US2584376A - Purification of benzene hexachloride - Google Patents

Description

Patented Feb. 5, 1952 PURIFICATION OF BENZENE HEXACHLORIDE Bill M. Williams, Midland, Mich., assignor'to' The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application October 22, 1947, Serial No. 781,479

1 Claim.

This invention relates to a treatment of hexachlorocyclohexane, also known as benzene hexachloride (CcHeCle), to alter the ratio of the isomers present and to obtain fractions greatly enriched with respect to particular isomers. It relates in particular to a method for obtaining fractions of benzene hexachloride greatly enriched as to their content of the gamma isomer and substantially free from irritating odor and contaminating color.

The insecticidal activity of benzene hexachloride has been widely publicized in recent years. It is known that this activity is centered principally in the-gamma isomer, and that the alpha, beta, delta and epsilon isomers have relatively little insecticidal effect. The usual methods of preparation of benzene hexachloride inevitably produce mixtures of the alpha, beta and gamma isomers together with one or both of the delta and epsilon isomers, and possibly others as yet unisolated. As first prepared, the alpha isomer is present in the greatest proportion while the amount of the beta isomer is commonly from to 20 per cent and that of the gamma isomer from 2 to 12 per cent. The mixed product is not only less effective as an insecticidal agent than is the gamma isomer alone, but it usually has a strong odor which may be quite disagreeable. That odor has been found to be absent from or at a minimum in the gamma isomer alone, and,

regardless of whether the odor is due to one or a mixture of the other isomers or is due to common impurities, it is deemed desirable to remove it from those fractions of the product which are to be used for household insecticidal purposes.

It is the principal object of the present invention to provide a method for altering the ratio of isomers present in a mixture of isomeric benzene hexachlorides. A particular object is to provide a process for treating a mixture of isometric benzene hexachlorides to produce a fraction greatly enriched as to the gamma isomer. A further object is to provide such a process whereby the objectionable or disagreeable odor may be removed from the insecticidally useful fractions of benzene hexachloride. It is another object to make such process as simple as possible, so that it may be carried out readily in standard industrial equipment.

In accordance with the present invention, there has been developed a practical process whereby the foregoing and related objects may be attained. This process comprises a selective solvent extraction, to minimize the amount of the beta isomer to be treated subsequently, followed by a vacuum distillation of the resulting mixture with a negligible beta isomer content. It has been found that, while a reasonably complete separation of the various isomers is possible by using a variety of solvents in a series of fractional crystallizations, such process is not as economical as might be desired. It has been found as well that a mixture of benzene hexachloride isomers containing more than about 5 per cent of the beta isomer cannot be fractionally distilled to obtain a practical separation, since the extremely high melting beta isomer causes mixtures containing it to sublime during attempted distillation, and the sublimate tends to plug the apparatus, increasing the pressure therein and reducing both the rate of distillation and the degree of separation below practical limits. When, however, the amount of beta isomer is reduced to a value below 5 per cent, and preferably below 4 per cent, the remaining mixture of isomers may be subjected to fractional distillation at reduced pressure and it is found that such treatment results in a marked change in the ratios of isomers present in the distilled fractions as compared with the ratio in the original mixture.

The first step in the process may be carried out using benzene, carbon tetrachloride, propylene chloride, or any of several other solvents to dissolve preferentially the gamma, delta and epsilon isomers and part of the alpha isomer from the original mixture, leaving undissolved a part of the alpha isomer and nearly all of the beta isomer. Of the various solvents which may be used in this step, the most practical and convenient one for commercial operation is benzene, since this material is usually present in large amounts in the reaction mixture in which crude benzene hexachloride is first obtained by addition of chlorine to benzene. Partial distillation of the excess benzene from such reaction mixture causes crystals rich in the beta isomer, and containing some of the alpha isomer, to separate from the initial solution. Similarly, treatment of solid crude benzene hexachloride with hot benzene, or other solvent for the other isomers which is a poor solvent or a non-solvent for the beta isomer, results in a practical reduction or elimination of the beta isomer from the rest. The solution is separated from the insoluble crystals, and is ready for the second step of the present process.

