiometric 
amount. The anhydride reactant is a halogen substituted 
tetrahydro-ortho-phthalo compound or a gem-dihalogen substituted 
hexahydro-ortho-phthalo- compound. Suitable reactants are available 
commercially or can be prepared by various known methods. For example, the 
Diels-Alder reaction of a maleic anhydride with a conjugated diene will 
produce an anhydride with a partially saturated six-membered ring. 
Depending on the desired anhydride product, the conjugated diene and/or the 
maleic anhydride may be selected which contain the appropriate halogen 
substituents. The anhydride reactants that may be employed in the process 
of this invention include, for example: 
4-chloro-1,2,3,6-tetrahydrophthalic anhydride; 
4-fluoro-1,2,3,6-tetrahydrophthalic anhydride; 
4-bromo-1,2,3,6-tetrahydrophthalic anhydride; 
4-chloro-1,2,5,6-tetrahydrophthalic anhydride; 
4-fluoro-1,2,5,6-tetrahydrophthalic anhydride; 
4-bromo-1,2,5,6-tetrahydrophthalic anhydride; 
4-chloro-1,2,3,6-tetrahytdrophthalonitrile 
4-fluoro-1,2,5,6-tetrahydrophthalonitrile 
4-bromo-1,2,3,6-tetrahydrophthaloyl chloride 
4-chloro-1,2,3,6-tetrahydrophthaloyl chloride 
4,4-difluorohexahydrophthalic anhydride; 
4,4-dichlorohexahydrophthalic anhydride; 
4-chloro-4-fluorohexahydrophthalic anhydride; 
4,4-dibromohexahydrophthalic anhydride; 
4,4-difluorohexahydrophthaloyl chloride 
4-chloro-4-fluorohexahydrophthalonitrile 
3-chloro-1,2,5,6-tetrahydrophthalic anhydride; 
3-fluoro-1,2,5,6-tetrahydrophthalic anhydride; 
3-bromo-1,2,5,6-tetrahydrophthalic anhydride; 
3,3-difluorohexahydrophthalic anhydride; 
3,3-dichlorohexahydrophthalic anhydride; 
3,3-dibromohexahydrophthalic anhydride; 
3,3-difluorohexahydrophthaloyl dichloride 
4,5-dichloro-1,2,3,6-tetrahydrophthalic anhydride; 
4,5-difluoro-1,2,3,6-tetrahydrophthalic anhydride; 
4,5-dibromo-1,2,3,6-tetrahydrophthalic anhydride; 
3,4-dichloro-1,2,5,6-tetrahydrophthalic anhydride; 
3,4-difluoro-1,2,5,6-tetrahydrophthalic anhydride. 
The corresponding iodo compounds may be employed, but are generally less 
stable and are not preferred. 
When the starting reactant is a saturated or partially saturated halogen 
substituted ortho-phthalonitrile or phthaloyl dihalide, the reaction 
product may be converted to an anhydride in a known manner. Thus, when a 
halogen substituted tetrahydrophthalonitrile, or gem 
dihalohexahydrophthalonitrile, is reacted with a brominating agent, in 
accordance with the invention, the resulting halogen substituted 
phthalonitrile may be hydrolyzed, in a known manner, for example, using 
aqueous acid, to form the dicarboxylic acid, which is then dehydrated 
chemically or thermally to form the corresponding halophthalic anhydrides. 
In addition, the halophthalonitrile may be used as an intermediate to 
prepare the corresponding amides or other useful end products. Using the 
halogen substituted tetrahydrophthaloyl dihalide, or 
gem-dihalohexahydrophthaloyl dihalide in the bromination reaction, results 
in the formation of the corresponding halo-phthaloyl dihalide which may 
then be hydrolyzed in a known manner to the corresponding diacid which, in 
turn, may be chemically or thermally dehydrated to form the corresponding 
anhydride. Furthermore, the halo-phthaloyl dihalides may be employed as 
intermediates in the formation of various esters, by alcoholysis, or in 
the formation of the corresponding amides by ammonolysis. 
In addition to the anhydride reactants and products set forth, the 
applicability of the present invention to various equivalent reactants and 
products is contemplated. Contemplated equivalents to the anhydride 
reactants and products of the invention include the corresponding 
dicarboxylic acids, salts such as alkali metal salts, esters such as 
phenyl or alkylesters, imides, diamides and the like. 
