Identification of Unknown Aldehydes and Ketones

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

1. Preparation of Test SolutionsExpand

   Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

   • Before you start, put on a lab coat, splash-proof safety glasses, and nitrile gloves. Note: DNPH is a shock explosive, so handle it gently with low friction tools. Phosphoric acid is also toxic and corrosive.
   • Prepare 100 mL of a 0.1 M solution of DNPH in a 50/50 mixture of 85% phosphoric acid and ethanol. Add a medium stir bar to a 250-mL beaker. In a fume hood, measure 3 g of DNPH and gently pour it into the beaker.
   • Measure 50 mL of 85% by weight aqueous phosphoric acid and slowly add it to the beaker. Place the mixture on a stir plate and gently stir it until the DNPH dissolves.
   • Measure 50 mL of ethanol and pour it into the beaker. Once the solution looks homogeneous, turn off the stir motor and retrieve the stir bar.
   • Vacuum filter the solution to remove residual solids before transferring it to a 100-mL container. Cap the container, label it ‘DNPH solution’, and place it in a hood for the test reagents.
   • Prepare 200 mL of iodoform reagent. Measure 50 g of potassium iodide into a clean 250-mL beaker. Add 200 mL of deionized water and stir the mixture on a stir plate until the potassium iodide has completely dissolved.
   • Measure 25 g of iodine and add it to the stirring solution. Once the iodine has dissolved, turn off the stir motor. Transfer the solution to a labeled 200-mL container and cap it. Place the iodoform reagent in the test solution hood.
   • Make 22 mL of Tollens’ reagent. Measure 0.6115 g of silver nitrate and pour it into a 50-mL beaker. Note: Prepare this solution right before the lab because explosive silver nitride will start to form as the reagent decomposes.
   • Add 12 mL of deionized water and stir until the silver nitrate has dissolved completely to form a 0.3 M silver nitrate solution.
   • Measure 6 mL of 3 M NaOH and add it to the stirring silver nitrate solution. Brown silver oxide will precipitate from the solution.
   • Measure 4 mL of 2 M aqueous ammonia. Use a Pasteur pipette to slowly add it to the stirring solution.
   • Once you have added all 4 mL of ammonia, remove the stir bar and vacuum filter the solution. Pour the filtrate into a 25-mL brown glass bottle.
   • Wrap the bottle in foil to shield it from light and label it ‘Tollens' Reagent’. Put the wrapped bottle with the other test solutions.
2. Preparation of the LaboratoryExpand
   • Make sure that the sinks have paper towels and test tube brushes.
   • Set up appropriate waste containers in a waste collection hood and place glass waste nearby.
   • Fill a wash bottle with acetone and place it in the waste hood so that students can rinse strong-smelling chemicals out of their Pasteur pipettes before disposing of them.
   • In a fume hood, pour about 5 mL of butanone in a labeled vial for each lab group.
   • Prepare and label vials containing 5 mL of benzaldehyde in the same way. These are the known compounds that every student group will need.
   • The unknown compounds are acetophenone, butanol, 3-methyl-2-butanone, pentanol, 2-pentanone, and 3-pentanone. For each compound, prepare two vials containing 5 mL of that compound and label them with a code name. Place the known and unknown vials in a central area.
   • Set out the following glassware and equipment at each student lab station (we suggest that students work in pairs):

     1    Lab stand

     1    Small 3-prong clamp

     1    Hotplate

     1    Vacuum line

     1    Test tube rack

     1    Test tube tongs

     1    600-mL beaker

     4    250-mL beakers

     1    150-mL beaker

     5    50-mL beakers

     3    10-mL graduated cylinders

     5    5-mL graduated cylinders

     1    250-mL filter flask

     1    Büchner funnel

     1    Filter adapter

     4    Small watch glasses

     1    Glass stirring rod

     1    Thermometer with beaker clip

     3    Pipette bulbs

     1    Package of pH paper

     1    Roll of lab tape and marking pen

     1    250-mL bottle of DI water

   • Put Pasteur pipettes, filter papers, and small test tubes in a central area.
   • Place 3 M NaOH and 1,2-dimethoxyethane in a hood reserved for solvents.
   • Set up the melting point analyzer and ensure that there are enough capillaries.
   • At the end of the lab, promptly dilute any unused Tollens' reagent with at least 20 mL of water and flush it down the drain with copious tap water. Note: If necessary, use concentrated hydrochloric acid to precipitate the silver from the Tollens' reagent waste first.
   • Clean silver-coated test tubes with concentrated nitric acid in a fume hood. Neutralize and dispose of the silver waste according to your lab's standard procedures.