US3472748A - Electrolytic process for preparing cyclohexylhydroxylamine - Google Patents

Description

United States Patent ABSTRACT OF THE DISCLOSURE Process of making cyclohexylhydroxylamine in an aqueous electrolyte system at a high overpotential cathode.

This invention relates to a process for the production of cyclohexylhydroxylamine. More particularly, this invention relates to an electrolytic process whereby cyclohexylhydroxylamine is produced by the electrolytic re duction of nitrocyclohexane in a continuous system.

Cyclohexylhydroxylamine is an important intermediate for the preparation of cyclohexylsulphamic acid. The salts of sulphamic acid, particularly the sodium and calcium salts, are commercially important sweetening agents. Heretofore, cyclohexylhydroxylamine has been produced by a chemical reduction method, as described in US. Patent No. 2,850,531, or by the catalytic hydrogenation method as described in US. Patent No. 2,969,393. With these methods, especially with the catalytic hydrogenation method, it is often difiicult to control the distribution of the products. For example, the reaction proceeds to the formation of cyclohexylamine, which is an undesirable by-product. Also, isolation of the main product is sometimes difficult and cumbersome; the catalysts employed are expensive; and there is always a safety problem attendant with the use of large quantities of hydrogen gas.

It is the object of this invention to provide a process for making cyclohexylhydroxylamine which is inexpensive as compared to other methods of preparation, while at the same time ensuring the production of a highly pure product in large yields. The efficiency of the process is increased to near 100% by eliminating the formation of undesirable by-products which is wasteful of starting materials and therefore costly. Finally, the process is designed to be operable on a continuous basis thereby enhancing its value for mass production requirements.

It has been found that cyclohexylhydroxylamine can be prepared by the electrolytic reduction of nitrocyclohexane in a cell comprising a high hydrogen overvoltage cathode and an anode which is isolated from the rest of the cell by a material possessing a low electrical resistance, such as a cellophane membrane. The electrolyte may contain, in addition to the electrolytes, a substance capable of solubilizing nitrocyclohexane so that the reduction will proceed as quickly as possible. Water soluble organic solvents such as ethyl alcohol and acetone can be used but are not preferred since they are electroactive, interfere to some extent in the distribution of products, and are expensive. An alternative is the use of an electroinactive surfactant such as that having the structural formula where n is preferably the integer 9. The use of this substance has the advantage of maintaining a rapid reduction rate while producing a high yield of pure cyclohcxylhydroxylamine. However, the reduction process may be carried out without the presence of such solubihzing substances and still produce a good yield of pure product. If it is desired to use a surfactant, it is preferable that such substances be either anionic or non-ionic.

The nature of the electrolytes is generally immaterial with the exception that such electrolytes cannot be reduced since this would interfere with the formation of the cyclohexylhydroxylamine, which is produced by reduction and forms at the cathode. Also, the use of chloride salts or compounds containing chlorine should be avoided because these compounds produce chlorine gas at the anode which interferes with the operation of the electrolytic cell. It is preferable to use sulfate salts or carbonate salts, such as ammonium carbonate, since these salts are not reductible in the cell and have the added advantage of keeping the electrolyte solution buffered at the desired pH level. The process is operable within a pH range of from 2 to 11, but at a low or acid pH, the cell tends to lose electrical efficiency, and the product remains soluble thereby necessitating an additional step of adding a base to the solution, to raise the pH and precipitate the cyclohexylhydroxylamine out of solution. It is preferable to keep the pH at or near neutral pH, within the range of 7 to 9.5. At this level, the cyclohexylhydroxylamine is insoluble and will therefore precipitate out of solution as it forms at the cathode.

The process is operable within a temperature range of from 20 to 60 C. At higher temperatures, that is, betwen 45 to 60, the reaction rate is notably accelerated, although the yield of pure cyclohexylhydroxylamine is somewhat lowered due to the formation of aci-nitrocyclohexane. However, at these high temperatures, cyclohexylhydroxylamine tends to sublime forming needlelike crystals along the cool surfaces of the system, hence, this process can be carried out at higher temperatures (50-60) on a continuous basis by removing the crystalline product as it forms and continually feeding the electrolyte with additional amounts of nitrocyclohexane. At lower temperatures, such as room temperature (23 C.), the reduction rate is slower, and allows the product to precipitate out of solution at the cathode segment of the cell. At this temperature, a high yield of pure cyclohexylhydroxylamine is formed as a precipitate which is easily removable from solution, thereby enabling the process to be continued by feeding additional amounts of nitrocyclohexane into the electrolyte.

By controlling the potential of the cathode, the reduction of nitrocyclohexane is prevented from proceeding further to cyclohexylamine. Furthermore, the production of hydrogen gas from water is prevented. The process is effectively operable within a voltage potential range of from 1.4 to l.7 volts, as measured against a mercurymercurous sulfate-saturated potassium sulfate reference electrode with a potentiostat. Within this range, the desired reduction proceeds with a theoretical efficiency of near while preventing the formation of other undesirable reduction products.

