Lab Photo: The 2,4-Dinitrophenylhydrazine Test for Aldehydes and Ketones

Photo showing a positve and negative result using the 2,4-dinitrophenylhydrazine test for aldehydes and ketones

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.

Cyclohexanone reacts with 2,4-dinitrophenylhydrazine

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

Sonjiala Jackson-Hotchkiss

Sonjiala (SON-ja-la) is currently pursuing an MS degree in chemistry at UC San Diego. As a member of the Bridges to Baccalaureate Program at San Diego Mesa College, her research in organic chemistry includes the synthesis of fatty acid esters of vitamin C that will be tested for their ability to inhibit glycolytic enzymes. In an additional research project she uses analytical chemistry techniques to determine the composition of World War II era California pottery.

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