CA1201129A - Process for producing pentachloronitrobenzene from hexachlorobenzene - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
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Disclosed is a process for producing pentachloronitrobenzene (PCNB) by reacting hexachlorobenzene (HEX) with sodium hydro-sulfide (NaSH) in the presence of sodium hydroxide (NaOH), sodium carbonate (Na2CO3), or mixtures thereof to produce the sodium salt of pentachlorothiophenol (PCTP), followed by reacting it (or PTCP
itself after acidifying) with nitric acid in the presence of sulfuric acid or oleum.

Description

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PROCESS POR PRODUCING
PENTAC}-]LORONI'l'ROBENZENE FE~OM ~IEXACHLOROBENZENE

Background of the Invention 1. Fie]d of the Invention This invention relates to the production of pentachloronitro-benzene .

2. Brief l~escription of the Prior Art Pentachloronitrobenzene (sometimes referred to herein as PCNB ) is widely used today as a soil fungicide . It is particularly useful in controlling plant diseases caused by botryt~s, fusarium, rhizoctonia and anthracnose.
Several methods are known for the prepara-tion of PCN~. For example, IJ.S. Patent No. 4,026,955, which issued on May 31, 1977 to Br-eaux, Newman and Quinnet-t, teaches on such process. That patent teaches reacting pentachlorobenzene and a mixed nitration acid in three stages having specific temperature requirements.
IJ . S . Paten t ~,057,590, which issued on November 8, 1977 to Gay, discloses a low temperature process for making PCNB by rèacting pentachlorobenzene with substan-tially pure nitric acid . Also, U . S .
Patent No. 4,138,43~, which issued on February 6, 1979 to Gay, teaches another multi-step reaction between pentachlorobenzene and a mi~ed nitration acid and HCI . And U . S . Pa tent No . 4,147,732, which issued on ~pril 3, 1979 to Mendiratta, discloses a process with a two-stage reactant mixing step wherein pentachlorobenzene is first mixed with sulfuric acid and then concentrated nitric acid is added .
While the processes disclosed by these four references rep-resent significant advances in producing relatively high purity PCNB, there is still a need in the art to be able t~ produce high purity PCNB from precursors other than pentachlorobenzene, which is not always commercially available. The present invention covers such a process for making PCNB through a precursor that was unthought of for this use until the presen-t invention.

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Brief Summary of the Invention The present invention, ~herefore, is direc-ted to a process for producing pentachloronitrobenzene comprising (1) reactin~ hexachlorobenzene (I-IEX) with sodium hydrosulfide (NaSH) in the presence of an inorganic base selected from -the group consisting of sodium hydroxide, sod-ium carbonate and mixtures -thereof to produce the sodium salt of pentachlorothiopheno.~ (PCTP); and (2) reacting this sodium salt of pentachlorothiophenol with a mixed nitration acid comprising nitric acid and sulfuric acid (and, preferably, sulfur trioxide to form oleum) at a temperature from about 35C to about 110C to form penta-chloronitrobenzene, the nitric acid being in molar excess of the pen ~achlorothiophenol .
A preferred embodiment of this invention is directed -to. acid-if ying the sodium salt of PCTP with a mineral acid (e . g . HCI) to form PCTP itself, which is reacted with the nitric acid in the man-ner mentioned in step (2) above to form PCNR. This acidifying step facilitates processing when a solvent is employed in step (1 ), above, and appears to result in a purer PCNB product.

Detailed Description An important advantage of this invention is that it allows the conversion of hexachlorobenzene, an unwanted by-product of con-ventional PCI;IB production methods, back to PCNB. Thus, the unwanted and possibly harmful by-product HEX is converted into the useful PCNB.
In the first step of the process of the present invention, hexachlorobenzene is reacted with NaSH in the presence of NaOlI or Na2CO3 or mixtures thereof. This reaction is illustrated in the following reaction equation (A) wherein NaOH is employed as the inorganic base:

C6C16 + NaElS + NaOH s C6C15SNa + NaC1 + H2O (A) i .