The electrolysis is carried out in an electrolytic cell having a cathode and an anode isolated from the rest of the cell by a material possessing a low electrical resistance, such as a cellophane membrane or a porcelain container. The anode material is unimportant, but the cathode must be made of a material having a hydrogen overvoltage potential high enough so that hydrogen gas does not form. Hence, metals such as zinc, lead, tin or mercury can be used, although lead and tin tend to dissolve, forming oxides and impeding the reaction. The use of mercury as the cathode is preferable since it has a sufiicient hydrogen overvoltage potential and does not form oxides during the operation of the cell. An appropriate electrolyte solution is added to both the anode and the cathode compartments, and if an electroinactive surfactant is to be utilized, it is added to the catholyte. An electrode surface area-to-electrolyte volume ratio is only important insofar as a factor affecting the reaction rate. A range of .30 cm.- to 1.00 cm? is employed with good results. The nitrocyclohexane is suspended in the catholyte and a stirrer is provided such that stirring is vigorous enough to disperse the oil uniformly throughout the solution. The cell is attached to an electrical power supply and enough current is provided so that a voltage of from 1.4 to l.7 volts, as measured against the aforementioned reference electrode, is obtained at the cathode.

As electrolysis continues, cyclohexylhydroxylamine, being insoluble at the preferred pH range (7-9.5) and at room temperature, precipiates out of solution in the cathode compartment in the form of a white solid material. When the current decreases to a minimal value, the precipitate is removed from the catholyte by filtration or other appropriate means. The electrolyte is then replenished with additional amounts of electrolyte solution, the pH is adjusted at the desired level and additional amounts of nitrocyclohexane are suspended in the catholyte, thereby continuing the process.

The precipitate which is removed from the solution is dried in a desiccator without heat or vacuum since the cyclohexylhydroxylamine tends to sublime. The yields range from 63% to higher than 90%, depending upon the electrode surface area-to-electrolyte volume ratio, pH, cathode potential and temperatures utilized.

The nature of this process will be better understood by reference to the following examples which are given to illustrate but not to limit the invention.

EXAMPLE 1 An electrolytic cell is provided composed of a cathode made of 0.006 inch lead foil cylinder, 7 cm. high and 4 cm. in diameter, perforated with nine 1 mm. diameter holes per square cm., and an anode made of a carbon rod 8 cm. long and 8 mm. in diameter, isolated from the rest of the cell by a cellophane membrane. The anolyte is a water solution containing 4% sodium bicarbonate and 4% ammonium carbonate, and the catholyte is a solution containing 3.3% sodium bicarbonate, 3.3% ammonium carbonate, 0.033% surfactant, and a suspension of 5 ml. of nitrocyclohexane, maintained at a pH of 8.5. Oxygen is removed by nitrogen degassing, and the system is maintained at room temperature.

The cathode potential is held contant at 1.5 volts as against a mercury-mercurous sulfate-saturated potassium sulfate reference electrode with a Wenking Potentiostat. The system is provided with a magnetic stirrer to keep the solution uniform throughout.

The electrolysis current has an initial maximum of 630 milliamps, falling to 48 milliamps after 17 hours of electrolysis. When electrolysis is discontinued and stirring ceased, white solid particles precipitate out of solution at the cathode. The solid material is separated from the solution by centrifugation and taken up in hot isopropanol. Upon cooling, white needle-like crystals appear, having a M.P. of 135 136" C. Microanalysis reveals that the compound contains 62.6% carbon, 11.3% hydrogen, 12.1% nitrogen and 13.6% oxygen as against theoretical values of cyclohexylhydroxylamine of 62.6, 11.3, 12.2 and 13.9, respectively.

EXAMPLE 2 An electrolytic cell is provided having a cathode made of a mercury pool with 31 cm. surface area and an anode made of platinum gauze, isolated in a porous porcelain cup. An electrode surface area-to-electrolyte volume ratio of .31 cm. is utilized. One hundred (100) ml. of electrolyte is used consisting of 6% NaI-ICO at a pH of 8.0. A mechanical stirrer is set above the mercury pool. The cathode potential is held constant at 1.6 volts against the reference electrode, and the temperature is maintained at 4 23 C. Initially 2.08 g. of nitrocyclohexane is added to the catholyte and stirred vigorously enough to disperse the oil uniformly throughout the solution. The current rises to 380 milliamps and is maintained there for 1 hour, then falling to 100 ma. after 5 hours 15 minutes. The solution is filtered and the product separated out.

After this, 8 g. NH HCO is added to the electrolyte and the temperature is raised to 50 C. by means of a steam bath. Another 2.08 g. of nitrocyclohexane is suspended in the electrolyte and electrolysis continued. In 53 minutes the current peaks at 830 ma. and after 2 hours, the current falls to 100 ma. Electrolysis is discontinued and the precipitate at the cathode is filtered off.

