US2889376A - Isolation of gamma benzene hexachloride - Google Patents

Description

United States Patent Ofitice England, assignors to Imperial Chemical Industries Limited, a corporation of Great Britain No Drawing. Application August 10, 1954 Serial No. 449,009

Claims priority, application Great Britain August 14, 1953 7 Claims. (Cl. 260-648) This invention relates to improvements in the production of organic compounds, particularly of the gamma isomer of benzene hexachloride, and more especially to a process for the isolation of the said gamma isomer from a mixture of the benzene hexachloride isomers.

Benzene hexachloride, as normally produced by simple additive chlorination of benzene in presence of light as catalyst, contains a mixture of stereoisomers in which the gamma isomer is present to the extent of about 10% to 14% of the, total chlorination product, while the alpha isomer represents about 60% of this total product. In addition, it is now known that the proportion of gamma isomer formed in this process is influenced by varying the conditions of the reaction, and the proportion of gamma isomer in the crude material has been increased to over 20% by chlorinating at low temperatures ,in presence of certain chlorinated hydrocarbon solvents.

yIn most of the well known solvents the gamma isomer is from two to four times as soluble as the alpha isomer and a number of processes for isolating the gamma isomer have been based on this fact. The procedure has generally been to extract the crude isomeric mixture with a limited amount of the selected solvent so that practically all the gamma dissolves but only a comparativelyfsmall proportion of the alpha. A part of the gamma isomer can be obtained directly from this extract by boiling to remove part of the solventand then allowing to cool slowly without disturbance, when gamma isomer crystallizes from a mother liquor containing alpha isomer in supersaturation. Less than half the total gamma content is recoverable in this way, however, and I in addition this type of crystallization, although elfective, present obvious disadvantages for large scale working.

We have now found that pure gamma isomer can be produced from the crude additive chlorination product of :benzene by a process which does not involve the retention of alpha isomer in a metastable state of supersaturation, and which is based on the discovery that the solubility ratio of gamma to alpha isomers in dimethyl carbonate is unusually low, being approximately 1:1, and. hence it is possible to recover gamma isomer by crystallization from this solvent of a gamma concentrate in which the ratio of gamma to alpha isomers is greater than unity. V V H v For convenience it will be interpolated at this point that by the term solubility ratio of gamma to alpha isomers in a particular solvent we mean the figure obtained by determining separately the solubility of the gamma isomer in grams per 100. grams of the solvent and similarly that of the alpha isomer and then dividing the former figure by the latter. I

'Theprocess comprises treating a mixture of isomers of benzene hexachloride inswhich both the gamma and alpha isomers are present and in which the ratio of gamma isomer to alpha isomer is greater than unity with dimethyl carbonate in at least sutficient amount and under such conditions of temperature as to dissolve Patented June 2, 1 95 9 and retain in solution substantially all the alpha isomer together with an approximately equal weight of gamma isomer while the remainder of the gamma isomer'which is undissolved or which, having been dissolved, is recrystallized from the solution as a result of bringing the latter to a supersaturated state with respect to gamma isomer is separated off by decantation, filtration, centrifuging or like means.

Obviously this process is equally adaptable to the production of a less pure gamma fraction, for instance, a product containing say or of gamma. The only modification needed is to use less dimethyl carbonate at the extraction or crystallization stage so that some of the alpha isomer either remains undissolved with the gamma or, in the casewhere the whole concentrate is first dissolved, crystallizes along with the gamma.

According to the invention, therefore, a process for the separation of the gamma isomer of benzene hexachloride from a mixture of at least the alpha and gamma isomers in which the amount of the gamma isomer exceeds that of the alpha isomer comprises treating the said mixture of isomers with dimethyl carbonate under conditions of temperature and concentration such that substantially all of the alpha isomer but only a part of the gamma isomer is taken into and finally retained in solution in the dimethyl carbonate and thereafter separating the gamma isomer that remains undissolved from the said solution.