The process is carried out in the liquid phase, either neat or in the 
presence of a solvent, at atmospheric pressure or under applied or 
autogenous pressure at temperatures ranging from about 0.degree. to about 
230.degree. Celsius or slightly higher and preferably about 70.degree. to 
about 170.degree. Celsius. Lower temperatures, such as 30.degree. C. and 
40.degree. C., can be used but they are not generally preferred due to 
long reaction times and/or lower yields. At temperatures substantially 
higher than about 230.degree. Celsius, some degradation of the reaction or 
the product of reaction may appear. Moreover, when the reaction mixture is 
heated to temperatures in excess of about 170.degree. C., it is important 
that the initial reaction with bromine occur at a temperature below about 
170.degree. C. 
Solvents that may be employed are preferably substantially non-reactive to 
bromine as well as to the organic reactant and preferably are 
characterized by a boiling point greater than about 100.degree. Celsius. 
Typical of the solvents that may be employed are bromobenzenes and 
chlorobenzenes. The most preferred solvent is monochlorobenzene. Lower 
boiling solvents, such as chloroform, carbon tetrachloride, or chlorinated 
ethanes may be advantageously employed when the process is carried out at 
lower temperatures, for example, in the presence of a free radical 
initiator. 
The process of the invention involves a free radical reaction which may be 
enhanced by the use of a free radical initiator such as visible or 
ultra-violet irradiation, or addition of catalytic amounts, typically less 
than about 5 percent by weight, based on weight of reactants, of 
initiators such as azo compounds, peroxides and the like. Typical azo 
compounds useful as free-radical initiators are azobis (alpha, 
gamma-dimethyl valeronitrile), 2,2'-azobis (2,4-dimethyl valeronitrile); 
and typical peroxides are benzoyl peroxide, diacetyl peroxide, diisopropyl 
peroxydicarbonate, lauroyl peroxide and the like. Azobisisobutyronitrile 
is particularly useful in the process of this invention. When the process 
is carried out in the presence of a free radical initiator, lower 
temperatures, typically in the range of about 0.degree. to about 
100.degree. Celsius, may be employed. 
When the process of the invention is carried out to substantial completion 
at a single temperature, or temperature range, it is preferred, based on 
yield and purity achieved, to carry it out at about 90.degree. to 
135.degree. and preferably about 90.degree. to 125.degree. C. However, it 
has been found advantageous to carry out the reaction in at least two 
temperature stages, by adding the bromine reactant at temperatures of 
about 90.degree. to 135.degree. Celsius, and maintaining the temperature 
in that range until the bromine is substantially consumed and then 
increasing the temperature to above about 160.degree. to remove any 
remaining dissolved HBr and convert any residual intermediates to the 
final product. When the bromine reactant has been substantially consumed 
at a lower temperature, the higher final temperature may, for example, be 
as high as about 250.degree. without substantial deleterious effect. 
However, since temperatures greater than about 190.degree. C. offer no 
particular advantage, it is preferred to employ final temperatures in the 
range of about 150.degree.-160.degree. to 190.degree. Celsius. 
In a preferred embodiment of the process of this invention, the addition of 
bromine to the reaction mixture is carried out in stages with associated 
increases in temperature. Preferably a major portion of the bromine, such 
as 65-80 percent, is added slowly while the reaction mixture is maintained 
at a temperature of about 90.degree. to 125.degree. Celsius until the 
bromine is substantially consumed. The temperature is then increased to 
about 130.degree. to 145.degree. and the remaining 20-35 percent of the 
bromine is added slowly while the temperature is maintained until the 
bromine is substantially consumed. The temperature is then increased to 
about 160.degree.-175.degree. and preferably maintained thereat for a 
period of time, such as about 3 to 8 hours to remove any remaining 
dissolved HBr and convert any residual intermediates to the final product. 
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 subsequent re-use).

The following specific examples are provided to further illustrate this 
invention and the manner in which is 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 temperature are in degrees 
Celsius. 
EXAMPLE I 
A mixture of 559.5g (3.0 moles) of 4-chlorotetrahydrophthalic anhydride and 
84.Og of monochlorobenzene was heated and maintained at 105.degree. C. 
while 720.Dg (4.5 moles) of bromine was 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.Og (1.5 moles) of bromine was 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 
(.1875 mole) bromine was added over a 35-minute period. Temperature was 
maintained for an additional 5 hours. Final analysis of the crude reaction 
mixture by gas chromatography indicated (in area %) 94.7% 4-chlorophthalic 
anhydride, 2.1% bromophthalic anhydride and no detectable 
4-chlorotetrahydrophthalic anhydride starting material. A simple 
up-and-over distillation at reduced pressure afforded a product of greater 
than 98% purity. 
