1. Field of the Invention
This invention relates to a method for purifying pentachloronitrobenzene (hereinafter referred to as PCNB), and more particularly, to an improved method for purifying PCNB under reduced pressure so as to decrease the amount of an impurity, hexachlorobenzene (hereinafter referred to as HCB), contained in PCNB produced industrially. PCNB can be used as a germicidal agent for soil in agriculture.
2. Description of the Related Art
It has been recently a worldwide problem that agricultural chemicals and impurities and other compounds present therein cause environmental pollution. PCNB is already widely used as a germicidal agent for soil and is also included in agricultural agents. It is highly desirable to reduce the contents of undesirable compounds contained therein such as HCB and the like to make the quality of PCNB much purer.
For example, in U.S.A., a Revised PCNB Registration Standard was issued in April 1982 by the U.S. Environmental Protection Agency to the effect that a new technique should be practiced to reduce the content of HCB in PCNB to 0.5 weight % or lower after March 1983 and to 0.1 weight % or lower after April 1988.
Heretofore, there are known various industrial methods for Producing PCNB. Main methods are processes for producing PCNB by chlorinating nitrobenzene or chloronitrobenzenes as disclosed in Khim. Prom., 1968, 44 (5), 334 etc. and processes for producing PCNB by nitration of pentachlorobenzene disclosed in U.S. Pat. Nos. 4,026,955, 4,057,590, 4,147,732 etc.
However, in the former method it is inevitable to produce HCB as a by-product by the perchlorination reaction and in the latter method HCB is formed as a by-product upon nitration and furthermore there is a significant problem that the starting material, pentachlorobenzene, is not always commercially available.
Recently Japanese Patent Application Laid-open No. 60-174748 disclosed a process for producing a highly pure PCNB comprising reacting HCB with sodium hydrogensulfide to form sodium pentachlorothiophenolate followed by reaction with a mixed acid to produce a highly pure PCNB containing a small amount of HCB.
However, there are problems in this process in that highly poisonous specific chemical substances are used as starting materials and HCB possibly remains depending upon the reaction conditions.
As is clear from the above discussion it is difficult at the present technical level to suppress the side reaction forming HCB in the reaction and to produce a highly pure PCNB containing only a very small amount of HCB.
On the other hand, there are mainly two methods for purifying PCNB containing HCB as an impurity to obtain a highly pure PCNB having a reduced HCB content.
One is a method comprising recrystallizing crude PCNB from a benzene-methanol system followed by subjecting the PCNB to column chromatography using an activated carbon column. A method for preparing a PCNB standard sample is disclosed in "Nogyo Kotei Kensaho Shokai" (Details of Agricultural Official Test Method), published by Nanko-do.
The other is a method comprising subjecting crude PCNB to silica gel column chromatography using carbon tetrachloride alone or a mixed solvent of carbon tetrachloride and a saturated hydrocarbon liquid (Japanese Patent Application Laid-open No. 53-95926).
Both purification methods are suitable for obtaining a highly pure PCNB in a laboratory, but are of less industrial value because large amounts of solvents and carriers are necessary.
In view of the present state of the art for producing a highly pure PCNB containing a small amount of HCB, the present inventors have found as a result of an intensive research on industrial purification methods that PCNB decomposes at temperatures higher than 250.degree. C. resulting in an increase in HCB while at 250.degree. C. or lower the decomposition hardly occurs and HCB and PCNB can be fractionated though the possibility is very narrow, and have proposed a method for purifying PCNB by distillation under reduced pressure at 150.degree.-250.degree. C. (Japanese Patent Laid-Open No. 62-230752).
According to the above-mentioned method, impurities such as tri- and tetrachloronitrobenzenes and the like as reaction intermediates as well as HCB contained in crude PCNB, have boiling points lower than PCNB and therefore, can be removed as initial distillate fractions by distillation under reduced pressure to enhance the purity of PCNB. After removing the impurities, the PCNB remaining in the bottom of column can be taken out as a pure PCNB or the PCNB is further subjected to distillation to obtain a highly pure PCNB. However, in this prior art purification method by distillation under reduced pressure, allowable impurities other than HCB are not mentioned in detail.
As mentioned above, there are known methods of producing PCNB such as chlorination of nitrobenzene or nitrochlorobenzenes and nitration of pentachlorobenzene. In the former method, chlorosulfonic acid is used as a reaction solvent while, in the latter method, a mixed acid is used as a nitration agent.
In each method, most of PCNB thus produced in the reaction is precipitated together with a by-product, HCB, outside of the reaction system, and therefore, after the reaction, the reaction mixture is poured into a large amount of water or the reaction mixture is directly subjected to a solid-liquid separation procedure, and then washed with water and, if necessary, washed with an alkaline aqueous solution followed by further dehydration or drying to isolate crude PCNB.
The crude PCNB thus isolated is treated in a purification step, but it is difficult to remove completely acids such as sulfuric acid, hydrogen chloride and the like included in the crystals. Therefore, inorganic acids such as sulfuric acid, hydrogen chloride and the like are present though the amounts are small, and sometimes sodium sulfate as well as organic impurities such as HCB and the like exist in the crude PCNB.
In fact, according to the investigation by the present inventors, it has been found that crude PCNB produced by the chlorination of nitrobenzene or chloronitrobenzenes contains about 1,000-5,000 ppm of sulfate ion and about 100-500 ppm of chloride ion and the pH is considerably acidic, that is, 4 or less.
In addition, it has been found that the presence of a small amount of acid accelerates the decomposition of PCNB depending on the temperature conditions upon purification by distillation under reduced pressure resulting in formation of HCB, and in particular, when pure PCNB is obtained as a bottom residue after distillation under reduced pressure, the quality of pure PCNB is deteriorated.
Furthermore, the presence of such acids in crude PCNB causes corrosion of metal materials such as stainless steel in the distillation step and therefore, materials to be used for apparatuses and machinery of the distillation equipment are limited accordingly.