chem (ii) 5

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ALLIANZE UNIVERSITY COLLEGE OF MEDICAL SCIENCES FOUNDATION OF MEDICAL STUDIES 2011 CHEMISTRY LABORATORY REPORT NAME : NURUL HASANAH BINTI NOR IKHSAN MATRIX NO. : CPM 33/11C GROUP : A SEMESTER : II DATE : 19.01.2012 EXPERIMENT 5: REACTIONS OF ALDEHYDES AND KETONES

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Page 1: CHEM (II) 5

ALLIANZE UNIVERSITY COLLEGE OF MEDICAL SCIENCES

FOUNDATION OF MEDICAL STUDIES 2011

CHEMISTRY LABORATORY REPORT

NAME : NURUL HASANAH BINTI NOR IKHSAN

MATRIX NO. : CPM 33/11C

GROUP : A

SEMESTER : II

DATE : 19.01.2012

EXPERIMENT 5:

REACTIONS OF ALDEHYDES AND KETONES

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OBJECTIVE

The purpose of carrying this experiment is to differentiate between aldehydes and ketones using qualitative analysis.

INTRODUCTION

There are two types of carbonyl compounds which are aldehydes and ketones in which both of them have a carbonyl group, C=O. An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl group (a carbon double bonded to oxygen) with a hydrogen atom attached to it and an R group which is any generic alkyl or side chain. The group without R is called the aldehyde group or formyl group. Aldehydes differ from ketones in that the carbonyl group is placed at the end of a carbon skeleton rather than between two carbon atoms. The word aldehyde is derived from parts of the words alcohol and dehydrogenated as aldehyde is prepared by removing two hydrogen atoms in a process dehydrogenation from ethanol. Examples of aldehydes are:

CH3-C-H CH3CH2CH2-C-H

Ethanal Butanal

In organic chemistry, a ketone is an organic compound with the structure RC(=O)R', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group, C=O bonded to two other carbon atoms. Thus, ketones never have a hydrogen atom attached to it. The word ketone derives its name from Aketon, an old German word for acetone. Examples of ketones are:

CH3CCH3 CH3CCH2CH2CH3

Propanone (Acetone) 2-pentanone

In this experiment, aldehydes will be differentiate from ketones by conducting a few experiments using a few reagents such as Brady’s reagent, Fehling’s solution, Schiff’s reagent and Tollen’s reagent. Usually, ketone is less reactive compared to aldehyde as it lacks the presence of hydrogen attach to the carbon-oxygen double bond. Hence, in this experiment, aldehyde shows more positive results for the mentioned reagent, where as ketone does not.

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MATERIALS

1. Unknown A2. Unknown B3. 2,4-dinitrophenylhydrazine4. Fehling's solution5. 250 ml Beaker6. Bunsen burner7. Wire gauze8. Tripod stand9. 2.5 M sodium hydroxide solution, NaOH10. 0.3 M silver nitrate solution, AgNO3

11. 5% ammonia solution, NH3

12. Droppers13. Schiff's reagent14. Distilled water15. Rubber stopper16. 10 test tubes17. Water bath (maintained at 70oC)18. Label stickers

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PROCEDURE

A. Brady's Test

1. 1 ml of unknown A and unknown B is placed in two separate test tubes. The tubes are labelled.

2. A few drops of 2,4-dinitrophenylhydrazine is added to each test tube.3. The test tubes are shaken and heated in the water bath for 5-10 minutes. The formation of

precipitate is observed.4. If no precipitate forms,2 ml of distilled water is added.5. All observation is recorded.

B. Fehling's Test

1. 1 ml of unknown A and unknown B is placed in two separate test tubes. The test tubes are labelled.

2. 2 ml of Fehling's solution is added in each test tube.3. The test tubes are shaken gently.4. The mixture is heated in boiling water for 15-20 minutes and any precipitate formation is

observed.

