GB1560608A

GB1560608A - Production of chlorothioformates

Info

Publication number: GB1560608A
Application number: GB9170/78A
Authority: GB
Original assignee: Stauffer Chemical Co
Current assignee: Stauffer Chemical Co
Priority date: 1977-03-09
Filing date: 1978-03-08
Publication date: 1980-02-06
Also published as: NL7802453A; YU41428B; BE864686R; NL188640C; AU3388178A; BG38486A3; IL54140A0; IT7848316A0; ZA781370B; IT1155865B; JPS6228783B2; DE2809776A1; NL188640B; YU56178A; FR2383172A1; DD134869A5; PL205160A1; DE2809776C2; BR7801423A; JPS53112816A

Abstract

Process for the preparation of a chlorothioformate of the formula <IMAGE> in which R is alkyl, lower cycloalkylmethyl, lower cycloalkyl-lower alkyl, phenyl, chloro-substituted phenyl, benzyl or chloro-substituted alkyl, the chloro substituent being located no closer to the sulphur atom than on the gamma -carbon atom, with the exception of ethyl chlorothioformate, in a two-step process and in the presence of an active carbon catalyst, by reaction of a mercaptan with phosgene in the liquid phase. Only a minimal amount of disulphide by-product is formed, and the production capacity is increased.

Description

(54) PRODUCTION OF CHLOROTHIOFORMATES (71) We STAUFFER CHEMICAL COMPANY, a corporation organised under the laws of the State of Delaware, United States of America, of Westport, Connecticut 06880, United States of America do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the production of chlorothioformate; more particularly, it relates to a process for the preparation of a chlorothioformate (RSC(O)C1) by the reaction of a mercaptan with phosgene in the presence of an activated carbon catalyst,

In the above, R represents alkyl, lower cycloalkyl-methyl, lower cycloalkyl, lower alkenyl, phenyl, chloro-substituted phenyl benzyl or chloro-substituted alkyl in which the chloro substituent is situated at least as far as the y - (gamma) carbon atom, with respect to the sulphur atom. By the terms "alkyl" or "chloro-substituted alkyl" are meant such groups having from 1 to 15, preferably from 1 to 10, most preferably from 1 to 6, carbon atoms, for example methyl, n-propyl, isopropy, n-butyl, sec.-butyl, isobutyl, n-pentyl, neopentyl, n-hexyl, neohexyl, n-heptyl,n-octyl, n-decyl, n-dedcyl, n-dodecyl and n-tetradecyl. By the term "lower alkenyl" is meant such groups having from 2 to 5 carbon atoms and at least one olefinic bond. By the term "lower cycloalkyl" is meant cycloaliphatic groups having from 3 to 7 carbon atoms, such as cyclopropyl and cyclohexyl. By the term "lower cycloalkylmethyl" is meant groups having from 3 to 7 carbon atoms in the cycleoalkyl portion, such as cyclopropylmethyl and cyclopentylmethyl. By the term "chlorophenyl" is meant both mono- and poly-chlorinated phenyl rings in which the chlorine atom or atoms may be variously substituted.

In a preferred embodiment of this process, R represents alkyl, lower cycloalkyl, lower cycloalkyl-methyl, benzyl, phenyl or chloro-substituted phenyl. Preferred embodiments for the various possibilities for R are: for alkyl, such groups having from 1 to 6 carbon atoms, particularly n-propyl isopropyl, n-butyl, sec.-butyl, n-pentyl and neopentyl; for lower cycloalkyl, cyclobutyl; for lower cycloalkyl, methyl-cyclopropylmethyl and cyclopentylmethyl; for lower alkenyl allyl; for chloro-substituted phenyl, p-chlorophenyl; for the haloalkyls, 3-chloropropyl.

Such chlorothioformates are useful intermediates for the production of herbicidally effective thiocarbamates and similar compounds. This reaction between mercaptans and phosgene to produce clorothioformates is described in U.S.Patent No. 3,165,544 which discloses the conduct of this process in laboratory size equipment. It is pointed out that reaction temperatures should be maintained as low as possible, consonant with reasonable reaction rates since at high temperatures a disulphide by-product begins to form in a significant amounts. Maximum temperatures are suggested for this reaction of between 70 and 140 C.