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The use of the inorganic bases, NaOH or Na2CO3, reduces the amount of expensive NaSH necessary to carry out the conversion and eliminates the generation of the highly toxic side-product, hydrogen sulfide (H2S).
The molar ratio of NaSH to ~IEX is preferably in the range of about 0.75:1 to ahout 1.25:1. More preferably, it is from about 0.9:1 to about 1.1:l. Most preferably, i-t is about 1:1.
The mole ratio of NaSH to the inorganic hase (NaOH, Na2CO3 or mixtures thereof) is preferably at least about 1:1. More prefer-ably, it is from about 1:1 to about 1.5:1.
This salt~forming reacting is preferably carried out in the presence of an inert organic solvent. Suitable organic solvents include N, N-dimethylacetamide and dimethylformamide . However, the presence of a solvent is not critical to this invention.
l 5 Any suitable reaction temperature may be employed for this reaction step. A preferable range is from about 50C to about 100C. The reaction time is dependent upon the temperature used;
suitable reaction time would ran~e from about 30 minu tes to about 600 minutes. However, the present invention is not limited to particular reaction -temperatures or times.
The sodium pentachlorothiophenolate may be reacted directly in a nitrating medium to form PCNB, or, according to a preferred embodiment, may be first acidified with a mineral acid such as HCl to form pentachlorothiophenol. This latter reaction is illustrated in the following reaction equation (B) wherein HCl is employed as the mineral acid.

C6C15SNa + HC1 s C6C15SH + NaC1 (B) Other mineral acids besides HCl may be used for this step.
These acids include sulfuric and phosphoric acids. ~enerally, the amount of mineral acid added should be sufficient to convert sub-stantially all of this sodium salt to PCTP.
Any reaction conditions normally used in similar acidification reactions may be used herein and the present invention is not to be limited to any specific reaction conditions for this step.

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3~ither sodium pentachlorothiophenolate or pentachlorothiophenol is reacted with nitric acid in the presence of H2SO4 or oleum to form PCNB. The exact mechanism of this reaction is not known;
bu~ it is believed that the PCTP reacts with HNO3 in the presence 5 of oleum by more than one reaction route. Two theorized routes are illustra~ed below by reaction equations (C) and (D):

C6C15SH ~ 7~INO3 + 3SO3 C6c15NO2 + 6N2 + 4H2~04 ( c ) C6C15SH ~ 3HNO3 + SO3 C6C15NO2 + 2NO + 2H2S4 (D) As can be seen, NO2 or NO is produced as a by-produc-t of each route. Since mixtures of NO2 and NO may be in the resul-ting reaction mixture, it is believed that the forma-tion of PCNB from PCTP occurs simultaneously by both reaction mechanisms (C) and 15 (D) and possibly others. Of course, the presen-t invention is not to be limited to any particular reaction mechanisms.
The mixed ni-tra-tion acid reactant for this step is, as indicated above, comprised of sulfuric and nitric acid. It is preferred tha-t sulfur trioxide also be present (as in commercial oleum). While it is 20 not be]ieved to be cri tical, i t is advan tageous to employ a wei~ht ratio of sulfuric acid to nitric acid of at least about 0.1:1 in order to achieve desirable yields of PCNB. It is more preferred to employ a v~eight ratio of at least about 0.2:1, most preferably, from about 0.25:1 to about 1.1:1, of these two acids for optimum yields.
25 Also, it is desirable to employ SO3 in amounts from about 1% to about 30%, more preferably, at least about 10% by ~eight of the H2SO4 employed.
Sufficient mixed nitration acid should be employed so as to have a molar excess of HNO3 over PCTP. As can be seen from 30 equations (C) and (D), above, the theorized reaction mechanisms require this molar excess. Advantageously, the mole ratio is preferred to be at least about 3:1. More preferably, it is desirable to employ sufficient nitric acid so that the molar ratio is from about 10:1 to about 40:1.

