CHM1024 Report 5 : Reactions of Aldehydes and Ketones

FOUNDATION IN MEDICAL STUDIES (JULY 2013 INTAKE) CHM1024 LABORATORY REPORT PRACTICAL 5 : REACTIONS OF ALDEHYDES AND KETONES

NAME

:

AKMAL ADIB BIN FADZIL

MATRIX ID

:

CPM0018_2013C CPM0018_2013C

GROUP

:

A

SEMESTER

:

TWO

DATE

:

21st JANUARY 2014

LECTURER

:

MR. MOHD YUSOFF HUSSAIN

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OBJECTIVE The objective of this experiment is to differentiate between aldehydes and ketones using qualitative analysis.

INTRODUCTION In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double bonded to an oxygen atom, C=O. Aldehydes Aldehydes and ketones are belonged to this group. While both molecules have a carbonyl group, they differ in what atom is bonded to the carbonyl carbon. The carbonyl carbon of an aldehyde is bonded to a hydrogen atom and one carbon atom. The carbonyl carbon of a ketone is bonded to two carbon atoms. Aldehydes and ketones are commonly found in sugars, flavours, steroids and intermediates in biological chemical production. Aldehyde

Ketone

One of the properties of aldehydes and ketones is intermolecular forces. The only intermolecular forces are Dipolar and LDF. The double connection makes the CO double bond even larger dipole than a CO single bond and therefore exerts a more attractive force than ethers. Another one of the physical properties of aldehydes and ketones is solubility. Aldehydes Aldehydes and ketones cannot give a H-bond but they can receive two H-bonds from water since the carbonyl has two lone pairs. Since they have a larger dipole than alcohols, water will form a stronger H-bond to them. This balances out to ake the aldehydes and ketones to have about the same solubility in water as a similar sized alcohol. Besides that, since they cannot give a H-bond the only forces that hold the molecules of the pure substance together is dipole-dipole attractions and LDF attractions. A carbonyl is more polar than an ether connection so we expect the melting and boiling point for similar sized aldehydes and ketones to be higher than ethers and lower than alcohols. Aldehydes and ketones are neutral (neither acidic nor alkaline).







premixed with oxygen in the air. Liquids and solids only burn at the surface and are limited by the amount of oxygen that gets to them. Gases mixed with oxygen can burn all at once. Besides that, aldehydes can be oxidized to carboxylic acids. Ketones are not easily oxidized. This is one way to distinguish between the aldehydes and the ketones. Furthermore, aldehydes and ketones can undergo addition reactions. An addition reaction is where a whole molecule is added across the double bond and it becomes a single bond. In addition, both aldehydes and ketones can be reduced. This type of addition reaction is where H2 is added to the carbonyl to give the corresponding alcohol. Aldehydes can be reduced, [H], back to primary alcohols and ketones can be reduced to secondary alcohols by the addition of H2 and a catalyst. Aldehydes and ketones can both react with alcohols (usually under acid conditions) to form an addition product. Any alcohol should react but usually the alcohols chosen are small like methanol or ethanol. Aldehydes add to the first alcohol to become a hemi-acetal and then add to a second alcohol to form a stable acetal and release water. Ketones add the first alcohol to become a hemi-ketal and then add a second alcohol to form a stable ketal and release water.

For this experiment, several tests will be conducted to identify two unknown solutions where at the end of this experiment, one of the solutions will be identified as aldehyde while the other one is ketone. ketone . The test are Brady’s Brady’s Test, Fehling’s Test, Tollens’ Test Test and Schiff’s Test.







APPARATUS AND MATERIALS 1. Unknown A. 2. Unknown B. 3. 2, 4 – dinitrophenylhydrazine (Brady’s reagent). 4. Fehling’s solution. 5. 250 ml beaker. 6. Bunsen burner. 7. Wire gauze. 8. Tripod stand. 9. 2.5 M sodium hydroxide, hydroxide, NaOH solution. 10. 0.3 M silver nitrate, AgNO 3 solution. 11. 5 % ammonia, NH3 solution. 12. Droppers. 13. Schiff’s reagent. 14. Distilled water. 15. Rubber stopper. 16. 10 test tubes. 17. Water bath maintained at 70 ℃. 18. Labelling stickers.







METHOD A. Brady’s Test. (2, 4 – 4 – DNPH) DNPH) 1. 1 ml of Unknown A and Unknown B are placed into two separate test tubes and labelled. 2. A few drops of of 2, 4 – – dinitrophenylhydrazine (2, 4 – – DNPH) are added into each test tube. 3. The test tubes are shaken and heated in the water bath for 5 – 5 – 10 10 miutes. The formation of precipitate is observed. 4. 2 ml of distilled distilled water water is added if no precipitate precipitate forms. 5. All observations are recorded. B. Fehling’s Test. 1. 1 ml of Unknown A and Unknown B are placed into two separate test tubes and labelled. 2. 2 ml of Fehling’s solution solut ion is added into each test tube. 3. The test test tubes are shaken gently. 4. The mixture is heated in boiling water water for 15 – 20 – 20 minutes. Any formation of precipitate is observed. C. Tollen’s Test. 1. Tollen’s reagent is prepared by adding one drop of 2.5 M NaOH solution into 2 ml solution of 0.3 M AgNO 3 in a test tube. 2. 5 % NH3 solution is added drop by drop until the precipitate dissolves. dissolves. 3. 1 ml of Unknown A and Unknown B are placed into two separate test tubes and labelled. 4. 1 ml of Tollen’s reagent is added into each test tube and the mixtures are shaken gently. 5. The mixtures are allowed to stand for 3 minutes. minutes. Both test tubes are then observed to see whether a silver mirror is formed in any of the t est tubes. 6. The mixture is warmed warmed in the water water bath at 70℃ for 5 minutes if the silver







D. Schiff’s Test. 1. 1 ml of Unknown A and Unknown B are placed into two separate test tubes and labelled. 2. 1 to 3 drops of Schiff’s reagent is added into each test tube. 3. The test tubes are shaken shaken gently. Any changes in colour are noted. 4. If any of the compounds does not dissolve, the test tube is closed with a rubber stopper and shaken vigorously until an emulsion is formed. Any observations are recorded.









