Identification of an Unknown Carbonyl Compound

 

1. Deternination of the boiling range for your unknown:  Pour the contents of your unknown

    bottle into your 50 mL round-bottomed flask.

 

2. Clamp the flask to a ringstand and prepare your simple distillation apparatus.

 

3. Setting your variable voltage source at approx. 70% of max. power, simple distill your

    unknown, taking temperature readings every 60 - 120 secs once the liquid begins to distill.

 

3. Preparation of the semicarbazone:  Place 10 mL of the semicarbazide reagent solution in

   a 50 mL Erlenmeyer flask and add 0.5 - 1 mL of the unknown carbonyl compound with a

   disposable plastic pipet.  Stopper the flask with a cork and shake the flask vigorously.*

 

5. Filter the crystals with suction using the small plastic Buchner funnel with a sintered

   plastic disk.  No filter paper is required with this type of funnel.  Rinse the

   Erlenmeyer flask twice with 5-10 mL of cold tap water each time and pour the rinsings

   through the Buchner funnel containing the filtered solid.  Do not attempt to remove solid

   adhering to the wall of the flask at this time.  Do not wash the flask, either, but save it

   for use in the recrystallization.

 

6. Recrystallization of the semicarbazone:

 

   a)            Transfer the crude solid from the funnel to the 50 mL Erlenmeyer flask.  (The yield of

            product is not important in this case since only enough is needed to determine the

            m.p.)

 

   b)      Add about 10 mL of 50% ethanol and carefully heat the mixture to boiling on a hot

            plate.  If all the solid does not dissolve, add another five mL of 50% ethanol.

            Repeat if necessary, but do not exceed a total of 25 mL of the solvent.

 

   c)      If there is obvious insoluble matter (e.g., debris or undissolved crude product), filter the

            hot solution through a fluted filter paper supported in a stemless funnel.

 

   d)      Cool the clear hot solution in the minimum amount of cold water in a small plastic

            bowl.  While the solution is cooling, wash the plastic Buchner funnel as it can be used

            to filter the purified product.  Also, pour any liquid still in the filter flask     down the

            drain.

 

   e)      If there are no crystals visible in the cooled liquid at this point, add a small amount

            of ice to the cooling bath, and stir the solution with a glass rod.  Put about 2 mL of

            50% ethanol in a test tube, and place it in an ice-water bath.

 

   f)       Filter the precipitated derivative with suction through the clean small plastic

            Buchner funnel.  Wash the filter cake with about 0.5-1.0 mL of ice-cold 50%

            ethanol.

 

   g)            Continue the suction for several minutes.  Then, carefully remove the purified

            product to  a sheet of 12.5 cm filter paper, spread the crystals around and allow

            them to air dry.

 

*NOTE:   If the semicarbazone has not precipitated after five minutes, and an oily layer is

               visible, add about 3 mL of 95% ethanol.  Stopper the flask and shake the mixture

               again.  If there is still no solid precipitate after two more minutes, add another 2 mL

               of 95% ethanol and continue shaking the mixture.  Should no solid separate at this

               point, consult the instructor. 

 

7.         Obtain the infrared spectrum for your unknown. ( See attached instructions. )

 

8.            Determine the m.p. of the semicarbazone.  Compare the melting range of the

            derivative you made with the m.p.'s of the semicarbazones listed below.  Compare the

            infrared spectra of the known compounds that you think could be identical to your

            unknown (see the special folder provided in the laboratory) with the spectrum of your

            unknown.  Make a positive identification of the unknown on the basis of the spectra

            comparisons. 

 

                     Ketone

           Boiling Range ( oC )

     Semicarbazone m.p. ( oC )

                 2-pentanone

                       101

                      112

          3-methyl-2-butanone

                        95

                      113

                 3-hexanone

                       123

                      113

      2, 6-dimethyl-4-heptanone

                       169

                      122

                  2 octanone

                       173

                      122

                 4-heptanone

                       145

                      132

         4-methyl-2-pentanone

                       118

                      132

                  2-butanone

                        80

                      146

               propiophenone

                       218

                      173

        3-methylcyclohexanone

                       170

                      179

                    acetone

                        56

                      190

        2-methylcyclohexanone

                       162

                      191

                acetophenone

                       202

                      198

        4-methylcyclohexanone

                       170

                      199

 

 

1The melting range you determine may be somewhat wider than what you would expect for a

 purified product.  Pay most attention to the upper limit of the range when attempting to

 compare your measured value with the listed m.p.'s.