School of Chemical Engineering and Chemistry

Mapua Institute of Technology

Organic Chemistry Laboratory 2 Final Report

Aldehydes and Ketones

Gian Vincent C. Dizon, Lorissa M. Egalam, Regina V. Entrolizo

Prof. Ureah Thea A. Sevilla

School of Chemical Engineering and Chemistry, Mapua Institute of Technology, Intramuros, Manila, Philippines

Experiment No., Submitted on November 10, 2010 at N402.

ABSTRACT

In this paper, we are to identify tests that may be used to classify aldehydes and ketones and explain the chemical basis of these tests. This paper also includes how to distinguish an unknown organic compound as either an aldehyde or a ketone. We first do so by having different compounds undergo oxidation reactions: Benedict’s test and Tollen’s test. Under acidic conditions, in this case, the Benedict’s test, the aldehyde is oxidized to a carboxylic acid and makes the solution red, yellow or yellowish-green in color. Under alkaline conditions, the Tollen’s test, this couldn't form because it would react with the alkali. A salt is formed instead and the aldehyde reduces the diamminesilver(I) ion to metallic silver. We also test the reactions of the carbonyl groups of an aldehyde and a ketone. Both aldehydes and ketones react to the hydrazine group of 2,4-dinitrophenylhydrazine and the yellow to orange precipitate confirms the reaction due to condensation reaction. They also undergo nucleophilic addition which explains why both formed precipitate during the sodium bisulfite test. Lastly, we are to observe the reactions of the alkyl groups in aldehydes and ketones where the yellow iodoform formed during the oxidation of the ketone to a carboxylic acid, indicating a positive test. Methyl ketones, but not other ketones, are oxidized by iodine in aqueous sodium hydroxide. This explains why only acetone showed a positive result during the experiment.

Keywords: aldehydes, carbonyl, condensation, iodoform, ketones, nucleophilic addition, oxidation

1 INTRODUCTION

Aldehydes and ketones share the carbonyl functional group which features carbon doubly bonded to oxygen. In the case of ketones there are two carbon atoms bonded to the carbonyl carbon and no hydrogens. In the case of aldehydes there is at least one hydrogen bonded to the carbonyl carbon; the other attachment may be to a carbon or a hydrogen. In all cases the carbon(s) that are attached to the carbonyl group may be aliphatic (not part of an aromatic ring) or aromatic (part of an aromatic ring). Since they share the carbonyl group, aldehydes and ketones share much of their chemistry, but they are different enough to be considered different classes of compounds. This situation is similar to that of alcohols and phenols which both share the -OH group.

In this experiment, we are to identify tests that may be used to classify aldehydes and ketones, to explain the chemical basis of each test used to classify aldehyde and ketones and to classify an unknown organic compound as neither an aldehyde or a ketone.

First test is the benedict’s test. The test reagent is made up of sodium citrate and hydrous sodium carbonate. Copper sulfate is then added to determine the test result. When there is precipitate formed, it is a positive test.

Aldehydes are also oxidized by Tollens’ reagent, a substance that contains Ag+1. The silver ion is, concomitantly, reduced to metallic silver. Silver ion is a weak oxidizing agent; aldehydes are very easily oxidized and are essentially unique in being able to reduce silver ion to silver metal.

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Methyl ketones, but not other ketones, are oxidized by iodine in aqueous sodium hydroxide. The ketone is oxidized to a carboxylic acid; yellow iodoform also forms. It is the yellow iodoform that is indicative of a positive test. Acetaldehyde, but not other aldehydes, give this test owing to its structural similarity to methyl ketones. It is also true that ethanol (oxidized to acetaldehyde) and secondary alcohols that can be oxidized to methyl ketones give this test.

