Process for the production of 2,5-dichlorotoluene

Dichlorotoluene that contains at least 55 percent of 2,5-dichlorotoluene is prepared by contacting ortho-chlorotoluene with chlorine in the presence of a chlorination catalyst that is either a metal sulfide or a mixture of a ring-chlorination catalyst and a co-catalyst that is a sulfur compound to form a reaction mixture containing ortho-chlorotoluene, dichlorotoluene, and trichlorotoluene and separating dichlorotoluene from the reaction mixture.

This invention relates to a process for the production of dichlorotoluenes. 
More particularly, it relates to a process for the chlorination of 
ortho-chlorotoluene whereby there is formed a mixture of dichlorotoluene 
isomers of unusually high 2,5-dichlorotoluene content. 
2,5-Dichlorotoluene is used commercially as an intermediate in the 
production of compounds that are useful as pesticides, lubricants, and 
dyestuffs. For example, it is used in the production of 
2,5-dichlorobenzoic acid, which is then converted to 
2,5-dichloro-3-aminobenzoic acid, which is a widely-used herbicide. 
In the past, 2,5-dichlorotoluene has been produced by the diazotization of 
5-chloro-2-aminotoluene and the treatment of the resulting intermediate 
compound with cuprous chloride. It has also been produced by the direct 
chlorination of toluene or orthochlorotoluene in the presence of iron or 
another known ring-chlorination catalyst. This process, which is described 
in my U.S. Pat. No. 3,000,975, yields a mixture of dichlorotoluenes that 
contains about 30 percent to 50 percent of 2,5-dichlorotoluene. In view of 
the increasing demand for 2,5-dichlorotoluene, it has become necessary to 
provide a more efficient process for the production of a dichlorotoluene 
product that contains a substantially larger amount of 2,5-dichlorotoluene 
than is obtained when ortho-chlorotoluene is chlorinated in the presence 
of the previously-known ring-chlorination catalysts. 
In accordance with this invention, it has been found that when 
ortho-chlorotoluene is chlorinated in the presence of any of the 
previously-known ring-chlorination catalysts and a co-catalyst that is 
sulfur or a compound containing divalent sulfur the dichlorotoluene 
mixture formed has a 2,5-dichlorotoluene content that is substantially 
higher than that obtained when the chlorination is carried out in the 
absence of the co-catalyst. 
The process of this invention is carried out by contacting 
ortho-chlorotoluene with chlorine in the presence of a ring-chlorination 
catalyst and a co-catalyst as hereinafter defined until the reaction 
product is a mixture of chlorotoluenes that contains at least 50 percent 
of dichlorotoluenes. This mixture is then fractionally distilled to 
separate the dichlorotoluenes from the monochlorotoluene and 
trichlorotoluenes that are present. The dichlorotoluene fraction, which 
contains at least 55 percent of 2,5-dichlorotoluene, may be subjected to 
fractional distillation or crystallization to obtain substantially pure 
2,5-dichlorotoluene. 
The chlorination of ortho-chlorotoluene is carried out in the presence of a 
ring-chlorination catalyst and a co-catalyst that is sulfur or a compound 
containing divalent sulfur. Any of the well-known ring-chlorination 
catalysts that are used in the production of chlorobenzenes and 
chlorotoluenes may be used. These include, for example, iron, iron 
chlorides, ferrocene, aluminum chloride, zirconium tetrachloride, thallium 
chloride, stannic chloride, gallium chloride, indium chloride, tungsten 
chloride, molybdenum chloride, iodine, and boron trifluoride and mixtures 
thereof. The preferred ring-chlorinated catalysts and the ones ordinarily 
used in the process of this invention are iron, ferrocene, zirconium 
tetrachloride, and aluminum chloride. 
The sulfur compounds that can be used as co-catalysts in the process of 
this invention include sulfur and a wide variety of organic and inorganic 
compounds that contain one or more divalent sulfur atoms and that are 
soluble to at least a limited extent in the reaction mixture. These 
include sulfur, sulfur monochloride, sulfur dichloride, carbon disulfide, 
thiophenes, thiophanes, alkyl-, cycloalkyl-, aryl-, and aralkyl mercaptans 
and dimercaptans, thioethers, and the like. The preferred co-catalysts are 
sulfur monochloride, sulfur, and the sulfur-containing compounds that are 
converted to sulfur monochloride under the conditions of 
ring-chlorination, for example, sulfur dichloride and carbon disulfide. 
