US2192142A - Process for preparing alpha-substituted acrylic acids - Google Patents

Description

Patented Feb. 27, 1940.-

PROCESS FOR PREPARING lit-SUBSTITUTED ACRYLIC ACIDS Erich F. Meitzner, Philadelphia, Pa., casino: to

Biihm 8: Hana Company, Philadelphia No Drawing. Application June 3, 1938,

Serial No. 211,802

Claims.

This invention relates to a process for the manufacture of alpha-substituted acrylic acids. It relates more particularly to a process whereby such acids are formed by oxidizing an alphasubstituted methyl vinyl ketone. bymeans of alkaline hypohalites. 3

It is. known that cinnamic acid can be prepared by oxidizing methyl p-phenylvinyl ketone with hypohalites and that the same oxidizing agent will oxidize methyl pfi-dimethylvinyl ketone to 5.,3-dimethyl acrylic acid but prior to the present invention no one had succeeded in oxidizing a methyl a-alkylvinyl ketone to the corresponding Tit-alkacrylic acid.

The ketones with which the present invention is concerned have the general formula in which R is an alkyl, aryl or aralkyl group and differ from those mentioned'above in that they contain a terminal methylene group and are susceptible to polymerization. The ketones which do not contain this terminal methylene group do not polymerize.

It has been found that ketones of the above general formula may be oxidized in an alkaline solution by means of hypochlorites to the corresponding a-SllbStitutQd acrylic acid of the general formula The ,5 unsaturated ketones may be prepared by condensing formaldehyde with a methyl alkyl ketone in the presence of an alkaline condensing agent and subsequently dehydrating the ketol thus formed.

The reaction by which the unsaturated ketone is converted to the a-alkacrylic acid may be expressed as follows:-

This method makes it possible to produce the higher a-alkyl homologs of acrylic acid from which various branched chain carboxylic acids may be prepared which hitherto have not been available. Many attempts have been made to convert the higher methyl alkyl ketones to With the exception of methacrylic acid such syntheses give only small yields of the desired a- .alkaline solution of the hypochlorite.

alkacrylic acid or else'the cyanhydrine reverts to the ketone and hydrogen cyanide when the attempt to dehydrate and hydrolyze it is made. In many cases the cyanohydrine could not be dehydrated and in others large amounts of the afi-dialkylacrylic acid were formed. Other attempts have also been made to prepare the higher a-alkacrylic acidsby treating an u-bromo aliphatic acid with zinc dust and formaldehyde and then dehydrating the fi-hy'droxy acid obtained according to the Reformatsky reaction but this reaction gives very poor yields and is exceedingly difllcult to control. It is entirely unsuited to large scale operations. v

On the other hand, the haloform reaction, viz: the oxidation of the e-unsaturated ketone by means of hypochlorites, is very easy to control and gives yields of acid ashigh as 70% ofthe theoretical. The reaction proceeds smoothly at low temperatures and thus the danger of loss due to polymerization, side reactions or shift of the double bond is reduced to aminixnum.

Practically all hypohalites are operative and give good yields but in general the alkali metal hypochlorites give better yields than the others and are cheaper. Potassium hypochlorite usually gives better yields than sodium hypochlorite. The hypobromites and hypoiodites generally give somewhat lower yields than the hypochlorites and are not so economical on account of their higher cost.

The reaction is carried out in general by adding the unsaturated ketone in small portions or in a'slow stream to a cooled and well-agitated The hypochlorite solution should contain between 12 and 20% of available chlorine and approximately 2% of free alkali at the start of the reaction. The chloroform which is formed in the reaction separates from the water and carries with it a fairly large amount of the ketone which is more soluble in it than in the water. For this reason the rate of reaction decreases as the amount of chloroform increases and in order to overcome this it is advisable to use an amount of ketone about 20% in excess of that theoretically required to react with all of the hypochlorite. The reaction mixture is well stirred throughout and after all the ketone has been added the agitation is continued until a testshows that all of the hypochlorite has been consumed. The mixture is then allowed to settle, the chloroform drawn oilf and redistilled to recover any unreacted ketone. The aqueous solution is then acidified with an inorganic acid and the a-alkyl acrylic acid extracted with. a water-immiscible solvent.

In carrying out the reaction it is preferable to add the ketone to the alkaline hypohalite solucooled to 10.

tion rather than vice versa because, when methyl isopropenyl ketone, for example, is agitated in a dilute alkaline solution, heavy oily condensation or polymerization products are formed. Slmultaneous addition ofthe two reactants to the reaction vessel oifers no particular advantageover adding the ketone slowly to the hypohalite solution.

In the case of the lower members of the ketone series, it is best to work at about l5-.25 C. whereas with higher ones temperatures up to 40 C. may be employed. with methyl isopropenyl ketone, for example, secondary reactions take place above 35 C.-leading to increased consumption of hypohalite and reduced yields of the methacrylic acid- The best concentration ofhypochloride is that corresponding to 12-20% ofavailable chlorine. Such solutions can be prepared by passing chlorine int'oan alkali hydroxide solution until the solution will just turn red litmus blue and then bleach it within two or three seconds. In such solutions there is a free alkali content of about 1 to 3%. If the solution contains less than about 12% of available chlorine or if the initial concentration of free alkali is much above 3%, the yields of the a-alkacrylic acid are reduced. The same is true if the alkali formed during the reaction is simultaneously neutralized with sulfuric acid.