2 METHODOLOGY

2.1 Materials

ReagentsAcetaldehyde acetone benzaldehydeAcetophenone Benedict’s reagentTollen’s reagent dilute H2SO4 95% ethanol8.0M NaOH saturated NaHSO3

2,4-dinitrophenylhydrazineIodine in potassium iodide solution

Apparatuspipets Test tube rackmicro test tubes with cork stopperdroppper hot platewarm water bath thermometer

2.2 Methods

We first let the following test compounds undergo oxidation reactions: acetaldehyde, acetone, benzaldahyde and acetophenone. We had them undergo the Benedict’s test by dissolving 1mL of each test compound in 2mL distilled water in separate micro test tubes. We mixed each with 2mL of Benedict’s reagent then carefully boiled the mixture.

The next oxidation reaction was using the Tollen’s reagent. We added 2mL of each test compounds to 1mL freshly prepared Tollen’s reagent in separate micro test tubes. When there was no reaction, we placed the solution in a warm water bath avoiding excessive heating that will decompose the reagent.

We then observed the reactions of the carbonyl group of acetaldehyde and acetone. The first is the condensation reaction performed by adding 2 to 3 drops of acetaldehyde and 1mL of 2,4-dinitrophenylhydrazine to 1mL of 95% ethanol. We shaked the mixture and let it stand. When there was no precipitate within 10 minutes, we added dilute H2SO4 dropwise until precipitation occurs. We repeated these procedures using acetone and recorded the observations.

The next carbonyl group reaction is through addition reaction where we added 1mL of saturated NaHSO3 to each test compounds in separate test tubes. We shaked the mixtures and observed the reactions for 3 to 5 minutes.

Finally, we observed the the haloform reaction or the reaction of the alkyl group of acetaldehyde and acetone. We did this by mixing 1mL of each compound in separate test tubes with 1mL of water. Into each, we added 2 drops of 8M NaOH. Dropwise, we added iodine in potassium iodide solution until a pale yellow color was observed. We warmed the solutions at 50°C and set them aside for 5-15 minutes and observed.

3 RESULTS AND DISCUSSION

3.1 Oxidation Reactions

CompoundObservations

Benedict’s Test Tollen’s Test

Acetaldehyde

Formation of 2 layers: yellowish

green precipitate as upper layer and blue lower kayer (+) (1)

Formation of 2 layers: cloudy

upper layer and clear silver

precipitate (+) (5)

AcetoneFormation of yellow to orange precipitate (-) (2)

No precipitate formed in the

solution (-) (6)

BenzaldehydeWith precipitate(+) (3)

Viscous dirty white lower layre

with silver precipitate (+) (7)

Acetophenone

Immiscible yellow upper layer and clear blue lower

layer (-) (4)

Viscous pale yellow upper

layer and clear lower layer

without precipitate (-) (8)

Table 1: Observations in the reaction of the test

compounds with Benedict’s reagent and with Tollen’s reagent

Reactions involved

(1)

(2)

(3)

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+ 2Cu+2 + 4OH- + 2Cu+2 + 4OH-

+ 2Cu+2 + 4OH- No rxn

+ 2Cu+2 + 4OH- + 2Cu+2 + 4OH-

(4)

(5)

(6)

(7)

(8)

Table 2: Reactions involved of the test compounds with Benedict’s reagent and and Tollen’s reagent

Benedict’s test and tollen’s test was used to classify carbonyl compounds as aldehyde or ketones.

The oxidation of aldehydes can be performed with a mild oxidizing agent, such as Ag+ in ammonia solution used in the Tollens test or Cu2+ in alkaline solution used in Benedict's test. If the Tollens test is performed in a scrupulously clean glass vessel, the silver metal is plated on the walls of the glass to form a silver mirror. Benedict’s reagent is a deep blue Cu2+ solution that forms a brick-red precipitate of Cu2O in the presence of aldehydes.

In the experiement, acetaldehyde and benzaldehyde are the two test compounds who have the positive test result in both Tollen’s and Benedict’s reagent. Acetaldehyde and benzaldehyde are both aldehyde while acetone is an alcohol and acetophenone is a phenone.