EXAMPLE II 
A mixture of 9.3g (0.05 mole) of 4-chlorotetrahydrophthalic anhydride and 
15.Og monochlorobenzene was heated to 80.degree. C. 17.8g (0.1 mole) 
N-bromosuccinimide was added in small portions over a 30-minute period. An 
exotherm to 118.degree. C. was observed. The reaction mixture was sampled 
and gas chromatograph analysis indicated 74.6% (g.c. area percent) 
4-chlorophthalic anhydride. Continued heating at 90.degree.-100.degree. C. 
for 1/2 hour produced 84.6% 4-chlorophthalic anhydride. An additional 4.2g 
(.OZ3M) N-bromosuccinimide was added over a 41/2 hour period at 
100.degree.-110.degree. C. to yield reaction product containing 91.9% 
4-chlorophthalic anhydride as determined by G.C. 
EXAMPLE III 
A mixture of 18.65 g (0.1 mole) of 4-chlorotetrahydrophthalic anhydride and 
73 g of chloroform was heated to 30.degree. C. and sparged with dry 
nitrogen for 15 minutes. Vazo 64 (azodiisobutyronitrile) (0.3g) was added 
and ultraviolet radiation applied. (GE F15T8 BLB, 15 watt black light 
bulb) Bromine (32g; 0.2 mole) was added slowly over a three hour period at 
which time a sample of the reaction mixture was analyzed by gas 
chromatography and found to contain 17.2% (g.c. area percent) 
4-chlorophthalic anhydride. An additional 0.8g of Vazo 64 was added and 
the temperature was increased to 40.degree. C. and maintained thereat for 
4 hours, at which time analysis indicated 43.1% 4-chlorophthalic 
anhydride. A further addition of 1g of Vazo 64 and 9g of bromine (0.056 
mole) was made and temperature was maintained at 40.degree. C. for 7 more 
hours. Limited solubility of the reaction mixture in the amount of 
chloroform and the temperatures employed resulted in solids formation and 
the reaction was stopped with yield of 52.2% 4-chlorophthalic anhydride. 
EXAMPLE IV 
37.3 g (0.2 mole) of 4-chlorotetrahydrophthalic anhydride was heated to 
115.degree. C. and maintained thereat, with stirring while 70.0 g (0.4 
mole) of bromine was added, sub-surface, over a four hour period. The 
temperature was increased to about 125.degree. C. and an additional 7.0 g 
(0.04 mole) of bromine was added over a period of about 30 minutes. The 
reaction temperature was increased to 165.degree.-170.degree. C. and 
maintained, with stirring for four hours at which time analysis of the 
reaction mixture by gas chromatography indicated (in area percent) 91.1% 
4-chlorophthalic anhydride as shown in Table I below. 
In view of the suggestion of the prior art that the aromatization reaction 
of 4-methyltetrahydrophthalic anhydride with bromine is enhanced by the 
presence of an acid acceptor, such as pyridine, the following comparative 
example was carried out. 
COMATIVE EXAMPLE IV-A 
The procedure of Example IV was repeated except that an acid acceptor, 
pyridine (1.6 g/0.02 mole) was added to the initial reaction mixture. 
Results of analysis by gas chromatography are set forth in Table I. 
TABLE I 
______________________________________ 
Example 
Reaction Mixture (g) IV IV-A 
______________________________________ 
4-chlorotetrahydrophthalic anhydride 
37.3 37.3 
Pyridine 1.6 
Bromine 77.0 77.0 
Analysis of crude Reaction Product (area %) 
4-chlorophthalic anhydride 
91.1 84.9 
4-bromophthalic anhydride 
3.2 1.6 
Phthalic anhydride 1.0 10.8 
Other products 4.7 2.7 
______________________________________ 
From a comparison of the data of Examples IV and IV-A, it will be seen 
that, despite the teachings of the prior art regarding the necessity of 
using an acid acceptor, such as pyridine, to improve yield and purity of 
product in the aromatization reaction of bromine with other substituted 
tetrahydrophthalic anhydrides, the present process provides excellent 
yields and purity of product, without an acid acceptor. In fact, 
surprisingly, the presence of an acid acceptor in the aromatization 
reaction of the present invention actually results in a substantial 
lowering of both yield and purity of product. The impurities generated in 
this type of reaction, such as phthalic anhydride, are particularly 
difficult to separate by usual physical separation means such as 
conventional distillation and require costly and tedious separation steps. 
EXAMPLE V 
33.2 grams of 4-chlorotetrahydrophthalic anhydride was heated to 
120.degree. Celsius and maintained thereat for 4 hours while 64.4 grams of 
bromine was added slowly, subsurface. The temperature was increased to 
130.degree. C. and 6.4 grams of bromine added. The temperature was then 
increased to 164.degree. C. and maintained thereat for 4 hours. A sample 
was analyzed by gas chromatography and found to contain 84.6% (g.c. area 
%) of 4-chlorophthalic anhydride. An additional 6.4 grams of bromine was 
added and after 4 hours at 164.degree. C., the reaction mixture contained 
91.1% 4-chlorophthalic anhydride. 
