2,891,090 United States Patent Office Patented June 16, 1959 1. 2 ous treated medium and produce an apparent loss in 2,891,090 yield of the derivative being purified. This apparent loss PURFICATION OF SALCYLCACD can be overcome by treating the derivative of salicylic DERVATIVES acid with an aqueous solution of rather high concentra tion of said water-soluble derivative also containing the John Robert Campbell, Webster Groves, Charles M. small amount of the polybasic acid required to accom Eaker, Kirkwood, and Emory H. Kimball, Webster plish the removal and/or destruction of the colored in Groves, Mo., assignors to Monsanto Chemical Corn purities. Water-insoluble liquid and relatively low melt pany, St. Louis, Mo., a corporation of Delaware ing derivatives of salicylic acid can be treated by the No Drawing. Application December 30, 1954 O process of this invention by adding the dilute aqueous Serial No. 478,884 solution of phosphoric acid as a water wash for the liq uid or liquefied product. In addition, where the process 5 Claims. (CI. 260-474) of this invention is employed to purify water-insoluble derivatives of salicylic acid which are normally solid, This invention relates to the purification of derivatives 15 the dilute aqueous solution of polybasic acid can be used of salicylic acid and more specifically pertains to an to wash the crystalline product at any one of the recrys improved process for the purification of derivatives of tallizing steps but preferably is used as a wash for the salicylic acid whereby colored impurities are removed crystalline product resulting from the last recrystalliza and eliminated from derivatives of salicylic acid. tion step. Derivatives of salicylic acid as prepared on an indus 20 Also, a water-soluble normally solid derivative of sali trial scale sometimes contain colored impurities which cylic acid can be treated by the process of this invention not only detract from the appearance of the finished by recrystallizing the derivative from a solution in a product, but also render the finished product unsatis water-miscible solvent by adding a dilute aqueous solu factory for its ultimate use since many of the ultimate tion of the polybasic acid at or just prior to the time uses of derivatives of salicylic acid require that the prod 25 crystals begin to precipitate from the solvent solution. uct be free from colored impurities. This is especially Where the process of this invention is carried out by add true in the case where derivatives of salicylic acid are ing the dilute aqueous solution of phosphoric acid to a used as a pharmaceutical such as in the case of salol solution of the derivative of salicylic acid, a water (phenylsalicylate) or in the case of aspirin (acetylsali miscible solvent, a sufficient amount of the dilute aque cylic acid). Colored impurities are also undesirable in 30 ous solution of phosphoric acid is added to provide suf phenylsalicylate when used as an ultraviolet absorption ficient water to promote substantially complete crystal agent in protective oils, creams, and lotions employed lization of the derivative being purified. to protect the skin from ultraviolet rays. When deriva The most convenient and preferred water-soluble poly tives of salicylic acid are prepared on a laboratory scale basic acids for use in the process of this invention are in all glass equipment, the colored impurities do not ap phosphoric, citric and tartaric acids. However, any of pear in the final product. However, when derivatives of the other water-soluble polybasic acids can be employed salicylic acid are prepared on the industrial scale where where the salicylic acid derivative is not to be used for although most of the equipment is glass or glass-lined, pharmaceutical preparations for internal use. The there are, of course, some non-glass members, and col amount of phosphoric, citric or tartaric acids to be ored impurities do appear. - 40 employed in the process of this invention will, of course, Also industrially prepared derivatives of salicylic acid, depend upon the amount of impurity present. However, especially those derivatives of salicylic acid which are in most cases where it is required to remove the impuri ordinarily solids such as those derivatives having a melt ties appearing as relatively small numbers of discolored ing point of above about 35 C., will not have a uniform crystals, it will be found that the use of no more than discoloration throughout each and every crystal or par 45 about 1% by weight of phosphoric, citric or tartaric ticle of the product, but rather will have individual crys acids will produce satisfactory results. The percentage tals that have an appearance of being from slightly dirty by weight of the acid utilized being based upon the sum to almost black. Where the derivative of salicylic acid of the weights of the aqueous solution containing the can be distilled, distillation appears to be of some help acid and the derivative being treated where it is not in in eliminating the presence of these colored impurities. 