Aldehydes & Ketones

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- Tollen’s Test

Tollen’s Test is a general test for the presence of aldehydes. In this reaction, an aldehyde is oxidized to a carboxylic acid while the Ag1+ is reduced to silver metal, which deposits as a thin film on the inner surface of the glass or the formation of a “silver mirror”.(Tollens' Test) A negative result in this test indicates that the solution is a ketone. (Identification of aldehydes and ketones)

This explains the formation of a “silver mirror” in both formalin and benzaldehyde which are both aldehydes while no silver mirror formed in acetone since it is a ketone not an aldehyde.[pic 4]

Figure 2. Reaction of Aldehydes with Tollen’s Reagent

- Schiff’s Test

Aldehydes react with Schiff's reagent to produce a color change (purple ormagenta-colored addition product). The Schiff’s reagent acts as a nucleophile that adds to the carbonyl group of an aldehyde. Because this nucleophile is extremely bulky, a ketone, which is more sterically crowded than an aldehyde at the carbonyl carbon, does not react with Schiff's reagent, and thus does not produce the change in color. The change of solution into a magenta color therefore indicates that the unknown is an aldehyde and not a ketone. (Identification of an Unknown-Alcohols,Aldehydes and Ketones)

Therefore since the acetone is acetone, this explains the no change in color and since formalin and benzaldehyde are aldehydes this two test compounds should have produced a purple or magenta color. However, only formalin produced a purple color while benzaldehyde did not change in color, it should then be noted that Schiff’s Test only produces a color change in aliphatic aldehydes. Formalin is an aliphatic aldehyde thus producing a purple color while benzaldehyde is an aromatic aldehyde thus clarifying why it did not change in color[pic 5]

Figure 3. Reaction of Aldehyde with Schiff’s Test

- Benedict’s Test

- Iodoform Test

- Effect of acid catalyst on 2,4-DNP test

Summary and Conclusion

References

Doyle, M. and Mungall, W.(2000). “Experimental Organic Chemistry”. New York: John Wiley and Sons.

Identification of aldehydes and ketones. (n.d.). Retrieved October 23, 2016, from copharm.uobaghdad.edu: http://www.copharm.uobaghdad.edu.iq/uploads/activities%202014/lect.2year%20tagreed/Identification%20of%20aldehyde%20and%20keton%D9%85%D8%B9%D8%AF%D9%84.pdf

Identification of an Unknown-Alcohols,Aldehydes and Ketones. (n.d.). Retrieved October 22, 2016, from umass.edu: http://www.chem.umass.edu/~samal/269/aak.pdf

Lawrence, G. (2010). Aldedhydes and ketones. Retrieved on October 21, 2016, from: http://myweb. brooklyn.liu.edu/lawrence/che4x/e4aldket.pdf.

Razote, B. (2014). Aldehydes and ketones. Retrieved on October 21, 2016, from: https://www.scribd. com/doc/248189422/Aldehydes-and-Ketones-Individual-Laboratory-Report.

Tollens' Test. (n.d.). Retrieved October 23, 2016, from ncsu.edu: https://ncsu.edu/project/chemistrydemos/Organic/TollensTest.pdf

Wenn, B. (2016). Aldehyde and ketones: characterization of the unknown. Retrieved on October 21, 2016, from: https://www.scribd.com/doc/28446740/Experiment-9.