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. Add a magnetic stirring bar to the flask, clamp the flask to the stirrer/hotplate ringstand.

 

3. Set up your apparatus for simple distillation, start the solution stirring and heat the flask, gently at first (~4).

    If boiling does not commence within 10 mins, increase the temperature to 7.  At the first drop of distillate record the

    temperature, and take subsequent temperature readings at 1-2 min intervals.  Continue the distillation until the

    temperature plateaus.

 

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

    a 50 mL Erlenmeyer flask and add 20 – 40 drops of the distilled sample 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 Buchner 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 2 mL of 50% ethanol and carefully heat the mixture to boiling on a hot plate.  If all the solid does not

        dissolve, add another 2 mL of 50% ethanol. Repeat if necessary, but do not exceed a total of 10 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.

 

c)            Cool the clear hot solution in a bath of cold water.  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.

 

d)            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.

 

e)            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.

 

f)              Continue the suction for several minutes.  Then, carefully remove the purified product to  a sheet of 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 (0C)
Semicarbazone M.P. (0C)
Propionaldehyde
48
89
Acetone
56
190
Isobutyraldehyde
64
125
Butyraldehyde
75
96
2-Butanone
80
146
Isovaleraldehyde
92
107
3-Methyl-2-butanone
95
113
2-Pentanone
101
112
3-Pentanone
102
139
Pinacolone
106
157
2-Ethylbutanal
117
99
4-Methyl-2-pentanone
118
132
2,4-Dimethyl-3-pentanone
124
160
2-Hexanone
128
125
Mesityl oxide
130
164
4-Heptanone
145
132
3-Heptanone
148
103
2-Heptanone
151
123
Cyclohexanone
156
167
2-Methylcyclohexanone
162
191
2,6-Dimethyl-4-heptanone
169
122
3-Methylcyclohexanone
170
179
4-Methylcyclohexanone
170
199
Octyl aldehyde
171
101
2-Octanone
173
122
2,5-hexanedione
194
224
Acetophenone
202
198
Propiophenone
218
173

 

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.

 

 

                   FTIFT Operating Instructions

 

1.      Slide open the sample compartment and make sure the compartment is empty.

 

2.       Close the sample compartment.

 

3.       Click the Col Smp button – the second button down - at the upper left of the left toolbar.

 

4.       Enter the filename you wish for you sample, subsequently hit the ENTER key.

          This filename MUST be in the form your last name and the name of your compound or unknown concatenated together

 

   SMTIHC=O17A

  

5.       A Background Confirmation box will now appear on the screen.  Hit the <ENTER key>.

 

6.       After the Background Spectrum has been taken a Sample Confirmation box will appear on the screen

 

      At this point, go out to the dessicator on the west counter and remove one of the IR windows.  Add a drop of your sample to the window with the pipet provided you last week – YOU STILL HAVE IT, DON’T YOU ? Smear the sample over the entire window with the pipet tip, return to the instrument and insert your sample into the sample compartment, close the compartment, and then hit the <ENTER key>.

 

 

7a. Click on the ANALYZE button near the center of the top tool bar, and subsequently click on the FIND PEAKS option,

 

7b. Set the peak threshold level by clicking on the tip of the highest peak to which you want a  wavenumber value assigned..

 

7c. Click on the REPLACE button to the right of the spectrum.

 

 

8.       Click on the REPORT button near the center of the top tool bar, and subsequently click on the PREVIEW - PRINT REPORT option,

 

9.       Click on the PRINT option.

 

10.     Remove the sample from the sample compartment, close the sample compartment, get your spectral printout.

 

11. Return to the lab and clean your plate and return it to the dessicator.