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Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA
In this lab, you'll use the DNPH test, the Tollens' test, and the iodoform test to identify two unknown aldehydes or ketones. You'll use butanone and benzaldehyde as known compounds to confirm that the tests are working as expected.
You'll start the lab with the DNPH test, which relies on the reaction between 2,4-dinitrophenylhydrazine, or DNPH, and aldehydes and ketones. When you combine DNPH and an aldehyde or ketone in an acidic environment, they undergo a condensation reaction to form the corresponding hydrazone. The hydrazone then precipitates from solution. If the precipitate is red to orange, the aldehyde or ketone was aromatic. If it's orange to yellow, the aldehyde or ketone was non-aromatic or aliphatic.
|Compound||DNPH||Tollens'||Iodoform||Solubility in water||Hydrazone m.p.|
When you combine an aldehyde with diammine silver(I), or Tollens' reagent, the reagent is reduced to metallic silver and ammonia. The silver either coats the inside of the reaction vessel or precipitates as a black solid, so it's easy to see that the reaction happened.
Most ketones are harder to oxidize than aldehydes and therefore don't react with the Tollens' reagent. For this lab, you can assume that if silver appears, the compound is an aldehyde. If silver does not appear, the compound is a ketone.
The Tollens' reagent solution contains a strong base and must be disposed of specially to avoid forming an explosive decomposition product. Handle it carefully and remember to put the Tollens' test waste in the designated container.
When you combine iodine and a methyl ketone in aqueous base, iodine replaces the alpha methyl's hydrogens. An addition-elimination reaction followed by proton transfer yields iodoform and a carboxylate anion. Iodoform is insoluble in water, so you'll see a yellow precipitate if the compound that you're testing is a methyl ketone. If it is not a methyl ketone, the test solution will eventually turn brown because of the unreacted iodine.