C. Tollen's Test

1. Tollen's reagent is prepared by adding one drop of 2.5 M sodium hydroxide solution, NaOH to 2 ml solution of 0.3 M silver nitrate, AgNO3 in a test tube.

2. 5% ammonia solution, NH3 is added drop by drop until the precipitate dissolves.3. 1 ml of unknown A and unknown B is placed in two separate test tubes. The test tubes are

labelled.4. 1 ml of Tollen's reagent is added to each test tube and the mixtures are shaken gently.5. The mixtures are allowed to stand for 3 minutes and then whether a silver mirror forms in

any of the test tubes are observed.6. If there is no change, the mixture is warmed in a water bath at 70oC for 5 minutes.7. Observations are recorded.

D. Schiff's Test

1. 1 ml of unknown A and unknown B is placed in two separate test tubes. The test tubes are labelled.

2. 1 to 3 drops of Schiff's reagent is added into each test tube.3. Each test tube is shaken gently and the colour change is noted.4. If any of the compounds does not dissolve, the test tube is closed with a rubber stopper

and is shaken vigorously until an emulsion is formed. Then, observation is recorded.

RESULTS

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DISCUSSIONS

TESTOBSERVATION

UNKNOWN A UNKNOWN B

Brady’s Test

The clear solution quickly changes colour to yellow colour when added 2,4-dinitrophenylhydarazine. However, there’s no change occur when heated in water bath and even after adding distilled water.

The clear solution quickly changes colour to yellow colour when added 2,4-dinitrophenylhydarazine. Unknown B emulsifies when heated in water bath for 8 minutes.

Fehling’s Test There’s no change in solution. There’s no change in solution.

Tollen’s Test There’s no change in solution.Silver precipitate is formed but there is no silver mirror.

Schiff’s Test The clear solution turns pink. The clear solution turns light purple.

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In this experiment, all the tests that have been done are a success. Based on the results, it can be shown that unknown A is ketone and unknown B is aldehyde. In Brady’s test, 2,4-dinitrophenylhydrazaline is used to qualitatively detect the carbonyl functionality of a ketone or aldehyde functional group. A positive test signals by a yellow or red precipitate (known as a dinitrophenylhydrazone). If the carbonyl compound is aromatic, then the precipitate will be red but if it is an aliphatic compound, then the precipitate will have a yellow colour. In this test, both unknown A and B turn yellow in colour when added with 2,4-dinitrophenylhydrazine proving that both solutions are aliphatic compound. The test continues by heating the test tubes containing unknown A and B in a water bath for 5-10 minutes to observe any formation of precipitate. Unknown B emulsifies after 8 minutes in the water bath. However, for unknown A there was no change even though 10 minutes has passed, thus 2 ml of distilled water was added but there is still no change. From the reactivity of unknown A and unknown B, it can be seen that unknown B is much more reactive. Hence, unknown B is an aldehyde while unknown A is a ketone.

In Fehling’s test, 1 ml of unknown A and unknown B is placed in different labelled test tubes before been tested with 2 ml of Fehling’s solution. Fehling's solution is a chemical test used to differentiate between water-soluble aldehyde and ketone functional groups, and as a test for monosaccharide. As the both test tubes are shaken, there was no change in the clear solution of both solutions. The test continued by heating both test tubes in boiling water for 15-20 minutes to observe any formation of precipitation. However, there were still no changes in both solutions after more than 20 minutes has passed. This show there was human error such as parallax error when measuring the chemicals. Thus, unknown A and B cannot be differentiated in this test but it is shown that in other test unknown A is ketone and unknown B is aldehyde.