A further feature of the invention is a modified process wherein there is produced a less pure product, for instance, one containing 80 or more of gamma isomet, by using appropriately less solvent at the extraction or crystallization stage.

The starting material for the process is thus a gamma concentrate in which the ratio of gamma isomer to alpha isomer is greater than unity and several procedures are known whereby such a concentrate may be produced.

For instance, if in the normal process of additive chlorination of benzene the chlorination is continued long enough, crystals of the alpha and beta isomers separate out; if these are filteredofi and the filtrate evaporated to dryness, there is obtained a mixture of benzene hexachloride isomers containing considerably more gamma than alpha isomer. Again, if the chlorination is interrupted at an earlier stage, before any solid has separated, and the solution is concentrated by evaporation so as to cause on cooling precipitation of alpha and beta isomers, filtration, followed by removal of benzene from the filtrate by evaporation yields products wherein the proportion of gamma exceeds that of the alpha isomer.

Products made by these methods, however, are liable to contain undesirably large amounts of the delta isomer and impurities resulting from over-chlorination of some of the benzene. Materials that are freer from these undesired constituents may be obtained by carrying out the chlorination at low temperature in the presence of certain chlorinated hydrocarbon solvents as described in US. patent application, Serial No. 207,702, filed January 25, 1951, now abandoned. Preferably also the concentration of free chlorine in the reaction system is carefully controlled as described in US. patent application, Serial No. 405,428, 'filed January 21, 1954, now abandoned. Working up of these products in the manner indicated above likewise yields concentrates that contain more gamma than alpha isomer.

Further, it is possible to take a crude mixture 'of benzene hexachloride isomers and obtain therefrom a concentrate in which the gamma/alpha ratio is greater than unity by treating the mixture with a limited amount of a solvent in which the solubility of the gamma isomer significantly'exceedsifor example, being between 2 and 4'times as great as) that of the alpha isomer, so that all of the gamma isomer is dissolved whereas part of the alpha isomer remains undissolved or having been dissolved crystallizes out again and so can be removed leaving a solution enriched in gamma isomer. This solution can be evaporated to dryness leaving a solid concentrate suitable for treatment with dimethyl carbonate according to the process of the present invention.

One such method of obtaining a gamma concentrate suitable for use as a starting material in the process of the present invention is to extract the crude benzene hexachloride, which may contain anything from to over of gamma isomer, with a limited amount of solvent such as methanol or ethanol so that the gamma isomer is dissolved and a considerable amount of the less soluble alpha and beta isomers remains undissolved and can be filtered off or otherwise removed, leaving a gamma-rich concentrate in solution. Evaporation of the solvent will then give a product in which the proportion of gamma to alpha is greater than unity. Amongst other solvents which may be used instead of methanol or ethanol are other lower aliphatic alcohols, liquid homologues of benzene, chlorinated aliphatic hydrocarbons and esters of aliphatic carboxylic acids.

The concentrate obtained by any of these methods may contain between 30% and 60% of gamma isomer together with about one-third to half this quantity of alpha isomer (delta isomer and other chlorinated products are of course also present), and by further treatment with dimethyl carbonate the gamma isomer in excess of the amount approximately equal in weight to the alpha present can be isolated in practically pure state, without any necessity for crystallization from a solution containing alpha in a metastable state of supersaturation. Thus it is advantageous to start with a gamma concentrated in which the gamma/ alpha ratio is as large as possible.

If the amount of gamma isomer crystallized from the dimethyl carbonate solution corresponds to the full amount present in excess of the alpha isomer, it will be found that the product contains a certain proportion of alpha as impurity. To produce gamma isomer of a purity of 99% or over, therefore, it is necessary to take out rather less than this actual excess and thus to use a little more than the amount of solvent which, from the known solubilities of the separate individual isomers, would be expected to retain all the alpha isomer in solution. Since, as is described later, the gamma isomer remaining in solution is preferably recycled, it will be apparent that the total yield of gamma isomer Will not be affected by the use of this additional solvent but that the amount of recycled material will become correspondingly greater.

