Identification of Unknown Aldehydes and Ketones

Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

1. DNPH Test for Aromatic Aldehydes and KetonesExpand

   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.

   • Put on a lab coat, safety glasses, and nitrile gloves. Note: Many chemicals in this lab are toxic, corrosive, or flammable, so you will perform this lab in a fume hood. Change your gloves if you get any chemicals on them.
   • Get a vial of each known compound and choose two unknown compounds to analyze. Write the codes for your unknowns in your lab notebook.
   • Label a 250-mL beaker for aqueous waste and a 600-mL beaker for used pipettes. Put several clean Pasteur pipettes in another beaker and get more whenever you need them.
   • Obtain four small test tubes in a rack and label them with the four compound names, or codes, and the name of the first test.
   • Bring a 50-mL beaker and a graduated cylinder to the reagent hood and obtain 8 mL of the DNPH solution.
   • Use four Pasteur pipettes to place 3 – 4 drops of each compound into its test tube. Note: Don't reuse pipettes during this lab because cross-contamination will invalidate the test results.
   • Add 2 mL of the DNPH solution to one of the test tubes and mix it well with a glass rod. Thoroughly rinse the glass rod with deionized water over your aqueous waste beaker to remove all traces of the ketone or aldehyde.
   • Add 2 mL of the DNPH solution to each of the other three test tubes in the same way. Make sure to thoroughly clean the glass rod after each one. When you're finished, let the solution sit for 15 min.
   • While you wait, set up for vacuum filtration and fill a 250-mL beaker with deionized water.
   • After 15 minutes, inspect the precipitates in the test tubes. Note: Benzaldehyde has an aromatic group, so you should see a red to orange solid in its test tube. Butanone is aliphatic, so you should see an orange to yellow solid in its test tube. Record the results in your lab notebook.

     Table 1: DNPH, Tollens', and Iodoform Results

     Compound	DNPH	Tollens'	Iodoform	Solubility in water	Hydrazone m.p.
     Butanone
     Benzaldehyde

     Click Here to download Table 1
   • Compare the solids from the unknown compounds to the known compounds. For each unknown, record whether this solid was more red-orange, like benzaldehyde, or more orange-yellow, like butanone.
   • Use a clean Pasteur pipette to wet the filter paper of the vacuum filtration setup with deionized water and collect the solid from one of your unknowns by vacuum filtration. Rinse out any solid left in the tube with deionized water.
   • Wash the solid on the filter paper with 10 mL of deionized water.
   • Pick up a tiny amount of the solid with a clean glass rod and smear it on pH paper. If the pH is below 6.5, rinse the solid with another 10 mL of deionized water. Keep rinsing the solid and checking its pH until it is between 6.5 and 7.
   • Label a small watch glass for the unknown and carefully transfer the filter paper and solid to the watch glass to dry.
   • Thoroughly rinse the Büchner funnel with deionized water, dispose of the filtrate in the DNPH solution waste, and reassemble the vacuum filtration setup with a new piece of filter paper.
   • Collect and wash the solids from the second unknown compound and from the two known compounds in the same way.
   • Leave the four solids in your fume hood to dry (1 h).
2. The Tollens’ TestExpand

   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.

   • Place your used test tubes in a beaker and label four new tubes with the compound names or codes and Tollens' test.
   • Bring a clean 50-mL beaker and a 5-mL graduated cylinder to the reagent hood and obtain 4 mL of Tollens' reagent.
   • Use clean pipettes to place 3 – 4 drops of each compound in its test tube.
   • Add 1 mL of Tollens' reagent to each test tube and mix the solutions well with a clean glass rod, thoroughly rinsing the rod with deionized water between tubes. Set the rack of tubes to the side and let it sit for 10 min.
   • While you wait, get about 100 mL of water in a 150-mL beaker and set it on a hotplate. Position a small 3-prong clamp above the beaker.
   • Check the benzaldehyde tube after 10 min. Note: Benzaldehyde should have a positive result, as indicated by a silver film or precipitate. If you don't see a positive result in the benzaldehyde tube, secure a thermometer to the test tube and heat in the water bath at 35 °C for 5 min. The silver film, or precipitate, should then appear. Record this result in your lab notebook.
   • Butanone won't react with Tollens' reagent. Note: The butanone tube is an example of a negative Tollens' test result. Record this in your lab notebook.
   • Check the tubes of your unknowns. If you see a silver mirror or precipitate, record a positive Tollens' test result for that unknown. If you don't, heat the tube in the water bath for 5 min and check again. If the test is still negative, record that result in your lab notebook.
   • When you're done, turn off the hotplate and dispose of the Tollens' test solutions in the appropriate waste container. Put silver-coated tubes in the collection box for acid cleaning and put the rest in your beaker.
3. Iodoform Test for Methyl KetonesExpand

   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.

