US3062879A - Process for preparing perfluoroalkanedioic acids - Google Patents

Description

United States Patent PROCESS FOR PREPARING PERFLUORGALKANE- DIOIC ACIDS John J. Drysdale, Wilmington, Del., assignor to E. I. (11! Pont de Nemours and Company, Wilmington, Del., a

corporation of Delaware No Drawing. Filed May 15, 1961, Ser. No. 109,822

7 Claims. (Cl. 260--537) This invention relates to a process for preparing perfluoroalkanedioic acids and more particularly for preparing such acids by the hydrolysis of certain bis(alkylthio)- perfluoroalkanes.

Perfiuorodicarboxylic acids of structure are a valuable series of known compounds. Those, wherein m is greater than one, are known to be useful intermediates for the preparation of polyester lubricants and the like which are highly stable thermally and oxidatively (OBrian et al., W.A.D.C., Tech. Rept. No. 56-11, A.D. 110674). Several methods are known for preparing perfluorodicarboxylic acids but they are, in general, rather poor, requiring starting materials which are not readily obtained or involving reactions which give poor yields at best. It is an object of this invention to provide an improved process for preparing perfluoroalkanedio-ic acids. Another object is to provide a process for preparing perfluoroalkanedioic acids in high yields from readily available starting materials. A further object is to provide a process which is simple and readily carried out and produces the perfluoroalkanedioic acids in a more economical manner. Another object is to advance the art. Still other objects will appear hereinafter.

The above and other objects may be accomplished in accordance With this invention which comprises heating a bis(alkylthio)perfluoroalkane of the formula wherein R is an alkyl radical of 1 to 6 carbon atoms and n is an integer of from 2 to 20 with from about 1 to about 4 parts by weight of concentrated sulfuric acid of from about 80% to 100% strength at a temperature bis(alkylthio)perfiuoroalkanes are readily available materials known to the art. The process is easy to operate and control and economically attractive.

The bis(alkylthio)perfluoroalkanes which may be employed .in the process of this invention have the formula 'RS(CF CF ),,SR wherein R is an alkyl radical of l to '6 carbon atoms and n is an integer of from 2 to 20. While R may be almost any saturated alkyl radical, it is desired (primarily for economical reasons) that the alkyl radicals contain not more than 6 carbon atoms, and preferably methyl. employed in this invention conveniently are prepared by the method described by Hanford in US. Patent 2,443,- 003, and particularly in Example 2 thereof. By such process, there is produced a series of compounds of the formula RS(CF CF ),,SR wherein n has a series of in- The bis alkylthio perfiuoroalkanes tegral values of 1 to 25 or perhaps greater. Representative bis(alkylthio)perfluoroalkanes which are suitable for the purpose of this invention are: l,4-bis(methylthio)- perfluorobut ane (CH S(CF SCH 1,6 bis(methylthio)perfiuorohexane (CH S(CF SCH 1,8-bis(methylthio)perfiuorooctane (CH S(CF SCH 1,10 bis- (methylthio)perfluorodecane (CH S(CF SCH 1,12- bis(methylthio) perfluorododecane (Cl-I S(CF SCH l, 14-bis(methylthio)perfluorotetradecane 1,15 bis(methylthio)perfluorohexadecane and 1,18-bis(methylthio)perfluorooctadecane.

The sulfuric acid employed should have a concentration of from about to 100% strength. Usually, the commercial concentrated sulfuric acid of about to about 98% strength will be employed. Also, the sulfuric acid should be employed in the proportion of from about 1 to about 4 parts by weight per part of the bis(alkylthio perfluoroalkane.

The temperature for carrying out the reaction may be from about 100 C. to about 350 C., and preferably from'about C. to about C. The reaction time usually will vary from about 1 to about 10 hours, depending upon the particular starting material, the temperature employed, and economical factors such as the cost of more extended periods of heating and the increase in yield to be obtained thereby.

A number of methods are available for recovering the product from the reaction mixture. One method, which was employed in the examples presented hereinafter and which is suitable for those acids which-are reasonably soluble in dilute aqueous sulfuric acid, is to add the cooled reaction mixture to cooled water, filter the resulting aqueous solution, and then extract the dicarboxylic acid from the aqueous sulfuric acid with a suitable solvent. The perfiuoroalkanedioic acids which contain up to about 12 carbon atoms are reasonably soluble in water and in dilute aqueous sulfuric acid solutions, but can be crystallized from the aqueous solutions as the dihydrates. During the reaction, certain amounts of sulfur and tarry material are formed which are insoluble in the dilute aqueous solution, and such materials and unreacted starting material are removed by filtration of the cold dilute sulfuric acid solution. The perfiuoroalkanedioic acid will be extracted from the aqueous solution by a lower dialkyl ether such as diethyl ether, dipropyl ether and dibutyl ether. The extracted solvent is then evaporated to leave a residue of the perfluoroalkanedioic acid. Such perfiuoroalkanedioic acid sometimes will contain a small amount of partially hydrolyzed material having the formula RS(CF CF ),,CO H which is Water soluble and is extracted with the ether. If necessary, the perfiuorodialkanedioic acid may be purified by recrystallization, distillation, or the like.

