Lab Report on Aldehydes and Ketones Experiment
Lab Report on Aldehydes and Ketones Experiment
The current experiment aimed at learning properties of some of the common aldehydes and ketones, as well as, the chemical tests used to distinguish the different classes.
Both aldehydes and ketones have the same functional group known as carbonyl group C=O. In the aldehyde compounds, the carbonyl group is located at the end the structure with a general structure shown below (McNaught & Wilkinson, 1997).
The carbonyl carbon is, therefore, the first carbon of the carbon chain. On the other hand, the carbonyl group in ketones is located anywhere on the carbon chain but not at the end. The general structure of a ketone molecule is as shown below.
Aldehydes and ketones with low molecular weight or a short carbon chain are usually soluble in water. The carbonyl group creates a hydrogen bond with the molecules of water making them soluble. Some of the tests that are available for testing the aldehydes include chromic acid, Benedict’s and Tollen’s tests. Ketones show negative results for the above tests. The main test that is done on ketones is the Iodoform Test that is done to test for methyl ketone. Methodology
- Solubility in water
The experiment was done in 7 clean, test tubes that were labeled using letters 1-7. In the test tubes, 2mL of water was placed in each of the test tube and 3 drops of acetone, benzaldehyde, propanal (propionaldehyde), formaldehyde, acetophenone 3-pentanone and the unknown added to each of the labeled test tubes. The observations were recorded in the table below and the test tubes saved for the Benedict’s Test.
- Benedict’s Test: Oxidation of aldehydes and ketones
- Chromic acid Test
The seven test tubes were labeled and placed in the test tube rack. In each tube, 10 drops of acetone, benzaldehyde, propanal (propionaldehyde), formaldehyde, acetophenone, 3-pentanone and the unknown sample to each of the labeled test tubes. To each tube, 5 drops of chromic acid reagent were added one drop at a time and mixed by tapping the test tube with a finger. The formation of a blue- green color solution indicated the presence of an aldehyde. The aliphatic aldehydes were expected to react immediately while aromatic aldehyde, such as benzaldehyde, to take longer time to show a color changes. Observations were made and recorded after 20 minutes.
- Iodoform Test for methyl ketones
In this experiment, 7 test tubes were labeled and placed in the test tube rack. In each tube, 10 drops of acetone, benzaldehyde, propanal (propionaldehyde), formaldehyde, acetophenone, 3-pentanone and the unknown sample. To each test tube, 5 drops of 10% NaOH were added. One test tube at a time was placed in a 60oC water bath. The tubes were removed from the water bath, and 10-15 drops of the iodine reagent added. A positive indicator for the test was the presence of yellow precipitate (cloudiness) and not yellow oily droplets. The observations were recorded in the table below.
- Tollens’Test
A set of 9 clean, test tubes were labeled and placed on a test tube rack. In each test tube, 10 drops each of the samples were added, and 2mL of the Tollen’s reagent added and stirred using a glass-stirring rod. The test tubes were allowed to stand for 15 minutes, and the formation of the Ag (silver mirror) observed. The observations were recorded in the table below.
Results
The results for the various tests that were conducted were as in Table 1 below. Benedict’s test showed positive results for acetone, acetophenone and 3-pentanone. In the chromic acid test and Tollen’s Test, benzaldehyde, propanal and formaldehyde showed positive results. In the iodoform test, acetone and acetophenone showed positive results.
Discussion
The unknown compound was an aliphatic aldehyde. This is because the Tollen’s test gave a positive result as an aldehyde. In addition, the chromic acid test gave positive results almost immediately indicating that the product was an aliphatic aldehyde. The structural and molecular formula for hexanal, 2-hexanone and 3-hexanone are as follows.
The compounds that are similar in their molecular formula but different in structural formula are known as isomers. The tests that can be performed to differentiate hexanal, 2-hexanone and 3-hexanone are iodoform Test and Benedict’s Test. Since hexanal is an aldehyde, Benedict’s test will differentiate it from the other two which are ketones. The iodoform test will differentiate the 2-hexanone, which is a methyl ketone from 3-hexanone (Clark, 2004).
A compound with a molecular formula C4H8O, dissolves in water gives positive results with the Iodoform Test but negative with Benedict’s and Tollen’s tests means it is a methyl ketone. The structural formula for the compound would be CH3-CH2-CO-CH3 and its name 2-butanone.
In conclusion, the experiment successfully enabled learning the properties of some of the common aldehydes and ketones, as well as, the chemical tests used to distinguish the different classes.
Reference List
Clark, J. (2004). Oxidation of Aldehtdes and Ketones. Retrieved February 11, 2013, from http://www.chemguide.co.uk/organicprops/carbonyls/oxidation.html
McNaught, A. D., & Wilkinson, A. (1997). IUPAC. Compendium of Chemical Terminology (2nd ed.). Oxford: Blackwell Scientific Publications.
