Aldehyde/Ketone Unknown

Preparation: M&B Sects. 18.11 and 18.23. Aldehydes and ketones, compounds with a carbonyl group (C=O not bonded directly to another O, N or halogen), react specifically with certain other compounds with an G-NH₂ group to give a solid product. The reaction is so consistent that it serves as a specific test for aldehydes and ketones. These solids (called derivatives) can be isolated and used to identify the original compounds:

The melting points of these derivatives are used to identify the aldehyde or ketone tested. This possible because all known aldehydes and ketones have been reacted previously and their melting points have been recorded and published. A list of the solid derivative melting points is in another file "List of Unknown Aldehydes-Ketones".

For this laboratory you will identify an aldehyde or a ketone from:

a) its boiling point

b) its IR spectrum (liquid between salt plates)

c) the melting point(s) of one or more of its derivatives.

d) the Tollens test which distinguishes between aldehydes and ketones.

Prepare a Table of Chemical Tests (Common Practices.. ) to organize the data gathered below.

Procedure: The tests below need not be run in order. Again, prepare to repeat tests and (if necessary) use known reagents to determine what a positive result looks like.

Boiling Point: If you use the ultramicro capillary technique, obtain a preliminary boiling point first from the Mel-Temps which are preset. Wait until all others have taken their preliminaries before changing the control on the Mel-Temps. The Thomas-Hoover is also available after preliminary reading. You may find it desirable to obtain your boiling point using a Hickman still. For compounds that boil over 220 ^oC consult your instructor.

IR Spectrum: Take the spectrum of the liquid between salt plates, don't use Nujol. Note: not all aldehydes and ketones are in the Aldrich catalog of spectra. The carbonyl peak is very prominent between 1690-1760 cm^-1 for aldehydes and ketones. Individual variations are summarized at the beginning of each section for aromatic and non-aromatic aldehydes and ketones.

Determine if your compound is aromatic. Peaks at 1500 and 1600 cm^-1 are diagnostic. Check typical spectra. M&B fig 17.2, 17.3, 17.5, 17.7, H and J of fig 17.37. In fig 18.4, (a) is 2-butanone and (b) is isobutyraldehyde. In fig. 18.7, KK is anisaldehyde (structure, p 683), LL is para-methoxyacetophenone,

H₃C-O-C₆H₄-CO-CH₃, and MM is iso-butyrophenone (isopropyl phenyl ketone).

Note also that aldehydes almost always have an extra peak or two between 2700-2820 cm^-1, just to the right of the main C-H stretch. Note the difference in figures 18.4 and 18.7. Don't forget that aldehydes or ketones which also contain OH groups will have large peaks between 3600-3200 cm^-1. Remember that the format of the Aldrich library is different than your spectrum. In order tm save time searching the Aldrich catalog of spectra, mark off two 3x5 cards that correspond to the sizes of the spectra in the Aldrich library. On each card mark off your main peaks. Now slide the card below the spectra of your candidates in the library for a fast comparison of your spectrum with those of the candidates.

Procedures for Making Derivatives: Below are several recipes.

2,4-Dinitrophenylhydrazones. Place in an 8 inch test tube 4 mL of methanol and 40 mg of 2,4-dinitrophenylhydrazine and 0.1 mL of 6 M HCl. Boil for a minute to affect solution. If the solid does not completely dissolve decant the clear solution into another large test tube. Add about 50 mg (about 1-2 drops) of your unknown to the clear solution and heat it to boiling. Remove it from the steam bath and allow it to stand for 5 min. If no precipitate forms, add water drop by drop to the hot mixture until it becomes cloudy. Caution: Sometimes unreacted 2,4-dinitrophenylhydrazine (m.p. 199-200 ^oC) precipitates from solution when water is added.

2,4-Dinitrophenylhydrazones, recipe II. Prepare a fresh 2,4-dinitrophenylhydrazine reagent solution by dissolving 1.0 g of 2,4-dinitrophenylhydrazine in 5.0 mL of concentrated sulfuric acid. This solution is slowly added to a mixture of 8 mL of water and 25 mL of 95% ethanol. In a separate test tube place 4 drops of your unknown in 1 mL of 95% ethanol; add, dropwise, 3 mL of the reagent solution you prepared.

