Identification of an Unknown Aldehyde or Ketone

In this laboratory you will be given a small quantity of an unknown compound which has the following characteristics:

  1. it is either an aldehyde or a ketone
  2. it is found in the tables at the back of the lab manual
  3. it is a liquid

Your assignment is to determine the exact identity of the unknown molecule.

General Procedure
1. Obtain unknown (caution: avoid contact) and purify carefully by simple distillation (caution: flame hazard). Record the boiling point as carefully as possible. This is critical to later steps.

2. Perform Baeyer's test to distinguish the aldehyde from the ketone possibility. Baeyer's test is positive for aldehydes and many substituted aromatic molecules (among other things) but not for non-aromatic ketones. Perform this test on three knowns (methyl ethyl ketone, acetophenone, and any available aldehyde) as well as your unknown. It is best to run all four tests at the same time.

Procedure

  1. Place 5 drops of the test sample into a micro test tube.
  2. Add five drops of KMnO4 solution (caution: avoid contact).
  3. Agitate the tube and note any immediate changes.

3. If the Baeyer test is positive you have either an aldehyde or an aromatic ketone. To help distinguish between these, you may wish to record an IR spectrum. (Note: this may not help since you might have an aromatic aldehyde, too; however, it will be very useful if you learn that the molecule is not aromatic.)

4. Consult the tables in the back of the lab book to determine a short list of candidate possibilities for your unknown.

5. Prepare 2,4-dinitrophenylhydrazone and semicarbazone derivatives of your unknown as per the procedures in the lab book (caution: avoid contact).

6. Isolate the derivatives. Determine melting points in the next lab period. If any melting points have a range of more than 2-3 °C, it is wise to recrystallize the sample prior to the final determination of the melting point.

7. Using all of your information, propose the identity of your unknown.

8. WARNING: you may not be able to confidently give the exact identity. It might be better to give a list of 2 or three candidates in some cases. Do not simply decide to say, "I think it's this one" unless you can defend that answer completely.

9. Bring your completed lab notebook which details all work from this laboratory plus a written description of the process used to arrive at your conclusion to the instructor for evaluation.