Method of preparing phthalide

An improvement in a process for preparing phthalide by reacting chlorophthalide with hydrogen in the presence of a catalyst at a temperature between 50.degree. and 350.degree. C., the improvement residing in carrying out the process in the absence of a hydrogen chloride acceptor, flowing the chlorophthalide continuously in fluid form through a solid catalyst bed in a reactor, passing the hydrogen through in such an excess so as to sweep off the hydrogen chloride formed and withdrawing phthalide in fluid form.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is directed to a process for preparing phthalide by reaction 
of 3-chlorophthalide with hydrogen. More specially, this invention is 
directed to a process for the synthesis of high quantities of phthalide by 
a catalytic reaction of 3-chlorophthalide with hydrogen wherein hydrogen 
chloride which forms as a by-product is effectively removed. The reaction 
takes place in accordance with the following equation: 
##STR1## 
2. Discussion of the Prior 
The preparation of phthalide has been performed hitherto either through the 
reaction of 0-disubstituted benzenes in a plurality of difficult steps 
with, to some extent, unsatisfactory yields, or through the reduction of 
phthalic acid anhydride in the presence of catalysts. In these catalytic 
reactions, the water that forms interferes with the reaction and leads to 
the formation of undesirable by-products which contaminate the phthalide. 
These disadvantages are partially avoided in the electrochemical reduction 
of phthalic acid anhydride or ammonium phthalamale (German OS No. 21 44 
419), but the performance of these electrochemical processes generally 
presents technical difficulties and involves a relatively large investment 
in apparatus. 
The replacement of organically bound halogen with hydrogen, hereinafter 
referred to as hydrogenolysis, in the presence of catalysts of Group VIII 
of the Periodic System is also known in itself. In this process, 
aromatically bound halogen is more easily substituted than aliphatically 
bound halogen. In all cases the reaction takes place all the more easily 
the less acid the reaction solution is. However, since hydrogen chloride 
is released in the hydrogenolysis, known hydrogen chloride acceptors are 
generally added to the reaction mixture to improve the reaction rate and 
the space velocity. 
The addition of acid-binding compounds of this kind, such as, for example, 
amines, sodium acetate, and alkail-containing methanol, has nevertheless 
the disadvantage of complicating the working up of the reaction product 
and of the catalyst. In particular, the recovery of the catalyst essential 
to the process can be accomplished only by a plurality of procedures, so 
that such a method of preparation is not technically feasible. 
Another method of intercepting the hydrochloric acid consists in performing 
the hydrogenolysis in the presence of a solvent having a high dissolving 
power for hydrogen chloride. Methanol, for example, is suitable as such a 
solvent in the hydrogenolysis of benzyl chloride to toluene. 
If it is desired to apply this method of hydrogenolysis in the presence of 
methanol to the preparation of phthalide from 3-chlorophthalide, however, 
several disadvantages are encountered. The phthalide reacts further in the 
presence of hydrogen chloride and methanol with the cleavage of the 
lactone ring to form o-hydroxymethylbenzoic acid ester, 
o-chloromethylbenzoic acid ester and o-toluic acid methyl ester, so that 
phthalide is produced in a yield of only 15 to 18%. 
It has also been proposed to react molten or dissolved chlorophthalide 
catalytically with hydrogen at elevated temperatures in the absence of a 
hydrogen chloride acceptor, allowing the gaseous hydrogen chloride to 
escape from the reactor. The disadvantages of this proposal are evident. 
If a catalyst supported on a support material of the conventional kind is 
used, the catalyst has to be kept in a quasi-homogeneous suspension in the 
liquid phase under the conditions of the reaction and must be vigorously 
mixed with the hydrogen phase in the reaction vessel by stirring. In order 
to be able to achieve this quasi-homogeneous suspension of the catalyst 
grains, a grain size spectrum of from 0.05 to 0.3 mm is indicated for the 
catalyst. Now, the required vigorous mixing of the liquid and gas phase in 
the reactor brings about a comminution of the catalyst grains. 