2,4-dinitrophenylhydrazones can be recrystallized from 95% ethanol to improve the accuracy of the melting point.

Semicarbazones. A solution of 0.5 g of aldehyde or ketone in 3 mL of 95% ethanol and a solution of 0.5 g of semicarbazide hydrochloride and 0.75 g of crystalline sodium acetate in 5 mL of water are prepared in separate test tubes. The two solutions are heated on a steam bath for 2-3 minutes, then the aqueous solution is added in one portion to the ethanol solution and the mixture is agitated well and then heated on the steam bath for 4-5 minutes with occasional shaking. The mixture is then poured into 5 mL of water contained in a 50 mL flask and the mixture is cooled in ice; if an oil forms instead of crystals, rub the oil against the flask with a glass rod. Collect the crystals by vacuum filtration using a Hirsch funnel - the white porcelain one - using a paper disk. Wash the crystals while they are still on the filter paper with a few mL of cold water.

Semicarbazones, recipe II. In a 3 mL conical vial place 12 mg of semicarbazide hydrochloride, 20 mg of sodium acetate, 10 drops of water and 12 mg of the aldehyde or ketone (about 1 drop). Cap the vial, shake vigorously, vent, and allow the vial to stand at room temperature until the crystallization is complete. Cool the vial in an ice bath if necessary. Collect the crystals as above.

Semicarbazone derivatives can be recrystallized from 95% ethanol.

Recrystallization: Place the crystals in the Craig tube and add a few drops of the solvent. Warm on the steam bath until the solid is dissolved. If needed add more solvent drop by drop and reheat the tube until the solid is dissolved. Avoid adding too much solvent. Remove the Craig tube from the heat and cool slowly. Once the crystals are formed, spin the Craig tube in the centrifuge to remove the solvent.

Tollens test: Aldehydes can be oxidized to carboxylic acids by silver ion :

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The reagent should be freshly prepared in a thoroughly cleaned test tube. Place 2 mL of 5% aqueous silver nitrate in the tube and add 2 drops of 1 M sodium hydroxide solution. Mix thoroughly and add ammonium hydroxide dropwise until the precipitated silver hydroxide just dissolves. Add 50 mg of the unknown, shake thoroughly, and let stand for 10 minutes. If no reaction has occurred, place the tube in a beaker of hot tap water for an additional 5 min. If the test tube was properly cleaned before use, metallic silver will usually deposit as a mirror on the inside of the tube. Caution: immediately flush the contents of the test tube down the drain and rinse the test tube with dilute nitric acid. If unsure, test a known aldehyde - one that another student obtained a positive test with, or use benzaldehyde.

Iodoform test: The iodoform test (see notes in Alkene/Alcohol/Alkyl Halide Unknown directions) is positive for methyl ketones and alcohols with a methyl group next to the carbon with the OH group.

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Table of Candidates: Follow the example in the Common Practices … file. Select candidates whose boiling points are 5 ^oC lower up to 10 ^oC higher than the unknown's. If no good fit appears, expand the list. Remember, data may not always point to the same conclusion. Be prepared to retest and redo derivative preparations.

Conclusion: "Based on the data summarized in the tables above I conclude that Unknown Number ___ is ___."

Questions:

1. Write a structure of the 2,4-dinitrophenylhydrazone derivative of acetophenone (look up this ketone's structure in M&B).

2. Write a structure for semicarbazide.

3. What is the IUPAC name for n-pentyl methyl ketone?

4. What 5 second test can distinguish between isobutyl methyl ketone, b.p. 119 ^oC from chloroacetone, b.p. 119 ^oC?

5. What part of one's data clearly distinguishes between salicylaldehyde, and m-tolualdehyde, ?

Reference:

"M&B": Morrison, R.T., and Boyd, R.N. Organic Chemistry, 6th ed., Prentice Hall: Englewood Cliffs, NJ, 1992

Rev. Fall, 1997