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Preferab~y, the nitric acid and sulfuric acid (oleum may be substituted for the latter) making up this mixed nitration acid are both in the most concentrated form as possible. Desirably, the nitric acid making up part of the mixed nitration acid is concen-5 trated ni-tric acid having at least about 65%, more preferably at least a~out 90%, by weigh-t of HNO3. The s~llfuric acid is prefer-ab~y in concentrated form containin~ at least about 85%, more pre-ferably 956, by weight of ~12SO~.
Sufficien-t sulfuric acid should preferably be present to act 10 simul-taneously as a solvent and catalyst and to absorb water formed during the reaction. In particular, regarding its catalytic effect, it is known that the presence of sulfuric acid protonates the nitric acid and, thus, makes the nitric acid a more reactive species for the present invention. The additional presence of SO3 is preferred 15 because it is believed that a higher yield and a purer product may occur .
The last reaction of the presen-t inven tion may be conducted by mixing together the PCTP and the mixed nitration acid in one or two stages. For example, one preferred embodiment is to mix the 20 PCTP and concentrated nitric acid in one reaction vessel and then adding concentrated sulfuric acid (or oleum) to this mix-ture. In ano-ther preferred embodiment, the PCTP is added to a mixture of nitric acid and sulfuric acid (or oleum). Al-terna~ively, the acid mixture may be added to the PCTP. I~owever, the mode of addition 25 is not a critical feature of this invention as long as desired reaction temperatures are maintained during the addition period.
The reaction between PCTP and the mixed nitra tion acid is highly exothermic. In order to control the temperature of the reaction mixture, it is preferred to add the PCTP to the acids, or 30 vice versa, at a rate sufficient to control the -temperature to within the Aesired temperature range. If external cooling is provided more rapid addi-tion may be utilized, but such cooling is not essen-tial. Also, it is preferred that the reaction mixture be well mixed by known stirring or agitation techniques to better ensure proper 35 overall temperature control.
If the above-noted two stage mixing process is employed, the addition of the PCTP to -the nitric acid, or vice versa, is preferred h ~

to be made at a rate sufficient to maintain the reaction mixture at a temperature from about 35C to about ~5C. At temperatures below about 35C, several produc~ion problems may be encountereù in-volving difficult temperature control and inadequate production rates. At an initial reaction temperature above about 65C, the rate of by-product formations, including the forming of undesired hexachlorobenzene, may occur. Accordingly, it is preferred to conduct this first stage addition within the specified range, more preferably, from about 45C -to about 60C. In the second stage, the sulfuric acid or oleum is added to this resulting mixture at a rate sufficient to keep -the reaction mixture from about 55C to about 100C. Temperatures below abou-t 55C for this stage are not preferred because the reaction rates would be too slow for most commercial modes. Likewise, allowing the reaction temperature to rise above about 100C may result in the formation of undesirable impurities like hexachlorobenzene. ~ore preferably, it is desired tha t this second stage be carried ou t a-t temperatures from about 60C to about 85C.
If the above-noted one stage mixing process is employed, the addition of the PCTP to the mixed acids, or vice versa, is prefer-ably carried out from about 55C to about 100C for the same reasons as s-tated above. More preferably, the reaction is con-ducted in a range from about 60C to about 85C for this single stage .
Regardless of whether one or two-stage mixing steps, or other mixing procedures are followed, the reaction mixture should be maintained at the above-noted temperature range(s~ for a sufficient amount of time to convert at least a portion of the PCTP to PCNB.
Preferably, the amount of time should be sufficient to convert substantially all (i.e. greater than ~5% by weight) of the PCTP. In order to achieve this desired conversion, it is preferred to allow the reaction mixture to react from about 15 minutes to about 180 minutes, or greater. Of course, the reaction time will depend upon the specific reaction ~emperatures employed and the mole and weight ratios of HNO3: PCTP and HNO3: H2SO4 employed, respectively . To minimiY.e the time period of the reaction, it is preferable to utilize a combination of reaction temperature(s) and mole and weight ratios ~ ,:

which results in the substantial complete conversion of PCTP while minimizing the amount of by-products produced.
After the ]ast reaction has achieved its desired completion, the solid PCNB crystals formed may be recovered or subjected to fur-5 ther chemica] reaction in -the production ol other chemicals. Prod-uct recovery can be achieved by any suitable technique such as any conventional liquid/solid separation means such as fil-tration, centrifugation, decanting and the like. Preferably, the temperature of the reaction mixture, after completion of the reaction, is cooled 10 to a temperature from about 0C to about 30C and then the PCNB
is separated from the reaction mixture. The preferred separation means is filtration. This may also be followed by washing with water or any other sui table solvent to remove residuals . Alter-natively, a ho-t filtration without cooling may be preferred in some 15 instances. A highly pure PCNB product may be made according to this invention with the levels of hexachlorobenzene preferabl~ being less than 1.0% by weight of the total PCNB product.
The process of this invention was totally unexpected and surprising because there is no open position on the PCTP molecule 20 for substitution with a nitro group. One would believe that the -SH and -Cl groups would not be readily reactive for substitution.
But, if so, a wide variety of co-products would be produced. In the prior art methods of making PCNB, the pentachlorobenzene precursor has one open position on the ring free for substitution 25 with a ni tro group . That is not the case here . Furthermore, one might expect that the reaction of the HNO3 with the -SH group on the PCTP compound would form other substituents, such as the "sulfonic acid group . "
The following examples further illustrate the present invention.
30 All parts and percentages are by weight unless otherwise expressly indicated .

PREPARATION OE PENTACHLOROTHIOPHENOL

Example 1 Hexachlorobenzene (25.0 g, 0.08~ mol) was added to 50 mL of dimethylformamide followed by the addition of 7.4 g (0.096 mol) of 73% sodium hydrosulEide and 7.9 mL (0.079 mol) of 40% aqueous sodium hydroxide. The stirred reaction mixture was heated to 85C
for 3.0 hr, cooled to room temperature, and poured into 200 mL of water. After 10 min of stirring, the insoluble material was removed hy filtration, and the filtrate acidified with 15 mL (0.18 mol) of concentrated aqueous hydrochloric acid. The resulting precipitate was collected by filtration, washed with 100 mL of water, and dried in vacuo giving 22.8 g, 92% of theoretical, of pen-tachlorothiophenol.

Example 2 The experiment of Example 1 was repeated only using 50 mL of N, N-dimethylacetamide instead of 50 mL of dimethylformamide . The resulting product weighed 21.7 g, corresponding to an 88% yield of pentachlorothiophenol .

Example 3 The experiment of Example 1 was repeated only using 50 mL of sulfolane instead of 50 mL of dimethylfs>rmamide. The resulting product weighed 13.6 g corresponding to a 55% yield of pentachlo-rothiophenol .

Example 4 The experiment of Example 1 was repeated only using sodium carbonate ~8.4 g, 0.079 mol) instead of sodium hydroxide. The resulting product weighed 16.9 g corresponding to a 6~3% yield of pentachlorothiophenol .

Example 5 PCNB from Pentachlorothiophenol Using 70% Nitric Acid and 30% Oleum Pentachlorothiophenol (10.0 g, 0.035 mol) was added to 60.0 g 5 of 70~O nitric acid a t room -temperature . The reaction mixture ~as heated at reflux 7 hr followed by stirring at room temperature 16 hr. At this point, 40 mL of 30% oleum (30% by weight sulfur tri-oxide in sulfuric acid) was added at such a rate that the tempera--ture was maintained between 55 and 60C. After the addition was 10complete (30 min), the reaction mixture was heated at 108C for 30 minutes. The reaction mixture was then cooled to room tempera-ture, fi]tered, the product washed with water and dried in vacuo to give 7.6 g (74% yield) of 99.0% PCNB with 1.0% hexachlorobenzene (GC assay).

15Example 6A
PCNB from Pentachlorothiophenol Using 99% Nitric Acid and 30% Oleum in a One-pot, Two-step Process Pentachlorothiophenol (10.0 g, 0.035 mol) was added to 60.0 g of 99% nitric acid over 20 min at 45-50C followed by heating at 20 reflux (55C) for 1.5 hr . At this poin-t 20 mL of 30% by weight oleum was added at such a ra-te that the temperature was kept at 75 to 80C. After the addition was complete (approximately 30 min. ), the reaction mixture was cooled to room temperature. The produc-t was collected by filtration, and thoroughly washed with 25 water to give after drying in vacuo, 8.6 g (85% yield) of 99.7%
PCNB with 0.17% hexachlorobenzene (~C assay). Variations on this one-pot, two-step process were examined and are summarized in TABLE I.