50 solution or the solution of the derivative. It will be But ordinarily, it is impractical to purify solid derivatives readily understood by those skilled in the art that where of salicylic acid by distillation. Practically all these solid much more of the colored impurities exist, larger amounts derivatives are recovered by crystallization processes and of the non-toxic water-soluble polybasic acid will be re purified by recrystallization from various solutions. quired. However, in treating the products resulting from However, even after the solid derivatives of salicylic acid 55 well-controlled, regulated industrial processes for pre have been subjected to recrystallization, the final product paring derivatives of salicylic acid, it will be found that still contains gray to almost black individual particles or the amount of the non-toxic water-soluble polybasic acid crystals even though all practical precautions have been to be used will be in the range of about 0.1% or less to taken to eliminate contamination of the product's dark 1% by weight based on the sum of the total weight of colored impurities. - 60 all materials employed in the step. - An improvement for the process for purifying deriva The process of this invention is not dependent upon tives of salicylic acid has now been discovered. This the use of any particular derivative of salicylic acid, but improvement comprises treating the derivatives of sali rather can be employed with any derivative of salicylic cylic acid with a dilute aqueous solution of a water acid as hereinbefore indicated. The problem of colored soluble acid, preferably a non-toxic polybasic acid such 65 impurities appearing in the desired derivative of salicylic as those used in foodstuffs as for example phosphoric acid appears to be in part linked with the free salicylic acid, tartaric acid and citric acid. The process of this acid content of the derivative rather than the nature of invention can be used in the purification of water-soluble the particular derivative. However, free salicylic acid as well as water-insoluble derivatives of salicylic acid. content of the derivative is apparently not the only cause Obviously, when employing the process of this inven 70 of the presence of colored impurities for a salicylic acid tion purifying water-soluble derivatives of salicylic acid, derivative can have an acceptable low free salicylic acid a portion of the derivative will be dissolved in the aque a content and still contain colored impurities. In this case, 2,891,090 3. 4. the colored impurities may have been caused by free Example III salicylic acid reacting with some substance during the Wet technical grade of acetyl salicylic acid crystals, operation or some derivative causing the formation of the product obtained by cooling in stainless steel equip a colored impurity. Regardless of the cause of the ment the reaction mixture resulting from the reaction of colored impurity, the process of this invention reduces acidic anhydride with salicylic acid and glacial acetic the free salicylic acid content of the desired derivative acid are employed in the process of this invention. This even more than does the recrystallization process involv technical grade crystalline product has a free salicylic ing the use of a water-miscible solvent and the addition acid content of 0.030% and a methanol solution of these of cold water to the solution. The process of this in crystals has a color corresponding to an A.P.H.A. No. vention also removes colored impurities present in a O of 12. There are combined in the following proportions: derivative of salicylic acid in the presence of or in the 100 parts of methanol and 0.1 part of decolorizing char substantial absence of free salicylic acid. coal for each 100 parts of the above technical grade of Hence, the process of this invention can be used to crystals. This mixture is heated to about 50° C. for purify but is not limited to the purification of the follow 10 minutes, filtered and then cooled. The cooling metha ing derivatives of salicylic acid. Esters of Salicylic acid nol solution is seeded with crystals of acetyl salicylic such as methyl salicylate, ethyl salicylate, butyl Salicylate, acid and then a cold aqueous solution containing 1% by propyl salicylate, isopropyl salicylate, 2,4-dinitrobenzyl weight of phosphoric acid is added thereto over a period salicylate, tolyl salicylates, naphthyl salicylates, trisalicylin of about one hour. After all the dilute phosphoric acid furfuryl salicylate, and cyclohexyl salicylate. Such ether had been added, the resulting mixture is cooled to about derivatives such as methoxy salicylic acid, ethoxy Salicylic 20 15° C., filtered and washed with ice water.