Qualitative Tests for Carbonyls: Aldehydes vs. Ketones

Various chemical tests identifying ketones and aldehydes are used in this experiment in order to identify an unknown carbonyl compound. The tests used are: 2,4-dinitrophenylhydrazone test, Tollen’s Test, Benedict’s Test, Chromic Acid Test, aka Bordwell-Wellman Test, Schiff’s Test, and Iodoform Test. These classification tests provide results based on color change or formation of precipitation, which is then used to determine the identity of the functional group.

The first test, the 2,4-dinitrophenylhydrazone test, determines the presence of a carbon-oxygen double bond. Brady’s reagent, another name for the test, is a solution of 2,4-dinitrophenylhydrazine, methanol, and sulfuric acid. When performing the test, a bright orange or yellow precipitate shows the presence of an aldehyde or ketone. This test proceeds via a nucleophilic addition-elimination reaction, as shown below.1

The Tollen’s test is used to distinguish between aldehydes and ketones. The reaction works due to the fact that unlike ketones, aldehydes can be readily oxidized. The Tollen’s test is also known as the silver-mirror test, because when the test is positive for aldehydes, a silver mirror forms on the side of the test tube. A sample reaction is shown below.

The Benedict’s Test and Chromic Acid Test both work in similar fashion as the Tollen’s Test. In the Benedict’s Test, cupric salts are used as the oxidizing reagent rather than the silver nitrate. An aldehyde is identified by a brick-red precipitate, while ketones have no reaction.

The Chromic Acid Tests, sometimes known as the Bordwell-Wellman Test, uses chromic acid to oxidize the aldehydes to carboxylic acids. Ketones do not react. When oxidized, the color changes from orange to blue-green.

The Schiff’s Test is a chemical test for the detection of aldehydes. When the sample is added to the decolorized Schiff reagent in the presence of an aldehyde, a bright purple color develops. Ketones do not react.

The last test, called the Iodoform Test, identifies the presence of a methyl ketone functional group via a halogenations reaction. A yellow precipitate tells us we have a methyl ketone.2

Discussion

Over the course of this two-week lab; students learned how to utilize six different chemical tests to obtain information regarding the tested compounds. This was a wonderful preparation for the second week, when students performed the tests on unknown compounds.

When working with the unknown compounds, students were only supposed to run the Tollen’s Test, the Chromic Acid Test, and the Iodoform Test. The Tollen’s and Chromic Acid Test both differentiate between aldehydes and ketones, and if the compound is determined to be a ketone, then the Iodoform Test is run. The Iodoform Test identifies whether the compound contains a methyl ketone.

The results for the two different unknowns were very clear; number 2 is an aldehyde, and number 4 is a ketone. According to the posted list of unknown compounds, unknown number 2 is heptaldehyde, and unknown number 24 is cyclopentanone. The chemical structures coincide with the chemical tests, as an aldehyde and a ketone were obtained.

Boiling points of the two unknown compounds also had to be determined. The obtained boiling point of heptaldehyde was 130.5 °C, in comparison to the literature value of 151 °C, and the boiling point of cyclopentanone was 125.5 °C, versus 130-131 °C lit.3

There are several things that may explain these changes in boiling points. There may have been contamination in the unknown vial, or the unknown vial could have been labeled incorrectly. The glassware used could have been cleaned improperly. Variance from standard temperature and pressure would have also affected the boiling point, since boiling point and the surrounding pressure are inversely related. There are almost a listless amount of possibilities that could have affected these deviances in boiling point.

Conclusion

This lab was very exciting, as there were many new and different concepts involved. Working with known substances helped to prepare the students to work with unknowns; they learned how to perform the tests and what to look for in the different situations of aldehydes and ketones. The difference in boiling point of the heptaldehyde is disappointing. However, the experiment as a whole was a success.

References

[1] Smith, J. Organic Chemistry, 2nd ed. New York: McGraw-Hill Companies; 2008.

[2] http://phoenix.liu.edu/~nmatsuna/che122/exp13.pdf

[3] http://www.sigmaaldrich.com