When the perfiuoroalk anedioic acids are insufficiently soluble in dilute aqueous sulfuric acid, i.e. contain more than 12 carbon atoms, the cooled reaction mixture can be added to cooled water and the resulting mixture made basic with alkali, such as sodium hydroxide, sodium carbonate, and the like, to form the water-soluble alkali salts. After filtering the salt solutions to remove sulfur,

tarry material and unreacted starting material, the solution can be reacidified to precipitate the periluoroalkanedioic acid. In this case also, partially hydrolyzed material (if present) will be carried into the perfluoroalkanedioic acid, which can be further purified by recrystallization, distillation, etc.

Representative acids which may be prepared by the present process include perfluorosuccinic acid perfiuoroadipic acid (HO C(CF CO H), perfluorosuberic acid (HO C(CF CO H), perfluorosebacic acid (HO C(CF CO H) perfluorododecanedioic acid (HO C(CF CO H), perfiuorotetradecanedioic acid (HO C(CF CO H), and perfluorooctadecanedioic acid (HO C(CF CO H).

In order to more clearly illustrate this invention, preferred modes of carrying it into effect, and the advantageous results to be obtained thereby, the following examples are given in which the parts and proportions are by weight except where indicated otherwise.

Example 1 1,6-bis(methylthio)perfluorohexane (29.4 parts) was heated with 40 parts of concentrated (95-98%) sulfuric acid at 150 C. for one hour. The reaction mixture was then cooled and added to ice-water. The aqueous phase was filtered to remove sulfur and tarry material, and the filtrate extracted with diethyl ether. The ether extracted with diethyl ether. The ether extract, on drying and evaporation, gave parts of perfiuoroadipic acid as di hydrate (62% yield). The acid was identified by converting it to several known derivatives such as the dianilinium salt, the dimethylester, the diethylester, and the diacid chloride.

Example 2 1,4-bis(methylthio)perfluorobutane (29.4 parts) was heated with parts of concentrated (95-98%) sulfuric acid at 165 C. for two hours. The perfluo'rosuecinic acid as the dihydrate (16.8 parts, 74% yield) was isolated as in Example 1. The acid was identified via its known di anilinium salt and dimethyl ester.

Example 3 l,8-bis(methylthio)perfluorooctane (39.4 parts) was heated with stirring with 40 parts of concentrated (95 98%) sulfuric acid for one hour at 155 C. to 160 C. The perfluorosuberic acid as the dihydrate (23.4 parts, 69% yield) was isolated as in Example 1, and identified via its dianilinium salt, M.P. 183 C., and the diacid chloride, B.P. 112 C./l mm.

Example 4 l,6-bis(methylthio)perfluorohexane (29.4 parts) was heated with 91 parts of concentrated (95-98%) sulfuric acid for six hours at 165 C. The perfluoroadipic acid as the dihydrate (19.4 parts, 78% yield) was isolated as in Example 1.

In place of the bis(methylthio)perfluoroalkanes used in the above examples, the bis(ethylthio), bis(propylthio), bis(butylthio), and the like perfluoroalkanes can be used with equal facility.

An alternate method of isolation of the perfluoroalkanedioic acids has been found which avoids the extraction with the highly flammable ethers as disclosed in the foregoing examples. In this alternate method, the reaction mixture is added to a cold mixture of water and a lower alcohol, such as methanol, ethanol, propanol and isopropanol, preferably methanol, preferably employing about a fourfold excess of alcohol over that theoretically required to convert the perfiuoroalkanedioic acid to the diester. Thereby, the perfiuoroalkanedioic acids are converted to their diesters, which are materially more soluble in the aqueous alcohol solutions. The cold mixture is then filtered to remove the insoluble by-products and the filtrate heated to about 60 C. and continuously extracted with an immiscible solvent in a continuous extraction apparatus. Suitable immiscible solvents are: 1,2-dichloroethane, chloroform, methylene dichloride, trichlorotrifiuoroethane, and dibutyl ether. The diester is collected by evaporation of the solvent, and separated from any partially hydrolyzed product by distillation of the residual ester. Using this method and the reaction procedure of Example 2, a 67% yield of dimethyl perfiuorosuccinate, B.P. 1l2-113 C./100 mm., is obtained. Other esters isolated in this manner are dimethyl perfluoroadipate, B.P. l20-121 C./50 mm., and diethyl perfiuoroadipate, B.P. 101 C./ 10 mm. The esters are then hydrolyzed, using dilute aqueous acid, to the acids which are collected by evaporation of the aqueous solution. For many purposes, the esters may be used in place of the acids for preparing useful derivatives.