The unrecrystallized solids obtained in the two runs are dried in a desiccator without heat or vacuum and the yields are as follows:

The same conditions are employed as in Example 2, except only 50 ml. of catholyte is used with a mercury pool of 31 cm. surface area, thereby raising the ratio of electrode surface to electrolyte volume ratio to 0.6 cmr 3.1 g. of nitrocyclohexane are added to the catholyte and electrolysis commenced at a temperature of about 50 C.

The product precipitates at the cathode and is filtered out of solution and dried. The yield is 1.71 g. representing a 63% yield after 3 /2 hours.

The process is continued by adding 3 g. of NH HCO to the electrolyte to adjust th pH and adding another 3.1 g. of nitrocyclohexane to the anolyte. Electrolysis is continued for another 3 /2 hours. The product is again recovered by filtration and the yield is 2.15 g., representing an yield.

The process can be continued indefinitely as indicated in this example.

EXAMPLE 4 An electrolytic cell is provided having an acetate medium as the catholyte at a pH of 4.1. Then 5.25 g. of nitrocyclohexane is added to the catholyte and suspended there by vigorous stirring. A cathode potential of 1.5 volts against the reference electrode is maintained and no heat is added to the system.

Electrolysis is continued for 6 hours after which time the current reaches background level. The pH of the solution is brought to 10.8 with NaOH and the solid Precipitates, it is collected, filtered and then dried.

The recovered material weighs 3.1 g. and represents a 70% yield.

Others may practice this invention in any of the numerous ways which will be suggested to one skilled in the art upon reading this disclosure. All such practice of the invention is considered to be covered hereby provided it falls within the scope of the appended claims.

I claim:

1. The process of making cyclohexylhydroxylamine by the electrolytic reduction of nitrocyclohexane in an electrolytic cell system comprising suspending nitrocyclohexane in an aqueous electrolyte system having substantially neutral pH, passing an electric current through the electrolyte and the suspended nitrocyclohexane, reducing the nitrocyclohexane at a high overpotential cathode to form cyclohexylhydroxylamine, and precipitating the cyclohexylhydroxylamine thus formed out of the system.

2. The process of making cyclohexylhydroxylamine by the electrolytic reduction of nitrocyclohexane in an electrolytic cell system having an anode compartment and a high hydrogen overvoltage potential cathode compartment isolated from each other by a low electrical resistance membrane and having electrolyt solution in each of the compartments comprising suspending nitrocyclohexane in the electrolyte solution of the cathode compartment having a pH value of from about 7 to about 9.5, passing an electric current through the anode, cathode, electrolyte and suspended nitrocyclohexane, maintaining the potential of the cathode at a value of from 1.4 to -1.7 volts, as against a mercury-mercurous sulfate-saturated potassium sulfate reference electrode, forming cyclohexylhydroxylamine at the cathode, precipitating the cyclohexylhydroxylamine thus formed at the cathode, and collecting the cyclohexylhydroxylamine by removing the product from the cathode compartment of th cell.

3. The process of making cyclohexylhydroxylamine according to claim 2 wherein the temperature of the electrolytic system is maintained at between 20 to 30 C.

4. The process of making cyclohexylhydroxylamine according to claim 2 wherein the high hydrogen overvoltage potential cathode is made of mercury.

5. The process of making cyclohexylhydroxylamine in a continuous electrolytic system by the electrolytic reduction of nitrocyclohexane comprising suspending nitrocyclohexane in an aqueous electrolytic system having substantially neutral pH, passing an electric current through the electrolyte and suspended nitrocyclohexane, reducing the nitrocyclohexane at a high overpotential cathode to form cyclohexylhydroxylamine, precipitating the cyclohexylhydroxylamine thus formed, removing the. product thus formed out of the electrolyte system, and feeding the electrolyte system with additional amounts of nitrocyclohexane to continue the electrolysis process with the formation of additional amounts of cyclohexylhydroxylamine.

6. The process of making cyclohexylhydroxylamine in a continuous electrolytic system by the electrolytic reduction of nitrocyclohexane comprising suspending nitrocyclohexane in an aqueous electrolyte system having substantially neutral pH, maintaining the temperature of the system at from to C., passing an electric current through the electrolyte and the suspended nitrocyclohexane, reducing the nitrocyclohexane at a high overpotential cathode to form cyclohexylhydroxylamine crystals, removing the cyclohexylhydroxylamine crystals thus formed by sublimation, and feeding the electrolyte system with additional amounts of nitrocyclohexane to continue th electrolysis process with the formation of additional amounts of cyclohexylhydroxylamine.

References Cited Allen, M. 1., Organic Electrode Processes, Chapman & Hall Ltd., London, 1958, pp. 49-55.

JOHN H. MACK, Primary Examiner H. M. FLOURNOY, Assistant Examiner (5/69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,472, 748 Date October 14. 1969 Inventofls) Herman H. Stein It is certified that error appears in the above- :[dentified patent and that said Letters Patent are hereby corrected as shown below:

' Column 5, Claim 2, line 2, insert "an aqueous" after "having".

SIGNED ML REALEU DEC 1% Q Atteat:

d E WartiM 0161161311: I. .I H. m Attestmg Offieer Gomissiom or Pam!