When ring-chlorination catalysts that possess only moderate activity, such 
as stannic chloride, thallium chloride, and indium trichloride, are used, 
the co-catalyst appears to improve the utilization of chlorine during the 
chlorination reaction as well as to shift the isomer ratio distribution of 
the dichlorotoluene fraction to favor 2,5-dichlorotoluene formation. 
In a preferred embodiment of this invention, the combination of 
ring-chlorination catalyst and co-catalyst is replaced by a metallic 
sulfide. The useful metal sulfides, which contain at least one divalent 
sulfur atom, are sulfides of the aforementioned metals whose chlorides are 
ring-chlorination catalysts. Particularly advantageous results have been 
obtained using ferrous sulfide, ferric sulfide, stannic sulfide, and 
indium sesquisulfide. The metal sulfides may be added to the reaction 
mixture as such, or they may be formed in situ by the reaction of the 
metal chloride that is being used as the ring-chlorination catalyst with 
the sulfur compound that is the co-catalyst. 
The catalyst system must contain at least 0.1 part by weight of the 
co-catalyst per part by weight of the ring-chlorination catalyst if the 
desired high proportion of 2,5-dichlorotoluene is to be obtained. In most 
cases, 0.5 part to 1.0 part by weight of the co-catalyst is used per part 
by weight of the ring-chlorination catalyst. The use of a larger amount of 
the co-catalyst does not result in a further increase in the yield of the 
dichlorotoluene fraction or its 2,5-isomer content. 
The amount of the catalyst system that is used in the process of this 
invention is not critical. It is that which will produce in good yield a 
dichlorotoluene fraction that contains at least 55 percent of the 
2,5-isomer. At least 0.1 gram of the ring-chlorination catalyst or metal 
sulfide is usually used per mole of ortho-chlorotoluene. An amount of the 
catalyst system that will provide from 0.5 gram to 1.0 gram of the 
ring-chlorination catalyst or metal sulfide per mole of 
ortho-chlorotoluene is generally preferred because it makes possible a 
reaction rate that is fast enough for commercial operation of the process 
while inhibiting side-chain chlorination and ring-addition reactions 
The chlorination of ortho-chlorotoluene is carried out by procedures that 
are well known in the art. For example, chlorine may be added to a 
reaction mixture containing ortho-chlorotoluene and the catalyst system 
until the increase in the weight of the reaction mixture or in its 
specific gravity indicates that the desired amount of chlorine has reacted 
with the ortho-chlorotoluene. When the chlorination is continued until 
from about 0.5 gram atom to 0.9 gram atom of chlorine has reacted per mole 
of ortho-chlorotoluene, the reaction product generally contains from 50 to 
75 percent by weight of dichlorotoluene, 20 to 45 percent by weight of 
monochlorotoluene, and 2 to 10 percent by weight of trichlorotoluene. It 
preferably contains at least 65 percent by weight of dichlorotoluene and 
not more than 6 percent by weight of trichlorotoluenes. The 
dichlorotoluene, which may be separated from the monochlorotoluene and 
trichlorotoluene by fractional distillation or other known technique, 
contains at least 55 percent and preferably 60 percent or more 
2,5-dichlorotoluene, the remainder being 2,3-, 2,4-, and 
2,6-dichlorotoluenes. 2,5-Dichlorotoluene may be separated from its 
isomers, for example, by fractional distillation or fractional 
crystallization. 
The chlorination is carried out at temperatures in the range of -20.degree. 
C. to 70.degree. C., with 20.degree. C. to 50.degree. C. the preferred 
range. At temperatures below -20.degree. C., the reaction takes place too 
slowly to be of commercial interest. At temperatures above 70.degree. C., 
there is a tendency for side-chain chlorinated reaction by-products to 
form. Since chlorination is an exothermic reaction, external cooling may 
be required to maintain the reaction temperature in the desired range. 
The rate at which chlorine is added to the reaction mixture does not have 
an appreciable effect on the yield of dichlorotoluene or on the isomer 
distribution of the product.