It will be understood that the preceding examples have been given for illustrative purposes solely and that this invention is not limited to the specific embodiments described therein. On the other hand, it will be readily apparent to those skilled in the art that, subject to the limitations set forth in the general description, many variations can be made in the materials, proportions, and conditions employed without departing from the spirit or scope of this invention.

From the preceding description, it will be apparent that the present invention provides a new and improved process for preparing perfiuoroalkanedioic acids readily and economically in a single reaction step from readily available starting materials. Accordingly, it will be apparent that this invention constitutes a valuable advance in and contribution to the art.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The process for preparing a perfiuoroalkanedioic acid which comprises heating a bis(alkylthio)perfluoroalkane of the formula RS(CF CF ),,SR wherein R is an alkyl radical of 1 to 6 carbon atoms and n is an integer of from 2 to 20 with from about 1 to about 4 parts by weight of concentrated sulfuric acid of from about to strength at a temperature of from about 100 C. to about 350 C., and recovering the perfluoroalkanedioic acid from the reaction mixture.

2. The process for preparing a perfiuoroalkanedioic acid which comprises heating a bis(alkylthio)perfluoroalkane of the formula RS(CF CF ),,SR wherein R is an alkyl radical of 1 to 6 carbon atoms and n is an integer of from 2 to 20 with from about 1 to about 4 parts by weight of concentrated sulfuric acid of from about 80% to 100% strength at a temperature of from about C. to about C., and recovering the perfiuoroalltanedioic acid from the reaction mixture.

3. The process for preparing a perfiuoroalkanedioic acid which comprises heating a bis(alkylthio)perfluoroalkane of the formula RS(CF CF SR wherein R is an alkyl radical of 1 to 6 carbon atoms and n is an integer of from 2 to 20 with from about 1 to about 4 parts by weight of concentrated sulfuric acid of from about 95% to about 98% strength at a temperature of from about 150 C. to about 175 C., and recovering the perfluoroalkanedioic acid from the reaction mixture.

4. The process for preparing a perfiuoroalkanedioic acid which comprises heating a bis(alkylthio)perfluoro alkane of the formula RS(CF CF SR wherein R is an alkyl radical of l to 6 carbon atoms and n is an integer of from 2 to 6 with from about 1 to about 4 parts by weight of concentrated sulfuric acid of from about 95% to about 98% strength at a temperature of from about 150 C. to about 175 C., and recovering the perfiuoroalkanedioic acid from the reaction mixture.

5. The process for preparing perfiuoroadipic acid which comprises heating a bis(alkylthio)perfluorohexane 5 of the formula RS(CF CF SR wherein R is an alkyl radical of l to 6 carbon atoms with from 1 to about 4 parts by weight of concentrated sulfuric acid of from about 95% to about 98% strength at a temperature of from about 150 C. to about 175 C., and recovering perfluoroadipic acid from the reaction mixture.

6. The process for preparing perfluorosuccinic acid which comprises heating a bis(alkylthio)perfiuorobutane of the formula RS(CF CF SR wherein R is an alkyl radical of 1 to 6 carbon atoms with from 1 to about 4 parts by Weight of concentrated sulfuric acid of from about 95% to about 98% strength at a temperature of from about 150 C. to about 175 C., and recovering perfluorosuccinic acid from the reaction mixture.

7. The process for preparing perfluorosuberic acid which comprises heating a bis(alkylthio)perfiuorooctane of the formula RS(CF CF SR wherein R is an alkyl radical of 1 to 6 carbon atoms with from 1 to about 4 parts by weight of concentrated sulfuric acid of from about 95% to about 98% strength at a temperature of from about 150 C. to about 175 C., and recovering 10 perfluorosuberic acid from the reaction mixture.

No references cited.

Claims (1)

1. THE PROCESS FOR PREPARING A PERFLUOROALKANEDIOIC ACID WHICH COMPRISES HEATING A BIS(ALKYLTHIO)PERFLUOROALKANE OF THE FORMULA RS(CF2CF2)NSR WHEREIN R IS AN ALKYL RADICAL OF 1 TO 6 CARBON ATOMS AND N IS AN INTEGER OF FROM 2 TO 20 WITH FROM ABOUT 1 TO ABOUT 4 PARTS BY WEIGHT OF CONCENTRATED SULFURIC ACID OF FROM ABOUT 80% TO 100% STRENGTH AT A TEMPERATURE OF FROM ABOUT 100* C. TO ABOUT 350*C., AND RECOVERING THE PERFLUOROALKANEDIOIC ACID FROM THE REACTION MIXTURE.