Preliminary Test: Dissolve a drop or a few crystals of the compound to be tested in 1 mL of 95% ethanol and add 1 mL of 1 M hydrochloric acid. Note the color produced when 1 drop of 5% ferric chloride solution is added to the solution. If a definite orange, red, blue, or violet color is produced, the following test for the acyl group is not applicable and should be omitted. Too much hydrochloric acid prevents the development of colored complexes of many phenols and all enols.
Heat to boiling a mixture of 1 drop or about 40 mg of the compound, 1 mL of 0.5 M hydroxylamine hydrochloride in 95% ethanol, and 0.2 mL of 6 M sodium hydroxide. After the solution has cooled slightly, cautiously add 2 mL of 1 M hydrochloric acid. Anhydrides, acyl halides, and esters would have undergone the reaction with the hydroxylamine to form the hydroxamic acid, as indicated in the above equations. If the solution is cloudy, add 2 mL of 95% ethanol. Observe the color produced when one drop of 5% ferric chloride solution is added. If the color caused by the drop of ferric chloride solution does not persist, continue to add the ferric chloride solution dropwise until the observed color permeates the entire test solution. Compare the color with that produced in the preliminary test.
A distinct burgundy or magenta color of the ferric hydroxamate complex is a positive test. Compare the color of this solution with the yellow observed when the original compound is tested with ferric chloride in the presence of acid to assure the color is due to the formation of ferric hydroxamate complex.
Some acids will give a positive test.
Primary and secondary nitro compounds, imides, some amides, most nitriles, and aldehydes (with no a-hydrogens give a positive test.
To 2 mL of a 1 M hydroxylamine hydrochloride solution in propylene glycol is added 1 drop or 30 mg of the compound dissolved in a minimum amount of propylene glycol. Add 1 mL of 1 M potassium hydroxide and the mixture is boiled gently for 2 min. Allow the mixture to cool to room temperature and add 0.5 to 1 mL of a solution of 5% alcoholic ferric chloride.
A red to violet color constitutes a positive test. Yellow colors indicate negative tests, and brown colors or precipitates are indeterminate.
Some sterically hindered amides fail to react.
To prepare the sulfonyl chloride from the sulfonic acid, combine 5 drops of thionyl chloride and 100 mg of the sulfonic acid in a test tube and heat in boiling water for 1 min. Allow the test tube to cool. To the test tube, 0.5 mL of a saturated solution of hydroxylamine hydrochloride in methanol is added. A drop of acetaldehyde is added. The sulfonyl chloride undergoes the reaction with the hydroxylamine to form an intermediate, which when treated with the acetaldehyde, forms the hydroxamic acid. Add dropwise a solution of 2 M potassium hydroxide in methanol until the solution is slightly basic, when checked with pH paper. Heat the solution to boiling and then allow it to cool. Acidify the mixture by adding, dropwise, 0.5 M hydrochloric acid until blue litmus paper turns red. Add a drop of 5% ferric chloride solution. Ferric chloride converts the hydroxamic acid to the ferric hydroxamate complex.
Sulfonyl chlorides can be treated directly with the hydroxylamine hydrochloride. The salts of sulfonic acids are first neutralized with hydrochloric acid, then evaporated to dryness. The residue is then treated with thionyl chloride as described above.
The magenta color of the ferric hydroxamate complex is a positive test