Instead of using a gamma concentrate produced by one of the methods described above, it is also possible to use crystalline products richer in gamma isomer which have been obtained by fractional crystallization from the solution produced by extracting the crude material with a limited amount of solvent. Such crystals may for instance contain between 60% and 90% of gamma isomer, and the treatment with dimethyl carbonate otters a convenient method of purification of such material from the contaminating alpha isomer.

The dimethyl cabonate treatment may be carried out in several ways, for example, by complete dissolution of the concentrate and subsequent crystallization of only the gamma isomer, or by extraction methods whereby the alpha isomer and other impurities are dissolved together with only a part of the gamma isomer, the rest of the gamma isomer remaining undissolved and thus being easily separable by methods such as decantation, filtration or centrifuging.

For instance, the gamma concentrate may be completely dissolved in the solvent at one temperature and the gamma isomer crystallized by cooling the solution to a lower temperature, the quantity of solventused being at least sufiicient to hold the alpha isomer in solution at the crystallization temperature. The concentrate may be dissolved in this amount of solvent by heating and the solution then cooled to bring about crystallization or alternatively the gamma concentrate may be dissolved in the solvent at room temperature and then the solution cooled to promote crystallization. Since however di methyl carbonate solidifies at just over 0 C., the tem perature drop obtained by this latter procedure is insufficient to bring about the crystallization of much of the gamma isomer and it is preferable also to increase the degree of supersaturation by removing a certain amount of solvent. This can be done by applying vacuum to bring about both the evaporation of solvent and the cooling simultaneously. This last procedure is, of course, merely a specific case of the general method of dissolving the concentrate wholly in the solvent then evaporating off such proportion of the solvent as it not required to keep the alpha isomer in solution at the crystallization temperature.

It is also possible to carry out the process without complete dissolution of the concentrate in the solvent, that is, by an extraction rather than a crystallization process. Thus an amount of solvent sufficient only to dissolve the alpha isomer and part of the gamma isomer may'be stirred with the solid gamma concentrate and the undissolved gamma isomer may then. be separated by known methods such as decantation, filtration or centrifuging.

It will be convenient to note at this point that not only is the ratio of the solubilities of the gamma and alpha isomers in dimethyl carbonate low as compared with most other solvents, but so also is the gamma/beta solubility ratio, so that gamma isomer separated from alpha by the process of this invention is also substantially freed from beta isomer.

It will be apparent that the quantity of gamma isomer separated by the dimethyl carbonate treatment will be only a fraction of the total gamma present, and that if the product is to be free from alpha isomer there will necessarily remain dissolved in the mother liquor at least as much gamma as alpha isomer. Further quantities of the gamma isomer left in the mother liquor can be recovered therefrom by removing the solvent and treating the product much in the Way that the original crude benzene hexachloride was treated. Thus it may be extracted with a solvent in which the gamma/ alpha solubility ratio is in the range 2:1 to 4:1, whereby more of the alpha isomer is eliminated, the residue then being subjected to another dimethyl carbonate treatment. Alternatively the mother liquor may be evaporated further so that the solvent remaining is insufficient to hold the greater part of the alpha isomer in solution and thus a crop of crystals containing approximately equal amounts of gamma and alpha isomers is obtained. These crystals are separated and recycled to the first stage of the process comprising the concentration of the crude material having a gamma/ alpha ratio of less than unity. The mother liquor from this second crystallization contains only a small fraction of the total gamma isomer originally present but most of the delta isomer and other impurities, and it is normally discarded as delta-residue.

The preferred embodiments of the invention are combined processes wherein, starting from a crude chlorination product, technically pure gamma benzene hexachloride of at least 99% purity is obtained with a minimum of extraction or crystallization procedures and a maximum degree of recovery in a pure state of the gamma isomer initially formed.