One process which has been utilized for commercial scale production of lower alkyl chlorothioformates by this reaction employs two catalytic beds of activated carbon arranged in series. The first bed is preferably contained in tubes of a multi-tube reactor; the second is in the form of a packed bed reactor containing a single catalyst bed. The first rector is operated as a continuous liquid phase reactor; more specifically as an upflow tubular catalytic reactor, with starting materials introduced at the bottom and products removed from the upper portion. The partially reacted mixture is then introduced into the top of the second reactor, which functions as a trickle-flow (downflow) packed bed. That is, the second reactor is operated in the continuous gas phase since gaseous hydrogen chloride product is continuously passing through the bed. Reaction products are removed from the lower portion of the second reactor and passed to downstream appartus for separating chlorothioformate. Operation of this process for production of ethyl chlorothioformate, however, has been found to produce this product in a purity of only between 91 and 95%.

The major impurity is diethyl disulphide, present in amounts of from 3 to 7%, with most of the remaining impurities being diethyl dithiocarbonate. When used to produce n-propyl chlorothioformate, the amount of disulphide by-product ranged from 1.5 to 13.7% and averaged just under 5% and the chlorothioformate purity averaged about 93%.

It is an object of the present invention to provide an improved process for the production of chlorothioformates by reaction of a mercaptan and phosgene in the presence of an activated carbon catalyst, which process possesses the following advantages: minimization of disulphide by-product,enhanced production capacity, good temperature control in the reactors and a good conversion of the mercaptan to chlorothioformate.

The present invention relates to a process for the preparation of a chlorothioformate corresponding to the following general formula:

wherein R represents alkyl, lower cycloalkyl, lower cycloalkyl-methyl, lower alkenyl, phenyl, chloro-substituted phenyl, benzyl or chloro-substituted alkyl in which the chloro substituent is situated at least as as far as the gamma carbon atom, with respect to the sulphur atom: by reacting the corresponding mercaptan (RSH) with phosgene in the presence of an activated carbon catalyst which comprises (a) contacting the mercaptan with phosgene in a first continuous liquid phase reaction zone in the presence of a catalyst comprising activated carbon: (b) removing the first reaction product from the first reaction zone: (c) contacting the first reaction product with a catalyst comprising activated carbon in a second continuous liquid phase reaction zone; and (d) removing a second reaction product comprising the chlorothioformate from the second reaction zone.

The present invention will now be more particularly described with reference to the accompanying Figure, which shows a generalized flow sheet for the conduct of the process.

Referring to the accompanying Figure, a mercaptan in line 1 is combined with phosgene in line 2 and the mixture introduced through line 4 into the lower portion of a first reactor 10. Reactor 10 is operated with reactants and products in a continuous liquid phase.

Preferably, reactor 19 is a tubular packed bed reactor containing a plurality of tubes filled with activated carbon of an appropriate particle size such that each tube functions in the conventional manner as a miniature packed bed reactor. The reactants in stream 4 are introduced into the lower portion of the reactor, thereby into the lower portions of the individual tubes, and pass upwards through the tubes. The average outlet temperature is generally between 0 and 70"C., preferably between 0 and 50"C. Pressures range between 0 and 150 psig, preferably between 0 and 50 psig.

The partially reacted products from the first reactor 10 are removed from the upper part of this reactor as overhead in line 6 and passed through line 8 into a second reactor 11. If desired, gaseous products from reactor 10 may be separated from the mixture in line 6 prior to its introduction into reactor 11. Reactor 11 contains a packed bed 12 of activated carbon.

The reaction is completed in reactor 11 in a continuous liquid phase. As shown in the Figure, this is accomplished by introducting reactants into the lower portion of ractor 11 so that this reactor operates in so-called "flooded upflow" condition. The reactor is generally operated at average outlet temperature of between 0 and 70"C., preferably between 10 and 50"C. Pressures range between 0 and 150 psig, preferably between 0 and 50 psig. Residence time of the reactants in reactor 11 is generally between 1 and 180 minutes, preferably between 5 and 90 minutes.

The reaction products are removed from reactor 11 through overhead line 9, passed to separation drum 13 and product chlorothioformate is removed in line 15 for further purification. Gaseous by-products (primarily hydrogen chloride with some unreacted phosgene) are taken off at line 14 and passed to downstream purification units (not shown) for recovery of unreacted starting materials for recycle and removal and further processing of hydrogen chloride.

