Positive (with precipitate) and negative result with the 2,4-Dinitrophenylhydrazine test for aldehydes and ketones. Photo by Sonjiala Hotchkiss/STEM Punk
Both aldehydes and ketones have a carbonyl group (a carbon double bonded to oxygen). The compound 2,4-dinitrophenylhydrazine (2,4-DNP or 2,4-DNPH) undergoes a reaction with the carbonyl group in aldehydes and ketones that gives a precipitate like the yellow one in the photo. Though esters, amides, and carboxylic acids also contain carbonyl groups, generally a precipitate does not form with the 2,4-DNP test.
When cyclohexanone, an unconjugated ketone, reacts with 2,4-dinitrophenylhydrazine, it gives a derivative with a characteristic melting point. Illustration by Sonjiala Hotchkiss/STEM Punk
My unknown in this lab was a carboxylic acid and gave the negative (no precipitate) result with the 2,4-DNP test shown in the photo. I also performed the 2,4-DNP test with a compound that would give a positive test result, but I don’t remember which compound I used for the test. How might I make an informed guess? The color of the precipitate gives a clue to the structure of the original compound. A yellow color is more indicative of an unconjugated compound like cyclohexanone shown above. An aromatic compound would likely have given a darker precipitate in the orange to red range.
The precipitate that results from the 2,4-DNP test is called a derivative. The 2,4-DNP derivatives have characteristic melting points. After determining the melting point, a 2,4-DNP derivative table may be used to figure out the identity of the original test compound.
Organic Chemistry II Lab, San Diego Mesa College