In one such process the crude benzene hexachloride slurry obtained by chlorinating benzene in presence of methylene chloride, at a temperature below 0 C., with a chlorine concentration in the range 0.1% to 0.3% weight/weight (compare United States application, Serial No. 405,428), which slurry contains benzene hexachloride isomers in proportions of 20% or more of gamma isomer, between 60% and of other isomers in addition to r nethylene chloride, unchanged benzene, and a small amount of dissolved hydrogen chloride, is neutralized by stirring with soda ash, recycle material (referred to more fully hereinafter) is added and the mixture is heated until all or nearly all the organic solids present have gone into solution. A quantity of the solvent (methylene chloride and benzene) is then evaporated and the solution cooled with stirring, when the greater part of the alpha and beta isomers separates out. These, together with any residual inorganic material, are filtered off, and the solvent is completely removed from the mother liquor, leaving a concentrate in which the gamma/alpha ratio is greater than unity. This concentrate in fact contains about 60% of gamma isomer.

This concentrate is then dissolved in 1.6 times its own weight of hot dimethyl carbonate and the solution cooled With stirring to 20 C., when pure gamma isomer crystallizes and is filtered ofi. Solvent is evaporated from the filtrate until the'solution contains 75% of benzene hexachloride and chlorinated impurities and the solution is 'cooledjto 20 C., when a further crop of crystals is precipitated which contains about 48% of gamma isomer (the remainder being mostly alpha). This is recycled to the first stage of the process, that is, it is added to the crude neutralized slurry befor'e evaporation of the solvent and separation ofthe alpha and beta isomers. The mother liquor is evaporated to recover the solvent and the mixture of benzene hexachloride isomers and impurities remaining is rejected.

The process can be Worked in batches but is particularly suitable for continuous Working. ,7

Another convenient way of carrying out the process, suitable for use as a continuous manufacture, is described in the following example which illustrates but does not limit the invention.

Example Benzene and methylene chloride in molar ratio 3:4 are passed with eflicient stirring through a reactor held at a temperature of 25 C., and illuminated with light from a mercury arc lamp the main constituent of which is of Wavelength 4358 A., into which reactor chlorine is also passed at such a rate that the concentration of free chlorine in the solution is maintained sensibly constant at a value in the range 0.1% to 0.3% by weight of the total Weight of the reaction mixture. The rate at which the benzene-methylene chlorine mixture passes through the reactor is adjusted so that 38% of the benzen is chlorinated. There is thus obtained a slurry consisting of the alpha and beta isomers in a liquid consisting of alpha, beta, gamma and delta isomers, some chlorinated impurities, unchanged benzene, methylene chloride and some dissolved hydrogen chloride. This slurry is neutralized by stirring with soda ash and at this point there is added a mixture of benzene hexachloride isomers and chlorinated impurities more fully described below and referred to as recycle material. The whole mass is well mixed and filtered, the solids, consisting of alpha and beta isomers and inorganic material, are rejected and the mother liquor is evaporated to yield a gamma concentrate suitable for treatment with dimethyl carbonate.

This gamma concentrate while still molten is dissolved in 0.7 times its own weight of dimethyl carbonate at 50 C., and the solution is then cooled to 20 C., when gamma isomer of a purity exceeding 99% crystallizes out and is filtered oif, washed with dimethyl carbonate and dried.

The mother liquor is evaporated and the solvent recovered. The benzene hexachloride residue is divided into a major portion which constitutes the recycle material referred to above and a minor portion that is rejected, the proportion of the latter being adjusted so that it carries away per unit of time as a purge an amount of the delta isomer and impurities equal to that formed in that time in the chlorination reaction.

In what has been described above the solvent has been consistently referred to simply as dimethyl carbonate. It is to be understood, however, that it is not essential, in order to secure the benefits of this invention, to use pure or substantially pure dimethyl carbonate. It is indeed possible to tolerate the presence of quite appreciable proportions of impurities or deliberate additions of other solvents without wholly losing the benefit of the low gamma/ alpha solubility ratio upon which the success of the invention is based. Thus, for instance, admixture with dimethyl carbonate of up to 40% of benzene, methylene chloride or methanol increases the gamma/alpha solubility ratio, but not beyond 2:1, so that even with such deliberately debased solvent the advantages associated with the use of dimethyl carbonate are still largely obtained.