Furthermore, catalyst losses occur during the filatration of the catalyst 
from the reaction product. Operation with granular catalyst in the sump 
phase is therefore uneconomical due to the catalyst losses which occur 
during the necessary filtration. If one operates without solvent, the 
difficulties that are involved in the filtration increase. 
SUMMARY OF THE INVENTION 
It is an object of this invention, therefore, to provide a method of 
preparing phthalide from chlorophthalide by hydrogenolysis such that 
phthalide will be produced without a hydrogen chloride acceptor, without 
catalyst losses, and without special process steps for the separation of 
catalyst and/or by-products, with a very complete conversion of the 
chlorophthalide. 
In accordance with the present invention, the solutions to the 
above-recited problems are solved by an improved process for preparing 
phthalide by contacting chlorophthalide with hydrogen in the presence of a 
catalyst at a temperature between 50.degree. C. and 350.degree. C., the 
improvement comprising passing the chlorophthalide through a reactor 
containing said catalyst fixedly disposed, passing hydrogen therethrough 
in an amount and at a rate sufficient to sweep out hydrogen chloride 
formed by said reaction and withdrawing phthalide so formed in fluid form. 
In accordance with the present invention, chlorophthalide is reacted in a 
liquid form, i.e., either in the form of molten chlorophthalide or in the 
form of dissolved chlorophthalide. The reaction is conducted at a 
temperature between 50.degree. and 350.degree. C., preferably between 
80.degree. C. and 180.degree. C. in the presence of fixedly disposed 
catalysts. Preferably, the process is carried out in the absence of a 
hydrogen chloride acceptor. 
The hydrogen which is introduced in the reactor is generally employed in a 
greater-than stoichiometric amount and in some cases is employed in great 
excess. The hydrogen is employ at such an amount and at such a rate so as 
to sweep out hydrogen chloride formed as by-product of the reaction. The 
hydrogen also acts as an interceptor of reaction heat. Phthalide is formed 
by such a reaction in high yields and is withdrawn continuously from the 
reactor in fluid form, i.e., in the form of molten phthalide or in the 
form of phthalide dissolved in an inner solvent. 
The grain size of the catalyst is between about one and twelve millimeters. 
It is not necessary that the catalyst fill up the entire interior of the 
reactor. It must, however, be present in such an amount that its catalytic 
action will suffice for the amount of the chlorophthalide in the reactor. 
Suitable catalysts are the noble metals of Group VIII of the Fifth and 
Sixth Period of the periodic system, which are also called the platinum 
metals, examples being rhodium, ruthenium or platinum. The metal is used 
on a support, which contains from 0.1 to 10%, preferably 1 to 2%, of the 
metal. It is also possible, however, to use support material having a 
higher or lower metal content. Granulated charcoal or kieselgur, for 
example, serve as support material. The preferred support material is 
granulated charcoal. 
It is furthermore of considerable importance to the method that the 
hydrogen be used in an excess. The excess must be at least so great that 
the gases leaving the reactor will contain hydrogen. The amount of 
hydrogen to be put in per unit of time is therefore always greater than 
the stoichiometric amount needed according to the above-given reaction 
equation. 
If both the chlorophthalide (in the molten state) and hydrogen are fed into 
the reactor from the top at temperatures of 80.degree. to 180.degree. C., 
the mass velocity for the gas phase (hydrogen+HCl formed) is generally 
between 10 and 100 kilograms per square meter per hour. The corresponding 
mass velocity for the liquid phase (the molten chlorophthalide and/or the 
molten phthalide) is between 100 and 2000 kilograms per square meter per 
hour. 
The method of the invention is distinguished by a high space velocity and 
by a quantitative reaction of the chlorophthalide. The phthalide obtained 
by an optimum conduct of the reaction has a purity of over 98%. In 
general, the percentage of the by-product is less than 4%. The crude 
phthalide is easily separated from these by-products by fractional 
distillation or by other known measures. 
The hydrogen leaving the reactor contains the hydrogen chloride that forms 
in the reaction. It can be separated therefrom by simple, known measures. 
Then, after scrubbing and drying, it is recycled to the process. 
In continuous operation, chlorophthalide is fed into the reactor in the 
amounts per unit of time which correspond to the phthalide withdrawn. 