   • Before the iodoform test, check the solubility of all four compounds in water. Label four clean test tubes with the compound names or codes and H₂O. Label another four test tubes with the compound names and CHI₃.
   • Use clean Pasteur pipettes to place 3 – 4 drops of each compound in its test tube.
   • Add 2 mL of deionized water to each labeled tube. Mix well with a clean glass rod. Thoroughly clean the rod with deionized water between tubes.
   • Wait a few minutes for the liquids to settle before evaluating them. Note: Butanone is moderately soluble in water, so you should see little to no separation. Benzaldehyde is sparingly soluble in water, and its density is a little higher than water's. You should see a small layer at the bottom of the tube.
   • Inspect your unknowns for liquid layers and record the results for all four compounds in your lab notebook.
   • Add 2 mL of deionized water to the butanone tube and any water-soluble ketones and mix them well with a clean glass rod. Note: If you have an unknown ketone that is insoluble in water, instead add 2 mL of 1,2-dimethoxyethane and mix it well with a clean glass rod.
   • Now, perform the iodoform test, which is for ketones only. Use a clean 50-mL beaker and a small graduated cylinder to obtain 4 or 6 mL of 3 M NaOH, depending on whether you have two or three ketones to test.
   • Pour ~10 mL of iodoform reagent into another 50-mL beaker.
   • Add 2 mL of 3 M NaOH to each ketone tube and mix well, thoroughly cleaning the glass rod with deionized water between each tube.
   • Obtain a clean Pasteur pipette and add the iodoform reagent dropwise to the butanone tube, frequently stirring until you see a yellow precipitate. Note: Butanone is a methyl ketone, so it will produce a positive result.
   • Rinse the glass rod with deionized water and start adding the iodoform reagent dropwise to your first, or only, unknown ketone, while stirring frequently. Continue adding the reagent until either yellow iodoform precipitates, like you saw with butanone, or the solution stays brown, even after you stir it well. Note: The latter is a negative result.
   • If you have another unknown ketone, clean the glass rod well and repeat the test with your second ketone.
   • When you're finished, dispose of your iodine-containing solutions in the appropriate waste container.
   • Once the solids from the DNPH test are dry, measure the melting point of each solid and record them in your lab notebook. Put the remaining solutes in the waste container for solid DNPH derivatives.
   • Dispose of the leftover organic liquids and the aqueous aldehyde mixtures in the usual waste containers.
   • Rinse the Pasteur pipettes used for the aldehydes and ketones with acetone before putting them in the glass waste. Pour your aqueous waste down the drain with running tap water. Clean your glassware, put away your lab equipment, and collect and throw out any trash in your fume hood before you leave.
4. ResultsExpand
   • Identify your unknown compounds. There are six options: acetophenone, butanal, 3-methyl-2-butanone, pentanal, 2-pentanone, and 3-pentanone.
   • Look at the DNPH results. If the precipitate was red to orange, the unknown aldehyde or ketone is aromatic. If it was yellow, the unknown is aliphatic. Note: Since only one possible compound is aromatic, red precipitate means that the unknown was acetophenone. As confirmation, you should also have seen a negative Tollens' test and a positive iodoform test.
   • A yellow precipitate in the DNPH test means that the unknown could be any of the other five compounds.
   • Look at the results of the Tollens' test. If you saw a silver mirror or silver-gray precipitate in the test tube, the unknown compound is an aldehyde. If you didn't see any change, then the unknown compound is a ketone. If your unknown is an aldehyde, it could be pentanal or butanal.
   • Compare the melting point of the unknown's hydrazone derivative to literature values for each aldehyde to figure out which one it was.
   • If Tollens' test was negative and the DNPH test showed an aliphatic compound, the unknown could be 3-methyl-2-butanone, 2-pentanone, or 3-pentanone.
   • Check the results of the iodoform test to narrow it down. If you saw solid yellow iodoform, the compound was a methyl ketone. If you saw a brown solution, it was not a methyl ketone.
   • Only one of the aliphatic ketones is not a methyl ketone, so if you saw a brown solution, the unknown was 3-pentanone. If you saw yellow iodoform precipitate from solution and the DNPH test also had yellow precipitate, then the unknown is 3-methyl-2-butanone or 2-pentanone.
   • Use the melting point of the unknown's hydrazone derivative to figure out which methyl ketone you had.