EXAMPLE VI 
To 3-necked, round bottom flask, fitted with a thermometer, condenser and 
additional funnel, was added 28.0 g (0.15 mole) of 
4-chlorotetrahydrophthalic anhydride and 28.0 g of chlorobenzene. The 
flask was heated to 80.degree. C. and 24.65 g. (0.15mole) of BrCl was 
added over a period of 3 hours. The temperature was increased to 
95.degree.-100.degree. C. over the last hour of addition. The temperature 
was then gradually increased to 165.degree. C. with the concurrent 
distillation of chlorobenzene over 2 hours. The product mixture consisted 
of 60.1% 4-chlorophthalic anhydride, 8.2% starting material and 13.5% 
intermediate dienes (GC area %). Another 8.5 g (0.05 mole) of bromine was 
added at 150.degree. C. and the temperature increased to 165.degree. C. 
and maintained thereat for 51/2hours. The final crude reaction mixture 
contained 84.5% 4-chlorophthalic anhydride. 
EXAMPLE VII 
To a 1-l three-necked flask, equipped with a mechanical stirrer, a 
condenser with a gas outlet and an equa-pressured addition funnel, is 
charged 241.5 g (1 mole) of 4-chloratetrahydrophthaloyl chloride and 36 g 
of chlorobenzene. The mixture is heated to 100.degree.-110.degree. C. with 
stirring. Bromine, 320 g (2 moles) is added dropwise into the solution in 
a sub-surface manner. The color dissipates quickly and an evolution of gas 
occurs. When approximately 240 g of bromine has been added, the pot 
temperature is raised to 135.degree. C. The addition of bromine continues 
until completion. The pot temperature is then raised to 
165.degree.-170.degree. C. for 2-4 fours. during this period, an 
additional amount of bromine, such as about 10 to 15 g, may be added to 
complete the conversion of 4-chlorotetrahydrophthaloyl chloride. A good 
yield of 4-chlorophthaloyl chloride will be obtained from the distillation 
of the mixture. 
EXAMPLE VIII 
To a 1-l three-necked flask, equipped with a mechanical stirrer, a 
condenser with a gas outlet and an equa-pressured addition funnel, is 
charged 166.5 g (1 mole) of 4-chlorotetrahydrophthalonitrile and 75 g of 
chlorobenzene. The mixture is heated to 100.degree.-110.degree. C. with 
stirring. Bromine, 320 g (2 moles), is added dropwise into the mixture in 
a sub-surface manner. The red color of bromine dissipates quickly and a 
gas evolution starts. After about 240 g of bromine is added, the pot 
temperature is heated to 135.degree. C. The addition of bromine continues 
until completion. The pot temperature is then raised to 
165.degree.-170.degree. C. for 2-4 hours. During this period an additional 
amount of bromine, such as about 10 to 15 g, may be added to complete the 
conversion of 4-chlorotetrahydro-phthalonitrile. A good yield of 
4-chlorophthalonitrile can be obtained by distilling out the 
chlorobenzene. 
EXAMPLE IX 
In a conventional acidic hydrolysis, the 4-chlorophthaloyl chloride or 
4-chlorophthalonitrile of Examples 7 or 8 is hydrolyzed to 
4-chlorophthalic acid which, upon dehydration at over 
180.degree.-200.degree. C., will afford a high yield of 4-chlorophthalic 
anhydride. 
EXAMPLE X 
A solution of 4,5-dichlorotetrahydrophthalic anhydride (1.0 g/0.0045 mole) 
in trichlorobenzene (1.0 ml) was heated to 90.degree.-100.degree. C. A 
solution of bromine (0.76 g/0.0048 mole) in trichlorobenzene (1.0 ml) was 
added dropwise. The first few drops immediately decolorized upon addition. 
Subsequently, the solution turned red. During the addition, the 
temperature gradually rose to 110.degree. C. Following the addition the 
temperature was increased to about 170.degree. C. and maintained thereat 
for about one and one-quarter hour at which time 5 drops of bromine was 
added and the temperature was increased and maintained at about 
180.degree. C. for an additional 45 minutes. Analysis of the final 
reaction product by gas chromatography indicated a 90% conversion of the 
4,5-dichlorotetrahydrophthalic anhydride to 4,5-dichlorophthalic 
anhydride.