of Identification an Aldehyde or Ketone Raymond Oldham, Michael Lester, Audrey Paulzak CH-238 QZ
Introduction: When determining if there is an aldehyde or ketone functional group, chemical test are the easiest way to detect them. A reaction with 2,4-dinitrophenylhydraznie to form a cooled solid product is a strong indication of a ketone or aldehyde. Because aldehydes are easily oxidized, Tollen's, Fehling's or Benedict's solution will give positive tests for aldehydes but not ketones. Once the function group has been determined, solid derivatives are produced to identify the compound by examing the melting point. Melting points can not identify a single compound alone, but the chances of two compounds having derivatives with the same melting point is very small.
Mechanism: Iodoform Test
O H R CH2
x3
+
HO
-
R
CH2
+
I
I
O
R
I
O
I I
O
+
HO
-
I CH3
I C
-
R
I
R
+
I
I I H
I
Semicarbazone
O R R R R R N R
2 + 1 1
O
-
R
1
H2O R
+
R
1
+
H2N
R
2
R
NH2
2
+
NH R R
1 2
+
H
H2O
R N R
2
Table of Reagents: Amt. Used 3.5 mL 1 mL 2M 0.5 g 1g 2g 10 mL 2 mL 0.5 g 0.2 g Molecular Wt. (g/mol) 39.9971 169.87 35.0456 32.04 126.904 166 Density (g/mL) 2.1
NaOH AgNO3 NH4OH CH3OH Iodine Potassium iodide 2,4Dinitrophenyhydrazine H2SO4 C2H4OH Hydroxylamine hydrochloride Semicarbazide hydrocholoride
MP C 318 212 -97 113.7 681
0.791
98.087 46 33.02 75.08
10 -116.3 33
1.84 0.799
Safety Information: 1) 2,4-Dinitrophenylhydrazine, semicarbazide and hydroxylamine are toxic. Silver nitrate can stain your skin and clothing blak. Sulfiric acid is corrosive. Sodium hydroxide is caustic. Wear gloves and avoid contact with your skin or clothing when using any of these reagents or the 2,4-Dinitrophenylhydrazine derivatives, and wash spills with water. 2) Petroleum ether is flammable. Dispose of all wastes in the bottles provided in the hood.
Procedure: To start the experiment, the unknown was first tested with the Tollen's test and the Iodoform test. Tollen's Test First, 10% sodium hydroxide was added to a clean test tube. Then 1 mL of 0.3 M aqueous silver nitrate and 0.5 mL of 3 M sodium hydroxide was added to the test tube.
Next, 2 M aqueous ammonium hydroxide was added drop wise to the test tube with constant shaking until the brown precipitate of silver oxide dissolved. After it dissolved, 3 drops of the unknown was added to the Tollen's reagent. The mixture was then shaken and left alone at room temperature for approximately 10 minutes.
Iodoform Test First, 3 drops of the unknown was dissolved in 0.5 mL of water within a large test tube. Then 0.5 mL of 3 M NaOH was added to the test tube. Next, 0.75 mL of iodine solution was slowly added drop wise and the solution was shaken until the brown color of the solution persisted.
After the two test were run to determine if the unknown was an aldehyde or ketone, two derivatives were prepared in order to take their melting points to identify the unkown.
Preparation of 2,4-Dinitrophenylhydrazone First, 2 g of 2,4-Dinitrophenylhydrazine was dissolved in 10 mL of concentrated sulfuric acid. Then 28 mL of water and 50 mL of 95% ethanol was added to the mixture with stirring. Next, 3 drops of unknown was dissolved in 2 mL of 95% ethanol in a separate flask. Then 2 mL of this solution was added to the 2,4-Dinitrophenylhydrazine solution. After the precipitate formed, the test tube was cooled in an ice bath to maximize crystallization. Afterwards the solid was isolated using vacuum filtration.
Preparation of Semicarbazone First, 0.2 g of semicarbazone hydrochloride and 0.3 g of sodium acetate were added together a in test tube with 2 ml of water. Then 0.5 mL of unknown was added to the test tube. Once a homogenous solution was obtained, the test tube was sealed with a stopper and shaken for one minute. The stopper was then removed and placed in a boiling water bath for 5 minutes. After the 5 minutes, the test tube was placed in an ice bath for 10 minutes to induce crystallization. The solid was then collected by vacuum filtration and washed with 5 mL of ice water.
After both derivatives were prepared, the melting point of both were determined and then compared to a chart of melting points to discover the identity of the unknown.
Results and Discussion The identification number of the experimental unknown was 5. When the Tollens' test was performed, silver ions did not precipitate and form a mirror coating on the inside of the test tube. Instead, a white non-homogenous solution resulted. This result clearly indicated that no aldehydes were present in the unknown solution. The second test performed, the Iodoform test, with the unknown. A solid, bright yellow precipitate formed at the top of the solution in the test tube. Since no acetone was used to clean the glass, the results indicate that there was a methyl ketone present in the unknown solution.
The 2,4-dinitrophenylhydrazone derivative of the unknown had a melting point of 117C. The semicarbazone derivative of the unknown had a melting point of 146 C. It is highly unlikely that any two ketones would have identical melting points in their derivatives, so this property can be used to identify which specific ketone our unknown is. According to the chart in the lab notebook, only 2-Butanone correctly fits the data we collected, and therefore must be the identity of the unknown number five. The unknown could also possibly be identified using such methods as 1H-NMR spectroscopy, 13C-NMR, and mass spectroscopy.
Questions
1) a) Acetaldhyde (aka ethanal) is the only aldehyde which gives a positive iodoform test, due to its structure CH3CHO which is both a methyl ketone and an aldehyde. b) Ethanol gives a positive iodoform test because it can be oxidized by the iodine used in the test to a methyl ketone, which yields a positive result. 2) As shown by the resonance structures, below, the cation can be formed and stabilized most effectively on the carbon and internal nitrogen, since internal double bonds are much more stable than external double bonds and secondary and tertiary cations are more stable than primary cations.
O NH H2N
-
O NH2
-
C H2N
+
O NH NH2 H2N
-
NH
+
NH2
O
NH H2N
+
NH2
These resonance forms help spread the charge out across the molecule and demonstrate why this cation forms and the semicarbazone bonds on one side. 3) Acetone is the most likely unknown since it has slightly closer melting and boiling points to the experimentally obtained values than the next closer compound, acetophenone. The student should follow this test with an electrophilic halogenation, such as Br2 with FeBr3, and then proceed to take the melting point and boiling point of the product. Since this type of substitution adds to an aromatic ring, if the reaction occurs the product will have a different melting point and boiling point. Therefore, if the reaction occurs, the unknown was acetophenone, but if it does not, the unknown was acetone. 4) The unknown compound must be either 3-Pentanone or 1-Phenyl-2-propanone. The student's best course of action may be to use pH paper in order to determine the acidity/basicity of the solution. The 1-Phenyl-2-propanone will be somewhat acidic, whereas the 3-Pentanone will be more basic.