3.2 Reactions of the Carbonyl Group

3.2.1. Condensation Reaction

Compound ObservationsAcetaldehyde Formation of orange precipitate

Acetone Formation of yellow to orange precipitate

Table 1: Observations in the reaction of the test compounds with 2, 4-dinitrophenylhydrazine

Dinitrophenylhydrazine is a substituted hydrazine, and is often used to qualitatively test for carbonyl groups associated with aldehydes and ketones. 2,4-dinitrophenylhydrazine can be used to detect the carbonyl functionality of a ketone or aldehyde functional group.

As seen in the results of the experiment, both acetaldehyde and the acetone reacted and formed an

orange precipitate with the 2,4-dinitrophenylhydrazine. This means that they are both positive to the dinitrophenylhydrazine test, and they contain a carbonyl group. This reaction can be described as a condensation reaction, with two molecules joining together with loss of water.

3.2.2. Addition Reaction

Compound ObservationsAcetaldehyde Two layers formed

Acetone Two layers formed (precipitate)

Table 1: Observations in the reaction of the test compounds with Saturated NaHSO3

When an organic compound with a carbonyl functional group is to react with Sodium bisulfite (NaHSO3), a bisulfite is produced in the form of a white precipitate. The Sodium bisulfite test is used to predict the presence or absence of a carbonyl group.

The formation of a bisulfite addition complex is suggesting a variety of carbonyl compounds. This reaction is greatly influenced by the spatial arrangement of the carbonyl group, thus this reaction does not occur with certain hindered ketones. The procedure works best with aldehydes. A positive test for aldehydes and ketones is indicated by the appearance of a white precipitate.

In the experiment, acetaldehyde and acetone both formed two layers. The two compounds both contain a carbonyl group.

3.3 Reactions of the Alkyl Group (Haloform Reaction)

Compound ObservationsAcetaldehyde Formation of orange precipitate

AcetoneColor change from clear to yellow,

yellow precipitate also formed

Table 1: Observations in the reaction of the test compounds with Iodine in basic solution

Methyl ketones can be distinguished from other ketones by their reaction with iodine in a basic solution to yield iodoform (CHI3) as a yellow colored precipitate. However, acetaldehyde (CH3CHO) and alcohols with their hydroxyl groups at the 2 position of the chain will also form iodoform under these same conditions. Alcohols of the type described are easily oxidized to methyl ketones under the conditions of the iodoform reaction (I2 is an oxidizing agent). The other product of the iodoform reaction is the sodium or potassium salt of a carboxylic acid.

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+ 2Cu+2 + 4OH- No rxn

+ 2Ag(NH3)2OH + 2Ag + +H2O +NH3

+ 2Ag(NH3)2OH No rxn

+ 2Ag(NH3)2OH + 2Ag + +H2O +NH3

+ 2Ag(NH3)2OH No rxn

The results of this part show that both acetaldehyde and acetone formed a precipitate. We may say that acetone and acetaldehyde both have a methyl substituent in them.

4 CONCLUSION

Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O. The carbon atom of this group has two remaining bonds that may be occupied by hydrogen or alkyl or aryl substituents. If at least one of these substituents is hydrogen, the compound is an aldehyde. If neither is hydrogen, the compound is a ketone. Since they share the carbonyl group, aldehydes and ketones share much of their chemistry, but they are different enough to be considered different classes of compounds. This situation is similar to that of alcohols and phenols which both share the -OH group.

The difference between an aldehyde and a ketone is the presence of a hydrogen atom attached to the carbon-oxygen double bond in the aldehyde. Ketones don't have that hydrogen. The presence of that hydrogen atom makes aldehydes very easy to oxidize while ketones don't have that particular hydrogen atom which makes them resistant to oxidation.

