D u a n e W. Knight1 and G e o r g e H. C l e l a n d
Occidental College 10s Angeles, California 90041
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The Detection of Carboxylic Acids by Formation of Ferric Hydroxamates An improved method
A
common test for the carboxylic acid function in qualitative organic analysis involves as a first step the reaction of the carboxyl group with thionyl chloride to give an acyl chloride (or in a few instances an acid anhydride). Subsequent reactions of these substances with hydroxylamine and then with ferric ion yield ferric hydroxamates, which a t low pH values show characteristic dark red, violet, or purple colors.a The methods commonly used for conversion of acyl chlorides to the corresponding hydroxamic acids vary somewhat in detaiP4 but they share in common the presence of one or several hydroxylic substances (water, alcohol, hydroxide ion) in the reaction mixture containing acyl halide and hydroxylamine. Certain failures have been encountered in organic laboratory courses here when these tests were employed. Some types of acids are known to give negative results3 because of ,alternate reactions with thionyl chloride, but it seemed reasonable that other failures were due to competitive reactions of acyl chloride with water, alcohol, or hydroxide ion to the extent that hydroxamic acid formation was reduced or eliminated. For this reason an alternate procedure was developed wherein, after forma tion in the usual manner, the acyl chloride was converted to the hydroxamic acid by treatment with hydroxylamine hydrochloride and then with an excess of pyridine. The advantages of this variation are: free hydroxylamine is generated by the pyridine; all competitive reactions of the acyl chloride with hydmxylic substances are eliminated; pyridine is a very effective agent for the promotion of many acylation reactionsin this case, of hydroxylamine by acyl chloride. The method has given excellent results during the past year, both in connection with certain research programs and in qualitative analysis in the first-year organic chemistry course. The reaction has been investigated, in connection with this article, with about fifty acids; the failures encountered were, in the main, predictable on the basis of previous observation^.^ Experimental The substance to be tested (20-30 mg) in a small test tube was treated with 3-4 drops of thionyl chloride, the mixture was refluxed for 3 0 4 0 sec, and excess thionyl chloride was removed by a stream of dry ilir.5.' Hydroxylamine hydrochloride (10-15 mg) was introduced and then pyridine was added dropwise until its odor was apparent after gentle warming. The mixture was refluxed for 2 0 4 0 sec, cooled, and treated with drops of concentrated hydrochloric acid until the pH was 1-3. Methanol (-2 mlI7 and then several drops of 10% (wt) aqueous ferric chloride
were added; water was introduced dropwise until a characteristic color developed.' The acids listed gave definite positive tests under these condit i o n ~ :acetic, ~ propianic, hutyric, hexanoic, heptanoic, decanoic, tetradecanoic, hexadecanoic; acrylic, methacrylic, crotonic, 2,4hexadienoic; succinic, adipic, suheric; pymvic; rnaleic; ~henoxvacetic: henzoie. and henzoic acid with the followine substituents in c+, m-,and p-positions: chloro, methaxy, nitro; phenylacetic, diphenylacetic; cinnamic, o-, m-, and p-nitrocinnamic, a-, m-, and pchlorocinnmnic; 1- and 2-naphthoic; phthalic, isophthalie, and terephthalic. The acids listed next gave tests which must be regarded as either inconclusive or else clearly negative: malanic, a-hramopropionic, oxalic, irichloroacetic, mandelic, tartaric, citric monohydrate, aceturic, glycine.
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This research was supported in part by grants from Research Corpor&m and from the National Science Foundation Undergraduate Research Participation program. 1 Present address: 2nd Lt. Duane W. Knight, Pilot Training Squadron, Vance Air Force Base, Enid, Oklahoma. "he color dependence on pH of the ferric hydroxamates is discussed by AKSNES,G., Aeta Chem. Scand., 11, 719 (1957). 8 CHERONIS, N. D., ENTRIKIN, J . B., AND HODNETT,E. M., "Semimicro Quditstive Organic Analysis," (3rd ed.), Interscience Publishers, (division of John Wiley & Sons, Inc.) New York, 1965, pp. 363, 366-7. 4 FEIGL, F., "Spot Tests in Organic Analysis," (6th ed.), Elsevier Publishing Co., (D. Van Nostrand Co., Inc.) Princeton, N. J., 1960, p. 250. 6 Each step from this point i s accompanied by shaking 07 stirring. 8 Acetic acid gave 8. positive test; this result was unexpected because of the relative boiling points of acetyl chloride and thionxl chloride. The removal of thionyl chloride by an air-stream must be conducted with same caution when it is suspected that the acyl chloride has a. fairly high vapor pressure. Many of the ferric hydroxamates tested gave characteristic colors at low pH values when treated with water. Some others required the pkor treatment with methanol in order to make them soluble enough to detect the characteristic color. Water enhances the typical color, methanol depresses the typical color, so that in some cases avery careful balance of water and methanol must be made. No ferric hydroxamate that gave a positive test with water alone failed to give a positive test when the methanol trestment was employed. It is sometimesprofitable to centrifuge a red oil or solid and then to treat the supernatant with a little water in order to observe the chmacteristic dark red, purple, or violet colors. It. is possible that the red oils and solids of themselves represent positive tests, hut the colors in solution are much more satisfrtctary evidences. 8 Formic acid gave a. positive test when the molar quantity of the acid was in excess over the amount of thionyl chloride employed, because of the facile reaction of this acid with hydroxylrtmine to give the corresponding hydroxamic acid. When molar amounts were reversed the test was negative since formic aoid was converted to products including hydrochloric scid and carbon monoxide.
Volume 47, Number 11, November 1970
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