Our invention, therefore, includes also processes wherein instead of dimethyl carbonate there is used a mixture of dimethyl carbonate with one or more of What we term, for convenience, the normal solvents for benzene hexachloride, that is, solvents for which the gamma/ alpha solubility ratio has the more usual value of 2:1 to 4:1, usually 3:1 to 4:1. Examples of such solvents are benzene and methylene chloride (small proportions of which may be easily carried forward into the dimethyl carbonate from earlier stages in the process, namely from the production of the gamma-rich concentrate), methanol, isopropanol and other solvents the use of which in producing gamma-rich concentrates is described in British specifications Nos. 573,693, 586,439 and 586,442, and also mixtures with water of suchof these solvents as are Water-miscible or form azeotropes with water or with water and dimethyl carbonate. Quite substantial proportions of such solvents can be tolerated with naturally some loss of efliciency but without losing entirely the favorable gamma/alpha solubility ratio that is characteristic of dimethyl carbonate. Indeed, the invention contemplates the use of dimethyl carbonate pure or substantially pure or of any solvent mixture containing dimethyl carbonate provided the proportion of dimethyl carbonate in the mixture is such that the gamma/alpha solubility ratio for that mixture does not exceed 2: 1.

What we claim is:

1. A continuous process for the isolation of the gamma isomer of benzene hexachloride which comprises photochlorinating benzene in presence of methylene chloride at a temperature below 0 C., with a chlorine concentration in the range 0.1% to 0.3% by weight of the total weight of the reaction mixture until there is formed a slurry containing alpha and beta benzene hexachlorides but substantially no gamma benzene hexachloride, neutralising the acid in the product, adding the mixture of gamma and alpha isomers referred to hereinafter as the recycle material, heating until all or nearly all the organic solids present have gone into solution and a portion of the solvent has been evaporated oif, cooling the solution to produce a precipitate containing alpha and beta isomers but substantially no gamma isomer, filtering off this precipitate together with inorganic material arising from the neutralisation step, evaporating the filtrate to remove unchanged benzene and solvent, dissolving the residue in hot dimethyl carbonate, cooling the solution with stirring and separating off the pure gamma isomer which crystallises and which constitutes the product, evaporating part of the solvent from the residual solution, cooling this solution and returning the crystals that separate out (composed of gamma and alpha isomers and conveniently termed the recycle material) to the cycle by adding them to the neutralised slurry prior to the first evaporation of solvent.

2. A continuous process for the isolation of the gamma isomer of benzene hexachloride which comprises photochlorinating benzene in the presence of methylene chloride at a temperature between 0 C. and minus 25 C., with a concentration of free chlorine in the reaction medium in the range of about 0.1% to 0.3% by weight of the total weight of the reaction mixture until approximately 38% of the benzene is chlorinated and a precipitate containing the alpha and beta isomers but substantially no gamma isomer is formed, neutralizing the acid in the mixture, adding the residue referred to hereinafter as the recycle material, filtering off and rejecting the solid material, evaporating the mother liquor to remove unchanged benzene and solvent, dissolving the residual concentrate in dimethyl carbonate at elevated temperature, cooling to cause crystallization of gamma isomer of a purity of at least 99%, filtering off these crystals which constitute the product, evaporating the mother liquor and returning a portion of the residue (termed the recycle material) to the cycle by adding it to the neutralized slurry immediately following neutralization, the proportion of the residue not so returned being adjusted so that it carries away per unit of time as a purge an amount of the delta isomer and impurities equal to that formed in that time in the chlorination reaction.