TAB~E 1 ONE-POT, TWO-STEP PCTP TO PCNB PROCESS

PCTP ADD'N
PCTP 99%TI~ (min)/TEMP REFLUX TIMF 30% OLEUM YIELD PRODUCT ASSAY (%~ b EXAMPLE (~) HNO3 (g) (C) (min) mL (g) %PCNB HEX
6B 10 7530 min (50-60C) 30 10 (18.8)8299.6c 0.08 6C 10 4530 min (50-60C3 30 10 (18.8)8499.3 0.2 6D 10 3030 min (50-60C) 30 10 (18.8)1999.1 0.2 6A 10 6020 min (45-50C) 90 20 (37.6)8599.7 0.17 6E 25 15040 min (50-60C) 30 30 (56.6)81~- 99.5 e~ 0.2 o 6F 25 15040 min ~50-60C) 30 25 (47.0)8299.6 0.07 6G 25 ~ 15040 min (50-60C) 30 15 (28.2)53;~- 99.5 c~ 0.2 6H 25 15040 min (50-60C3 30 10 (18.8)10~ 99.5 c~ 0.2 a Procedure and work-up outlined in Example 6A.
b Measured by GC internal area nor~alization assay.
c Elemental analysis. Calc'd for C6C15NO2: C, 24.40; Cl, 60.02; N, 4.74.
Found: C, 24.18; Cl, 59.68; N, 4.84.

Example 7A
PCNB from Pentachlorothiophenol Using 99% Nitric Acid and 30'~6 Oleum in a One-pot, One-step Process A 10 mL aliquot of 30% oleum was added to 60 . 0 g 99% nitric 5 acid resulting in a temperature rise to 65C. Pentachlorothiophenol (10.0 g, 0.035 mol) was added a-t such a rate that the temperature was maintained between 65C and 70C, with a total addition time of 1.0 hr. The reaction mixture was cooled to room temperature, and the produc~ collec-ted by filtration, followed by a thorough water 10 wash and drying in vacuo. The product, 8.5 g (82% yield), anal-yzed (GC) as 98.6% PCNB with .09% hexachlorobenzene. Variations on this one-pot, one-step process are summarized in TABLE II.

TABLE II

ONE-POT, ONE-STEP PCTP TO PCNB PROCESS'~

EXA~I- PCTP HNO 30% OLEU~ PCTP ADD'N YIELD PRODUCT ASSAY (%)b PLE (g) ~ )3 mL (g)TI~lE(min) (%) PCNB HEX
7A 10 60 10 (18.8) 60 82 98.6 0.09 7B 25 113 25 (47) 80 83 97.5 0.10 7C 25 75 25 (47) 80 74 97.3 0.09 7D 25 75 25 (47) 80C 72 not not determined determined a Procedure and work-up outlined in Example 7A.
Measu~ed by GC inte~nal area normalization assay.
c Th~ addition of PCTP wa~ followed by a 60 mi~ heaLing step at 70C.

Example 8 PCNB from Pentachlorothiophenol Using 99% Nitric Acid and C:oncentrated Sulfuric Acid Pentachlorothiophenol (10.0 g, 0.035 mol) was added to 60.0 g 99% nitric acid over 0.5 hr followed by a 0.5 hr reflux at 55C. At this point, 10 mL of concentrated sulfuric acid was added over a 10 min period, and the reaction mixture heated at 65-70C for 15 min.
The reaction mix~ure was cooled to room temperature, the product -12- ~

collected by filtration, and thoroughly washed with water giving after drying in vacuo 6.5 g (62% yield) of 98.6% PCNB with 0.54%
hexachlorobenzene (GC assay).