The addition of inert water-miscible solvents to increase the solubility of the ketone and retain the chloroform in solution does not increase the yield of alkacrylic acid. In fact, when such solvents as dioxane or ter-butanoi are employed, the yield is actually decreased and the recovery of the product is unnecessarily complicated.

The following examples will serve to illustrate the invention which, however, is not limited to the exact conditions and materials shown as it may be otherwise practiced within the scope of the appended claims.

Example 1 Potassium hypochlorite solution was prepared by dissolving 360 g, of 85% potassium hydroxide in about 1 liter of water. The solution was cooled and, after adding some ice, chlorine was passed in until litmus paper was first blued, but bleached after about 2-3 seconds. The available chlorine was then 14%, the total volume 1230 cc. The solution contained an excess of free alkali of about 2.5-3%. It is important that the hypochlorite solution contains an excess of alkali at the beginning of the-haloform reaction.

Eight hundred fiity cc. of this solution of potassium hypochlorite, containing 14% of available chlorine, was placed in the reaction vessel and While stirring, 48 g. of methyl a-methylvinyl ketone was added gradually over a period of two hours, care being taken that the temperature did not exceed After about five hours, the test for hypochlorite became negative. Stirring was then discontinued and the chloroform (43 g.) drawn ofl at the bottom. The aqueous layer was then acidified with dilute sulfuric acid, until a pH of 3 was reached. The potassium sulfate, which crystallized, was removed by filtration, and the filtrate extracted repeatedly with ethylene dichloride. After adding a suitable inhibitor (a mixture of p-naphthol and sulfur), the ethylene dichloride was distilled.

.The residual methacrylic acid was purified by distillation; B. PHYS-80 0.]35 mm. The yield was 27- g., or 55% of the theoretical.

condensation of para-formaldehyde and methyl n-propyl ketoneand subsequent dehydration of the resulting ketol.

Fifty-one grams of the ketone was added during one hour to 1050 cc. of a solution of potassium hypochlorite containing 12.4% of available chlorine. when the test for hypochlorite had become negative (after about four hours) the chicroform layer was separated. It weighed 43.5 g.: the unchanged ketone can be recovered by fractionation. The aqueous layer was acidified with dilute sulfuric acid, the potassium, sulfate removed by filtration, and the a-ethyl acrylic acid was extracted with ethylene dichloride. After distilling the ethylene dichloride, 17 g. of a-ethyl acrylic acid (33% of the theoretrical) was obtained, boiling at 90-92 C./ mm.

I Example 3 Methyl a-isopropylvinyl ketone,

' on|o0-o=cm B. P. 80-84" C./175 mm., was obtained by condensation of methyl isobutyl ketone and paraformaldehyde and dehydration of the resulting ketol.

Sixty grains of methyl e-isopropylvinyl ketone was gradually added during one hour to 600 cc. of a solution of potassium hypochlorite containing 18% of available chlorine. The temperature was kept at -40 for four hours. The top layer (37 g.) consisting of a mixture of chloroform and unchanged ketone was separated. The aqueous layer was acidified with dilute sulfuric acid and extracted withethylene dichloride. The ethylene dichloride was distilled and the residual acid purified by distillation. Yield 24 g., B. P. 90-95 C./ 17 mm. The chloroform layer was fractionsium hypochlorite containing 18.6% of available chlorine. The temperature was kept at 30-40 C. After stirring for five hours, the chloroform layer g.) was separated. The aqueous solution was acidified and the a-butyl acrylic acid was extracted with ethylene dichloride. 'By fractional distillation, the a-butyl acrylic acid was obtained, boiling at 113-115 C./ 18-20 mm. The yield was 20.3 g., or 41% of the theoretical.

I claim:

1. The process of producing a-alkacrylic acids,

which comprises oxidizing an unsaturated ketone of the general formula capo-c o-cm inwhichnisamemberofthegroupconsisting of alkyl, aryl and aralkyl radicals with an aqueous alkaline solution of a metal hypcchlorite containing from about 12 to about 20% of available chlorine and initially less than about 3% of free alkali.

2. The process 01' producing a-alkacrylic acids, which comprises oxidizing an unsaturated ketone of the general formula 7 ceeding about25 c.

in which R is a-member of the group consisting of alkyl, aryl and aralkyl radicals with an aqueous alkaline solution of a metal hypcchlorite containing from about 12 .to about 20% 01' available chlorine and initially less than about 3% 01 free alkali at temperatures not exceeding about 40 C.-

3. The process 01' preparing methacrylic acid which comprises oxidizing methyl a-methylvinyl ketone with an alkaline solution 01. potassium hypcchlorite containing from about 121m about 20% of available chlorine and initially less than about 3% of free alkali at temperatures not exceeding about 25 C.

4. The process of preparing a-ethyl acrylic acid which comprises oxidizing methyl a-ethylvlnyl ketone with an alkaline solution of potassium hypcchlorite containing from about 12 to about 20% of available chlorine and initially less than about 3% of free alkali at-temperatures not ex- 5. The process oi! preparing a-n-butyl acrylic acid which comprises oxidizing methyl a-n-butylvinyl ketone with an alkaline solution of potassium hypochlorite containing from about 12 to about 20% of available chlorine and initially less than about 3% of free alkali at temperatures not exceeding about 40 C.

"' ERICH F. Mm'rzm. 2o