The filtrate from the first step is heated to distill therefrom the volatile solvent present. The high boiling residue may be crystallized and then distilled, or it may be subjected directly to vacuum distillation, and the distillate may be separated into fractions of increasing boiling points.

hereas the crude mixture and that resulti am the solvent treatment to remove the beta )mer are both characterized by a strong and rite-ting odor, the distillate is practically odorss with the exception of the lowest boiling fracin thereof. Similarly, while the crude products e somewhat colored, the vacuum distillates are 'lorless and solidify on cooling to form pure aite crystals. Most of the original color is conntrated in the still residue. Analysis of the icuum distillates by comparison of their infrad absorption curves with those of known pure :imers shows the distillation to have effected substantial separation of the isomers and to we produced fractions greatly enriched as to Le insecticidally active gamma isomer. The eld of purified product is high and the process much more economical than the relatively inef- :ient processes depending entirely on fractional main crop of tan colored crystals separated which, I

whenidried, contained and had a melting range from 60 to 100 C. A portion of this material Weighing 463 grams was taken for distillation. When heated under vacuum, 74 grams of low boiling solvent was removed as a first fraction, leaving 389 grams of \\benzene hexachloride to be distilled. Distillation was effected at 5 millimeters (Hg) absolute pressure with a reflux ratio of to 1. The weights and analyses of the fractions obtained are given in the following table. Before distillation, the crystals had the characteristic odor of crude ystallization. 20 benzene hexachloride.

Boiling Weight Infra- Red Analysis Fraction No. 0 Range,

0J5 Grams Per Cent Alpha Beta Gamma Delta Epsilon l 73-150 74 16. 0 0 0 0 0 2. 150-151 80. 6 17. 4 57 0 12 28 1 3 151-152 44. 3 9. 6 46 0 26 24 4 4 152-154 41. 9 9. 1 0 25 5 5. 154-156 55. 5 12. 0 20 0 43 28 6 6. l56-l57 48. 9 10.6 10 0 42 35 7 7. 157-159 61.8 13. 3 0 0 40 13 Residue 56.0 12. 0 0 33 6 24 12 Totals. 463. 0 100.0

1 Trace.

The following examples illustrate the practice the invention:

Example 1 A crude benzene hexachloride, having a dark rown color and containing the following isoieric forms of the compound:

Per cent lpha-isomer' 55 eta-isomer 12 amma-isomer l5 elta-isomer 9 ndetermined (solvent) 9 It is seen that by careful vacuum distillation it is possible to concentrate the initially small amount of the beta. isomer in the undistilled residue, as well as to obtain distilled fractions ranging from those which are high in the alpha isomer to those which have little or none of the alpha or beta isomers and are considerably enriched in the gamma isomer. Fractions 2 through 7 were light colored materials which solidified as masses of white crystals essentially free from odor and having none of the characteristic irritating odor of the crude product. The residue was brown in color.

Example 2 Some of the benzene hexachloride tan colored crystals which were used as the material to be distilled in Example 1 were given further successive treatments with isopropyl alcohol and then with trichloroethylene, with intervening crystallization. The so-recrystallized material was analyzed and found to contain 34 per cent of the alpha, none of the beta, and 63 per cent of the gamma isomer. After heating 197 grams of this material under vacuum to remove remaining solvent, 182 grams were left for fractional distillation. The results are tabulated as before.

Boning Weight Infra- Red Analysis Fraction No. 0 Range,

Grams Per Cent Alpha Beta Gamma Delta Epsilon s. 6 4.8 is o 73 0 s Totals. 182. 0 100. 0

This distillation ran smoothly with no evidence of sublimation or plugging of the apparatus, and

it is seen that the gamma. isomer can be freed mer are not limited to fractional distillation.

While the latter results in concentration of the alpha isomer in lower boiling fractions and of the gamma, delta and epsilon isomers, when pment, in the higher boiling fractions, a straight vacuum distillation in which only a small forefraction and a still residue are separated from the main distillate is equally eilective in producing a substantially odorless and colorless product.

The distillations specifically reported in the examples, were carried out in glass apparatus but the invention, is equally applicable to distillations carried out in-iron, nickel or other industrialldis tillltion equipment, under reduced pressures of the order'oi'fll to 100 millimeters- The method which comprises suspending ;a mixture of isomeric forms of benzene, hexachlojride which cannot befractionally distilled to ob tain a practical separation, in a liquid which is a better solvent for the gamma isomer than for the beta isomer, separating therefrom a crop of crystals rich in the beta isomer, recovering from the liquor a mixture of isomeric forms of benzene hexachloride containing less than per cent of 6 the beta isomer, fractionally distilling the mixture at a pressure of from 1 to millimeters of mercury, and recovering the lower boiling fractions enriched as to the alpha isomer and the higher boiling fractions enriched as to the gamma isomer, said principal fractions being improved both in odor and color as compared with the mixture subjected to distillation.

BILL M. WILLIAMS.

' REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Morton, Laboratory Technique in Organic Chemistry, published 1938 by McGraw-Hill Book Company, New York, New York, page 99.

Slade, The Gamma-Isomer of Hexachloro Cyclohexane," Chemistry and Industry, Oct. 13, 1945, pages 314-319.