Example 9 PCNB from Sodium Pentachlorothiophenolate The sodium salt of pentachlorothiophenol was prepared by heating at 80C for 3 hr a reaction mixture consisting of 25.0 g (0.088 mol) hexachlorobenzene and 15 g (0.195 mol) sodium hydro-sulfide (73% assay) in 100 mL DMF. The solvent was removed by 10 distillation in vacuo and the resul-ting residue (36.3 g) added to a solution of 45 g of 30% oleum in 150 g of 99% nitric acid which was preheated to 65C. The addition was made in 0. 5 hr while the reaction temperature was maintained at 60-65C by ice-bath cool-ing. Tlle reaction mixture was cooled to room temperature, fil-15 tered, and the residue washed with water. After drying in vacuo21. 3 g of product was obtained which assayed (GC) at 97% PCNB, 0.26% hexachlorobenzene.

Claims (14)

WHAT IS CLAIMED IS:

1. A process for producing pentachloronitrobenzene com-prising:
(a) reacting hexachlorobenzene with sodium hydrosulfide in the presence of an inorganic base selected from the group consisting of sodium hydroxide, sodium carbonate, and mix-tures thereof, to form sodium pentachlorothiophenolate; and (b) reacting said sodium pentachlorothiophenolate with a mixed nitration acid comprising nitric and sulfuric acid at a temperature from about 35°C to 110°C to form pentachloro-nitrobenzene, said nitric acid being in molar excess of said pentachlorothiophenol .

2. The process of claim 1 wherein said mixed nitration acid further comprises sulfur trioxide.

3. The process of claim 1 wherein the molar ratio of said sodium hydrosulfide to hexachlorobenzene is from about 0.75:1 to about 1.25:1.

4. The process of claim 1 wherein said molar ratio of sodium hydrosulfide to inorganic base is at least about 1:1.

5. A process for producing pentachloronitrobenzene com-prising:
(a) reacting hexachlorobenzene with sodium hydrosulfide in the presence of an inorganic base selected from the group consisting of sodium hydroxide, sodium carbonate, and mix-tures thereof, to form sodium pentachlorothiophenolate;
(b) acidifying said sodium pentachlorothiophenolate with a mineral acid to form pentachlorothiophenol;
(c) mixing said pentachlorothiophenol with nitric acid at a temperature from about 35°C to about 65°C, said nitric acid being in molar excess of said pentachlorothiophenol;
(d) mixing the resulting mixture with sulfuric acid at a temperature from about 55°C to about 100°C, the weight ratio of said sulfuric acid to said nitric acid being at least 0.1:1, and (e) maintaining said reaction mixture within said tem-perature range for sufficient time to convert at least a portion of said pentachlorothiophenol to pentachloronitrobenzene.

6. The process of claim 5 wherein said mineral acid in step (b) is HCl.

7. The process of claim 6 wherein the molar ratio of said sodium hydrosulfide to hexachlorobenzene is from about 0.75:1 to about 1.25:1.

8. The process of claim 7 wherein the molar ratio of said sodium hydrosulfide to inorganic base is at least 1:1.

9. The process of claim 8 wherein said mixed nitration acid further comprises sulfur trioxide.

10. A process for producing pentachloronitrobenzene com-prising:
(a) reacting hexachlorobenzene with sodium hydrosulfide in the presence of an inorganic base selected from the group consisting of sodium hydroxide, sodium carbonate, and mix-tures thereof, to form sodium pentachlorothiophenolate;
(b) acidifying said sodium pentachlorothiophenolate with a mineral acid to form pentachlorothiophenol;
(c) mixing said pentachlorothiophenol with a mixed nitration acid comprising sulfuric acid and nitric acid at a temperature from about 55°C to about 100°C, said nitric acid being in molar excess of said pentachlorothiophenol and the weight ratio of said sulfuric acid to said nitric acid being at least 0.1:1; and maintaining said reaction mixture within said temperature range for sufficient time to convert at least a portion of said pentachlorothiophenol to pentachloronitroben-zene.

11. The process of claim 10 wherein said mineral acid is HCl.

12. The process of claim 11 wherein the molar ratio of said sodium hydrosulfide to said hexachlorobenzene is from about 0.75:1 to about 1.25:1.

13. The process of claim 12 wherein the molar ratio of said sodium hydrosulfide to said inorganic base is at least about 1:1.

14. The process of claim 13 wherein said mixed nitration acid further comprises sulfur trioxide.