When, as in the prior process, the second reactor 11 is operated as a continuous gas phase reactor (e.g. as a trickle-flow packed bed reactor) the average outlet temperature may also be maintained at between 0 and 70"C., as in the present process. However, operation according to the prior process results in an uneven temperature profile across the reactor due to poor heat transfer, providing localized high temperature zones, or "hot spots". It is known, from U.S.Patent No. 3,165,544, that undesirably high temperatures contribute to the formation of by-product disulphide. The presence of hot spots in reactor 11, therefore, increases the possibility of formation of this by-product.

When the process is practiced according to the present invention, however, the operation of the second reactor 11 as a continuous liquid phase packed bed reactor results in a marked decrease in disulphide formation since such operation provides better heat transfer and a more uniform temperature distribution throughout the catalyst bed.

Operation according to the present invention, with reactor 11 a continuous liquid phase reactor, results in an increase in the residence time in the second reactor at the same flow rate as the previous process, by a factor of at least 10. In the previous process, for instance, the residence time in this reactor was often of the order of from 4 to 5 minutes. In the present process the residence time may be between 5 and 180 minutes, or even longer, according to the flow rate. Preferably, the residence time is between 45 and 180 minutes, more preferably between 45 and 120 minutes, most preferaby between 45 and 90 minutes. It could reasonably be expected that operation using such longer residence times could result in increased by-product formation; however, it was found, surprisingly, that operation using such long residence times does not result in increased by-product formation so long as the temperature is maintained under good control. Alternatively, the flow rate of materials may be increased to permit operation using lower residence times in this reactor and increased capacity, as well as an increased conversion of mercaptan to chlorothioformate.

Preferably the flow rate may be increased up to 2 or 2l/2 times that used previously. Using increased flow rates, residence time in the first reactor 10 is also decreased.

The desired temperature control in reactor 11 and in the overall process may be augmented by introduction of excess liquid phosgene into the system, either as part of the feed in line 2 or separately, into the reactor 10. Part or all of this excess will vaporize until the normal operatjing conditions of reactor 11, the vaporization absorbing heat generated during the reaction.

As an alternative method of temperature control and also to assist in increasing the overall production of chlorothioformate, a relatively cold recycle stream 5, obtained from downstream processing units (not shown) and comprising primarily unreacted starting materials, may be introduced into the system. The recycle stream in line 5 may be introduced into reactor 11 via lines 7 and 8; its presence contributes to the maintainance of a desirably low temperature in reactor 11, preferably one below 50"C. Alternatively, recycle stream 5 may be introduced via lines 3 and 4 into the first reactor 10. Most preferably, temperature control is maintained by a combination of utilization of excess liquid phosgene and introduction of the recycle stream into reactor 11.

As an alternative to the "flooded upflow" type of reactor shown in the accompanying Figure, reactor 11 may be operated as a continuous liquid phase reactor in any other manner as may be convenient, for example as a downflow flooded packed bed reactor.

Operation according to the present invention, as will be further seen from the Example which follows, results in conversion of approximately 940/0 of starting mercaptan and production of a product of about 98% purity, containing generally less than 1% diethyl disulphide. Additionally, the use of a continuous liquid phase reactor, by the increse in residence time, provides greater capacity than a similar unit operating using a downflow or trickle flow packed reactor, in which the residence time is substantialy shorter.

The following Example illustrates the present invention.

EXAMPLE A two-reactor system is utilized as shown in the accompanying Figure, having a capacity for production of about 74,000 pounds per day of n-propyl chlorothioformate. The first reactor is a tubular upflow reactor, with the tubes packed with activated carbon catalyst.

The second reactor is a packed bed reactor containing a bed of carbon catalyst and is operated as an upflow reactor.

Into the first reactor, corresponding to a reactor 10 of the accompanying Figure, are fed 24.6 Ib.-moles/hr. of phosgene and 22.4 Ib./- moles/hr. of n-propyl mercaptan. A recycle stream containing about 11 lb.-moles/hr.phosgene and about 5 Ib./moles/hr. n-propyl chlorothioformate is also introduced into reactor 10. The reactor is operated at an inlet temperature of from 15 to 40 C.. an outlet temperature of from 40 to 55"C., and an outlet pressure of from 26 to 30 psig. The partially reacted products from the first reactor are fed into the lower portion of the second reactor. The second reactor is operated at an inlet temperature of from 40 to 55"C.. an outlet temperature of from 40 to 55"C., an outlet pressure of from 22 to 26 psig and a residence time of about 75 minutes.