This being said, we performed useful tests for aldehydes, the Tollens' test and Benedict's test, taking advantage of the ease of oxidation by using Ag(+) and Cu(2+)

as oxidizing agents. When silver cation is the oxidant, the aldehyde reduces this ion to metallic silver in the course of the reaction, and this deposits as a beautiful mirror on the inner surface test tube. At the same time, the aldehyde is oxidized to a carboxylic acid. During this test, it was satisfying to get a silver mirror. Life isn’t fair though, because in one of our test tubes, we only had a grey precipitate which still indicated that the test worked.

The Benedict’s test uses cupric cation as the oxidant. This deep blue reagent is reduced to cuprous oxide, which precipitates as a red to yellowish-green solid. In this reaction, the aldehyde reacts with the alkali and the ammonium salt of carboxylic acid is formed. It was not a pleasant thing to wait for the colorful precipitate but it was fun to see the yellowish-green with red precipitate appear from the blue solution. In both tests, acetaldehyde and benzaldehyde showed positive results for the tests.

Identifying the carbonyl group of aldehydes and ketones is easy since both react to the hydrazine group of 2,4-dinitrophenylhydrazine forming a condensation compound which came out as a yellow to orange precipitate. The precipitate indicated the presence of a carbonyl group. In both less acidic and more acidic conditions, both acetaldehyde and acetone came out as

positive with the tests showing a bright orange precipitate formation.

Under less acidic conditions, the reaction with 2,4-dinitrophenylhydrazine, a nucleophile donates a pair of electrons toward the carbonyl carbon forming a single bond to it. At the same time the double bond between the carbonyl carbon and oxygen becomes a single bond as one bonding pair of electrons in the double bond moves to become an unshared pair on the oxygen. The oxygen now has one bond to it and it holds three pairs of unshared electrons, so it has a negative charge. Consequently, the oxygen picks up a proton and becomes an -OH group. This product undergoes an elimination reaction in which the -OH is removed from the carbon to which it is attached and the hydrogen is removed from the nitrogen, resulting in a double bond between the nitrogen and carbon and a molecule of water. The final product is known as a 2,4-dinitrophenylhydrazone.

The presence of a carbonyl group was also tested using the sodium bisulfate test. Both were positive to this test showing the formation of a precipitate. Under a more acidic condition, the reaction with saturated NaHSO3, a proton attaches itself to one of the unshared pairs of electrons on the oxygen. The carbonyl group now has a +1 charge and is very inviting to even a weak nucleophile. So, a nucleophile attacks the carbonyl carbon forming a bond and the doubly bonded oxygen of the carbonyl becomes an -OH, as before.

Methyl ketones, but not other ketones, are oxidized by iodine in aqueous sodium hydroxide. The ketone is oxidized to a carboxylic acid; yellow iodoform also forms. It is the yellow iodoform(CHI3) that is indicative of a positive test. In the experiment, only acetone, a ketone with methyl, was positive for the test. The yellow precipitate, however, was hard to distinguish since they were too tiny to see.

ACKNOWLEDGMENT

We extend our gratitude and love to all of our parents and friends who never fail to support and help us especially in the accomplishment of this written report. We also thank Prof. Sevilla for the discussion, assistance and advice that helped us in this paper. We thank everybody who is always patient with us even though we suffer sleep deprivation and sometimes get cranky because of stress inflicted by this report.

Most of all, we thank the Lord for His guidance and never-ending knowledge shone upon us.

REFERENCES

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[1] Retrieved November 6, 2010, from http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/aldket1.htm

[2] Aldehydes and Ketones. (n.d.). Retrieved November 6, 2010, from

http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA5/MAIN/1ORGANIC/ORG12/MENU.HTM [3]

Chem 211 - Tests for Aldehydes and Ketones. Retrieved November 6, 2010, from http://www.wellesley.edu/Chemistry/chem211lab/Orgo_Lab_Manual/Appendix/ClassificationTests/aldehyde_ketone.html

[4] Oxidation of aldehydes and ketones. Retrieved November 6, 2010, from

http://www.chemguide.co.uk/organicprops/carbonyls/oxidation.html

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