In particular, the feeding of substances to the reactor can be performed in 
a number of variant ways. Molten chlorophthalide can be proportioned into 
the upper part of a vertical reactor and trickled through the catalyst 
bed. In such a case, the hydrogen is fed through the free interstices in 
the same direction or in the opposite direction from the chlorophthalide 
feed. It is also possible, however, to force the liquid phase into the 
reactor bed from the bottom together with hydrogen. 
In all these variants, the chlorophthalide can also be used in dissolved 
form. Suitable solvents are all those which do not react under the 
conditions of the reaction with chlorophthalide, phthalide, hydrogen 
chloride and hydrogen. Examples of such solvents are toluene and other 
aromatic hydrocarbons. 
Surprisingly, in none of the above-described variants do by-products form 
which are difficult to separate, particularly when great excesses of 
hydrogen are used with respect to the percentage content of the hydrogen 
chloride being transported away by it.

DESCRIPTION OF SPECIFIC EMBODIMENT 
Referring to the accompanying drawing, a reactor 1 is charged with fixedly 
disposed catalyst 2. Into the reactor 1 there is introduced molten 
3-chlorophthalide through conduit 3 and hydrogen through conduit 4. Liquid 
product (molten phthalide) is withdrawn from the bottom of reactor 1 
through conduit 5 while gaseous product comprising hydrogen and hydrogen 
chloride are taken off from reactor 1 and enter hydrogen chloride 
absorption column 6 which is fed with scrubbing water through line 7. An 
aqueous hydrogen chloride solution is withdrawn from hydrogen chloride 
absorption column 6 via conduit 8. Unabsorbed gases rise from hydrogen 
chloride absorption column 6 and enter the drying tower 9 where water is 
removed therefrom. This permits recycle of the hydrogen gas to the reactor 
1. 
The advantages of the method of the invention over the state of the art are 
obvious. In a simple reaction, which can be performed with or without 
pressure, chlorophthalide is converted to phthalide of high purity in a 
single pass through the reactor, and can be used directly for any 
application. The catalyst losses which occur in filtration are avoided. 
Even after several days of operation the catalyst does not lose its 
activity. The hydrogen chloride that is formed is withdrawn from the 
hydrogen circuit of the process in the form of aqueous hydrochloric acid. 
The working temperatures and rates of reaction can easily be optimized by 
known measures. 
In order to more fully illustrate the nature of the invention and a manner 
of practicing the same, the following examples are presented. Where 
numbers are used in the examples following reference to a reactor conduit 
or the like these refer to the components of the reaction scheme shown in 
the accompanying drawing. 
EXAMPLE 1 
In a vertical reactor 1 of 3 cm inside diameter and 1.5 m length, filled 
with 435 g of catalyst 2 (average grain diameter 3 mm, 2% palladium on 
granulated charcoal), 0.41 mole per hour of molten 3-chlorophthalide 3 and 
220 liters per hour of hydrogen are introduced at the top through a 
proportioning apparatus. The reaction temperature is adjusted to 
85.degree. C. and kept largely constant during the reaction. The reaction 
product 5 flowing downward from the reactor is collected in a tank. From 
there it is delivered for refinement by distillation. The gas phase 
emerging from the bottom of the reator contains hydrogen and hydrogen 
chloride. The latter is absorbed in a hydrogen chloride absorption column 
6 in water which is introduced into the absorption column at 7. The 
aqueous hydrochloric acid that is produced leaves the column at 8. 
The hydrogen freed of hydrogen chloride also passes through a drying tower 
9 and is combined with the fresh hydrogen stream 4. 
The 3-chlorophthalide conversion amounts to 100%. The phthalide obtained 
has a purity of 97.3%. The distillation of the raw product yields 
phthalide with a purity of 99.9%. 
EXAMPLE 2 
Repeating the procedure described in Example 1, the throughput of molten 
3-chlorophthalide is increased to 1.5 moles per hour at 300 liters of 
hydrogen per hour. The reaction temperature is increased to 115.degree. C. 
A complete conversion of 3-chlorophthalide is obtained; the percentage of 
phthalide contained in the raw product is 96.7%.