Conversion of n-propyl mercaptan to the chlorothioformate was 94%. The product was produced in from 98 to 99% purity.

In U.K. Patent No. 1,498,559, there is described and claimed a process for the production of ethyl chlorothioformate which comprises: (a) contacting ethyl mercaptan with phosgene in a first continuous liquid phase reaction zone in the presence of a catalyst comprising activated carbon; (b) removing a first reaction product from the first reaction zone; (c) contacting the first reaction product with a catalyst comprising activated carbon in a second continuous liquid phase reaction zone; and (d) removing a second reaction product comprising ethyl chlorothioformate from the second reaction zone.

We make no claim herein to such a process for the production of ethyl chlorothioformate or to ethyl chlorothioformate when produced by such a process.

Subject to the foregoing disclaimer, WHAT WE CLAIM IS 1. A process for the preparation of a chlorothioformate corresponding to the following general formula:

wherein R represents alkyl, lower cycloalkylmethyl, lower cycloalkyl, lower alkenyl, phenyl, chloro-substituted phenyl. benzyl or chloro-substituted alkyl in which the chloro-substituent is situated at least as far as the gamma-carbon atom, with respect to the sulpher atom (the above groups being as hereinbefore defined): which comprises: (a) contacting a mercaptan corresponding to the following general formula RSH wherein R is as defined above: with phosgene in a first continuous liquid phase reaction zone in the presence of a catalyst comprising activated carbon: (b) removing a first reaction product from the first reaction zone; (c) contacting the first reaction product with a catalyst comprising activated carbon in a second continuous liquid phase reaction zone; and (d) removing a second reaction product comprising the chlorothioformate from the second reaction zone.

2. A process as claimed in claim 1 in which in the general formulae, R represents n-propyl.

3. A process as claimed in claim 1 or claim 2 in which (c) is carried out at an average outlet temperature of between 0 and 70"C.

4. A process as claimed in claim 3 in which (c) is carried out at an average outlet temperature of between 10 and 50"C.

5. A process as claimed in any of claims 1 to 4 in which (c) is carried out using a residence time of between 5 and 180 minutes.

6. A process as claimed in claim 5 in which (c) is carried out using a residence time of between 45 and 180 minutes.

7. A process as claimed in any claims 1 to 6 in which (a) and/or (c) is carried out using an excess of liquid phosgene.

8. A process as claimed in any of claims 1 to 7 further comprising recovering unreacted starting materials from the product of (d) and recycling the unreacted starting materials to (a) and/or (c).

9. A process as claimed in any of claims 1 to 8 further comprising recovering the chlorothioformate from the product of (d).

10. A process as claimed in any of claims 1 to 9 in which (c) is carried out by introducing the first reaction product into the lower portion of a packed bed reactor containing a bed of activated carbon catalyst.

11. A process as claimed in any of claims 1 to 10 in which (c) is carried out using a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. pressure of from 22 to 26 psig and a residence time of about 75 minutes. Conversion of n-propyl mercaptan to the chlorothioformate was 94%. The product was produced in from 98 to 99% purity. In U.K. Patent No. 1,498,559, there is described and claimed a process for the production of ethyl chlorothioformate which comprises: (a) contacting ethyl mercaptan with phosgene in a first continuous liquid phase reaction zone in the presence of a catalyst comprising activated carbon; (b) removing a first reaction product from the first reaction zone; (c) contacting the first reaction product with a catalyst comprising activated carbon in a second continuous liquid phase reaction zone; and (d) removing a second reaction product comprising ethyl chlorothioformate from the second reaction zone. We make no claim herein to such a process for the production of ethyl chlorothioformate or to ethyl chlorothioformate when produced by such a process. Subject to the foregoing disclaimer, WHAT WE CLAIM IS

1. A process for the preparation of a chlorothioformate corresponding to the following general formula:

flooded upflow packed bed reactor.

12. A process as claimed in claim 1 substantially as herein described.

13. A process as claimed in claim 1 substantially as herein described with reference to the Example and/or the accompanying drawing.

14. A chlorothioformate when prepared by a process as claimed in any of claims 1 to 13.