3. A process for the fractional extraction of the gamma isomer- 0f benzene hexachloride from a mixture containing the alpha isomer of benzene hexachloride and the gamma isomer of benzene hexachloride wherein the amount of said gamma isomer in said mixture is greater than the amount of said alpha isomer, which comprises in combination the steps of (1) treating said mixture at a temperature of at least about room temperature with an amount of dimethyl carbonate suflicient to dissolve and retain in solution said alpha isomer, and an amount of said gamma isomer approximately equal to the amount of said dissolved alpha isomer, but not all of said gamma isomer, and (2) after all of said, alpha isomer is dissolved,separating the purified undissolved gamma isomer from the mother liquor.

4. A process for the fractional extraction of the gamma isomer of benzene hexachloride from a mixture containing the alpha isomer of benzene hexachloride and the gamma isomer of benzene hexachloride wherein the amount of said gamma isomer in said mixture is greater than the amount of said alpha isomer, which comprises in combination the steps of (1) dissolving said mixture in an amount of dimethyl carbonate sufficient to completely dissolve all of said mixture and form an initial solution at a temperature of at least about room temperature up to the boiling point of the dimethyl carbonate, 2) super-saturating this initial solution with respect to the said gamma isomer, (3) crystallizing said gamma isomer from the super-saturated solution, alpha isomer remaining in the mother liquor solution, and (4) thereafter separating said crystallized purified gamma isomer from the mother liquor.

5. The process of claim 4 wherein said initial solution is supersaturated by cooling.

6. The process of claim 4 wherein said initial solution is supersaturated by removing a portion of said dimethyl carbonate therefrom.

7. The process of claim 6 wherein after said removal of said dimethyl carbonate, the residual solution is cooled.

References Cited in the file of this patent Slade: Chemistry and Industry, Oct. 13, 1945, pages 314-9.

Claims (1)

1. A CONTINUOUS PROCESS FOR TH ISOLATION OF THE GAMMA ISOMER OF BENZENE HEXACHLORIDE WHICH COMPRISES PHOTOCHLORINATING BENZENE IN PRESENCE OF METHYLENE CHLORIDE AT A TEMPERATURE BELOW 0*C., WITH A CHLORINE CONCENTRATION IN THE RANGE 0.1% TO 0.3% BY WEIGHT OF THE TOTAL WEIGHT OF THE REACTION MIXTURE UNTIL THERE IS FORMED A SLURRY CONTAINING ALPHA AND BETA BENZENE HEXACHLORIDES BUT SUBSTANTIALLY NO GAMMA BENZENE HEXACHLORIDE, NUE TRALIZING THE ACID IN THE PRODUCT, ADDING THE MIXTURE OF GAMMA AN ALPHA ISOMERS REFERRED TO HEREINAFTER AS THE RECYCLE MATERIAL, HEATING UNTIL ALL OR NEARLY ALL THE ORGANIC SOLIDS PRESENT HAVE GONE INTO SOLUTION AND A PORTION OF THE SOLVENT HAS BEEN EVAPORATED OFF, COOLING THE SOLUTION TO PRODUCE A PRECIPITATE CONTAINING ALPHA AND BETA ISOMERS BUT SUBSTANTIALLY NO GAMMA ISOMER, FILTERING OFF THIS PRECIPITATE TOGETHER WITH INORGANIC MATERIAL ARISING FRM TEH NEUTRALISATION STEP, EVAPORATING THE FILTRATE TO REMOVE UNCHANGED BENZENE AND SOLVENT, DISSOLVING THE RESIDUE IN HOT DIMETHYL CARBONATE, COOLING THE SOLUTION WITH STIRRING AN DSEPARATING OFF THE PURE GAMMA ISOMER WHICH CRYSTALLISES AND WHICH CONSTITUTES THE PRODUCT, EVAPORATING PART OF THE SOLVENT FROM THE RESIDUAL SOLUTION, COOLING THIS SOLUTION AND RETURNING THE CRYSTALS THAT SEPARATE OUR (COMPOSED OF GAMMA AND ALPHAS ISOMERS AND CONVENIENTLY TERMED THE RECYCLE MATERIAL) TO THE CYCLE BY ADDING THEM TO THE NEUTRALISED SLURRY PRIOR TO THE FIRST EVAPORATION OF SOLVENT.