Aldehydes & Ketones

Experiment 7

ALDEHYDES AND KETONES

Aquino, D.M., Focasan, S.J., Gutierrez, J.M., Remot, D.M.

College of Science

University of the Philippines Baguio

Abstract

The objective of this experiment is to distinguish and qualitatively analyse aldehydes and ketones through various tests. In the reaction with 2,4-dinitrophenylhydrazine (2,4-DNP),  the acetone and benzaldehyde both reacted confirming the presence of ketones and aldehydes respectively. In the Tollen’s test, both formalin and benzaldehyde formed a silver mirror. In the Schiff’s test and Benedict’s test, formalin reacted forming a deep purple color and a formation of brick red precipitate, respectively. In the Iodoform test, there was a formation of yellow precipitate in both ethyl alcohol and acetone. And in the effect of acid catalyst on 2,4-DNP test, all three formed a precipitate with 1 M HCl having the most precipitate, followed by 0.1 M HCl and 0.1 M acetic acid the least. This experiment is important to help students be able to distinguish and characterize aldehydes and ketones by observing and comparing their reactions through several tests.

Introduction

Aldehydes and ketones both contain the carbonyl functional group, which features a carbon that is doubly bonded to an oxygen atom. However, they behave differently because of the difference in the groups attached to their carbonyl carbons (Razote, 2014).

         In aldehydes, at least one hydrogen atom and an alkyl group is attached to the carbonyl carbon. As compared to the ketones, aldehydes can be easily oxidized because of the presence of a hydrogen atom bonded to the carbonyl group. Figure 1 shows the structure of an aldehyde.

[pic 1]

Figure 1. An aldehyde

 Mild oxidizing agents such as  Ag+ in ammonia solution (used in Tollen’s test) or Cu2+ in alkaline solution (used in Fehling's test) can be used in their oxidation (Lawrence, 2010).

In ketones however, two alkyl groups are attached to the carbonyl carbon. Because of the absence of a hydrogen atom, ketones are usually resistant to oxidation. They can only however be oxidized using powerful oxidizing agents (Wenn, 2016). Figure 2 shows the structure of a ketone.

[pic 2]

Figure 2. A ketone

Our objective is to qualitatively analyze aldehydes and ketones using various tests. These tests includes the Tollen’s test, to test the presence of aldehydes. Tollen’s reagent is composed of silver nitrate complexed with ammonia along with Benedict’s test, with a reagent composed of solution of copper ions complexed with citrate in sodium bicarbonate. Aldehydes can also be distinguished from ketones using the Schiff’s test, whose reagent is a pink fuchsine dye which decolorizes into a purple-violet colored solution in the presence of aldehydes. Iodoform test, is used to test for the presence of a methyl group attached to the carbonyl group of the ketone. This test results to a yellow solid substance when methyl ketones react with iodine in the presence of a base (Doyle, 2000). We aim to establish a given compound’s identity and to distinguish whether it is an aldehyde or a ketone using the tests stated.

Results and Discussion

Table 1. Observed Results of the Reactions of Test Compounds

The table above (Table 1) shows the recorded observation on the reactions of Ketone and Aldehyde test compounds from the experiment. The data recorded is divided into six sections based on the test used.

1. Reaction with 2,4-DNP

Ketones and Aldehydes react with 2,4-dinitrophenylhydrazine reagent to form yellow or orange precipitates, whereas alcohols do not react. The formation of a precipitate therefore indicates the presence of a ketone and aldehyde. (Identification of an Unknown-Alcohols,Aldehydes and Ketones)

Thus explaining why no precipitate formed in the test compound with only ethyl alcohol while an orange precipitate formed in ethyl alcohol + acetone and a yellow-orange precipitate formed in ethyl alcohol + benzaldehyde because both test compounds contain either an aldehyde or a ketone.[pic 3]

2,4-dinitrophenyl hydrazine, HCl and, ethanol solution is also known as Brady’s reagent. Brady’s reagent is used for determining the presence of aldehydes or ketones in a given unknown solution. Acetone and Benzaldehyde reacted with 2,4-dinitrophenylhydrazine reagent and formed yellow to yellow-orange precipitate. The formation of a precipitate indicates the presence of a ketone and aldehyde in the solution (Identification of an Unknown-Alcohols,Aldehydes and Ketones).

        The lone pair in the amino group of 2,4-DNP makes the molecule a strong nucleophile. The reaction starts by the nucleophilic attack of the 2,4-DNP to acetone’s and benzaldehyde’s carbonyl group. As a product the reaction produced hydrazone and water. The reaction is an example of a condensation reaction, where two molecules fuse together and lose a water molecule in the process (“Why doesn’t the Brady’s test (2, 4-DNPH) work on carboxylic acids?,” 2016). The reaction is shown in the figure below.

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