In Tollen’s test, preparation of the Tollen’s reagent must proceed first before any other steps. The Tollen’s reagent is prepared by adding one drop of 2.5 M sodium hydroxide solution, NaOH to 2 ml of 0.3 M silver nitrate solution, AgNO3 in a test tube. Then, 5% of ammonia solution, NH3 is added drop by drop into the test tube until the precipitate dissolves. Tollen’s reagent is a chemical reagent most commonly used to determine whether a known carbonyl-containing compound is an aldehyde or a ketone. It is usually ammoniacal silver nitrate, but can also be other mixtures, as long as aqueous diamminesilver(I) complex is present. A positive test with Tollen’s reagent results in elemental silver precipitating out of solution. Occasionally onto the inner surface of the reaction vessel, it will produce a characteristic and memorable "silver mirror" on the inner vessel surface. Then, 1 ml of unknown A and unknown B are placed in two separate test tubes before 1 ml of the Tollen’s reagent is added into it and is shaken gently. The mixtures are then allowed to stand for 3

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minutes to observe if any silver mirror is formed but there was no silver mirror. Hence, the test proceeds by warming the mixtures in water bath at 700C for 5 minutes. As the results, unknown A shows no changes while silver precipitate is formed in unknown B but no silver mirror is formed. This is automatically proved that unknown A is ketone and unknown B is aldehyde.

Last but not least, Schiff’s test is conducted. It is a chemical test for detection of aldehyde by decolourizing the Schiff’s reagent and a characteristic magenta or purple colour develops which show the presence of aldehyde. Thus, when both 1 ml of unknown A and B are added with 3 drops of Schiff’s reagent in different test tubes respectively, it can be seen that both solution changes colour. Unknown A changes from a clear solution to a pink colour while unknown B changes from a clear solution into a magenta or purple colour. Therefore, it is shown that unknown B is aldehyde due to the change in colour to purple solution when added with Schiff’s reagent. Unknown A is ketone. In this test, there was no need for a rubber stopper as both compounds in the test tubes dissolve.

PRECAUTIONS.

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1. Students must always wear lab coats all the time when conducting the experiment.2. Students must wear gloves when handling with dangerous chemicals such as sodium

hydroxide solution.3. Ensure that all the measurement of the solution is accurate to ensure that the solution will react

and form the correct product.4. Students are required to carry out the mixing of chemicals step under the fume chamber as

some chemical produces gas that is poisonous.5. Ensure that the temperature of the water bath is constant at 70oC.6. All the unused or used waste materials must be poured into a specific waste container prepared in

the lab room to prevent pollution to the environment.

CONCLUSION

In this experiment, the experiment is a success as unknown A and B are known in which unknown A is ketone and unknown B is aldehyde respectively. However, one of the tests is not a success that is the Fehling’s test due to human error. In this experiment, Brady’s test is actually not suitable to differentiate aldehyde and ketone as it is commonly used only to qualitatively detect the carbonyl functionality of a ketone or aldehyde functional group. The other tests like Fehling, Tollen and Schiff can be used for differentiating purposes by differentiating between water soluble aldehyde and ketone, testing the presence of aldehydes, which are later oxidized to carboxylic acids, and the detection of aldehydes respectively.

QUESTIONS

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1. What causes the silver mirror to form in carbonyl compounds in Tollen’s test?The silver mirror is formed because of the reduction of silver nitrate ions, [Ag(NH3)2] +

by the aldehyde to a grey metallic silver compound.

2. Explain the iodoform test on carbonyl compounds.For aldehyde, ethanal is the only one which will give positive result on triiodomethane test where as for ketones, methyl ketones are the one which will give positive result.

3. What other test can be carried out to differentiate aldehydes and ketones?Oxidation of aldehyde with potassium permanganate solution, which will turn from purple to colourless or with potassium dichromate solution which will change from orange to green colour where as ketone will not react in the oxidation process.

REFERENCES

Other than the laboratory manual, the sources below has also contributed in making this report:

Chemistry notes, Chapter 8: Aldehydes & Ketones by Miss Shyamala A. Internet source I : http://en.wikipedia.org/wiki/Ketone Internet source II : http://en.wikipedia.org/wiki/Aldehyde Internet source III : http://www.chemguide.co.uk/organicprops/carbonyls/background.html Internet source IV : http://en.wikibooks.org/wiki/Organic_Chemistry/Ketones_and_aldehydes

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