RESULTS OBSERVATION TEST UNKNOWN A The colour changes from colourless to yellowish – yellowish – orange orange Brady’s Test

precipitate. The precipitate remains r emains unchanged when heated in water bath and added with distilled water.

Fehling’s Test

Tollens’ Tollens’ Test

Schiff’s Test

The blue colour of the mixture remains unchanged.

The mixture remains unchanged. unchanged.

The colour of the solution turns from light pink to t o magenta.

UNKNOWN B The colour changes from colourless to yellowish – yellowish – orange orange precipitate. The precipitate emulsifies when heated in water bath. The colour of the mixture has changed from blue into red. A brickred precipitate is formed. Silver precipitate is formed and eventually a silver mirror is formed. The colour of the solution turns from light pink to dark purple. An emulsion is formed.







DISCUSSIONS Brady’s Test. Test. For the first test, a few drops of 2, 4 - DNPH placed in two separate test tubes containing Unknown A and B respectively. The test tubes are shaken to allow the mixture to dissolve. This step is done to qualitatively detect the carbonyl functionality of the ketone or aldehyde functional group. A positive test is signalled by a yellow, orange or red precipitate known as 2, 4 - dinitrophenylhydrazone. If the carbonyl compound is aromatic, then the precipitate will be red; if aliphatic, then the precipitate will have a more yellow colour. In this test, both of the unknowns turn yellowish – yellowish – orange orange in colour when added with 2, 4 – DNPH – DNPH which means both solutions are aliphatic compounds.

Mechanism of reaction of an aldehyde with 2, 4 – DNPH – DNPH to form 2, 4 – 4 – dinitrophenylhydrazone. dinitrophenylhydrazone.

Mechanism of reaction of a ketone with 2, 4 – DNPH – DNPH to form 2, 4 – 4 – dinitrophenylhydrazone.







The test to t o identify the unknowns continues by using derivatization derivatization technique. Both test tubes (containing 2, 4 – – dinitrophenylhydrazone) are heated in a water bath for 5 – – 10 minutes. Mixture containing Unknown B emulsifies while the other one remains unchanged even when added with distilled water. In the end, it can be said that the Unknown B is an aldehyde while Unknown A is ketone.

Fehling’s Test. For this test, 2 ml of Fehling’s solution i s added to two separate test tubes containing Unknown A and B respectively. Both mixtures in the test tubes are then shaken and heated in boiling water for 15 – – 20 minutes. After the heating process, it is observed that the blue colour of the mixture containing Unknown B has changed from blue into red and a brick – red – red precipitate is formed while the mixture containing Unknown A remains unchanged. Therefore it can be said that Unknown B is an aldehyde while Unknown A is a ketone. The reason for the above statement is that Fehling’s solution contains bistartratocuprate bistartratocuprate (II) complex which is an oxidising agent (which is also the active reagent in the test) and therefore it needs a reducing agent for a redox reaction to occur. Aldehydes are a reducing agent so it will react with the bistartratocuprate bistartratocuprate (II) complex in the Fehling’s solution to form a redox

reaction.

The

bistartratocuprate(II)

complex

oxidizes

the

aldehyde

to

a carboxylate anion, and in the process the copper(II) ions of the complex are reduced to copper(I) ions. Red copper(I) oxide then precipitates out of the reaction mixture which forms into the brick – red – red precipitate that has been observed above. The carboxylic acid produced reacts further with the alkali to for a salt, carboxylate (RCOO (RCOO -) and water. Mechanism of reaction of aldehyde and Fehling’s solution.







Tollens’ Test. Before starting this experiment, Tollens’ reagent is needed to be prepared by mixing one drop of 2.5 M NaOH solution into 2 ml solution of 0.3 M AgNO 3 in a test tube and adding 5% NH3 solution is added drop by drop until the precipitate dissolves. This is because Tollens’ reagent is not commercially available due to its short life. Because of that, this reagent is prepared freshly in the lab and used for the experiment immediately. To start this experiment, 1 ml of Unknown A and Unknown B is added into two separate test tubes and followed by adding 1 ml of Tollens’ reagent into each test tube. Both test tubes containing the mixture are shaken gently and allowed allowed to stand for 3 minutes. After 3 mi nutes, it is observed that both of the mixtures remained unchanged. Therefore, both test tubes are warmed in a water bath for 5 minutes. At the end, it is observed that silver precipitate formed in the test tube containing Unknown B and eventually silver mirror is formed in the test tube. The other test tube which contains Unknown A remains unchanged. Therefore, it can be deducted that Unknown B is an aldehyde while Unknown A is a ketone. This is because aldehydes are more readily oxidised compared to ketones which is due to the

carbonyl-containing

carbon

in

aldehydes

having

an

attached

hydrogen.

The

diamminesilver (I) complex in the mixture is an oxidizing an oxidizing agent and is the essential reactant in Tollens' reagent. When Tollens’ reagent reacts with an aldehyde, the diamminesilver (I) complex oxidizes the aldehyde to a carboxylate ion and in the process is reduced to elemental silver and aqueous ammonia. The elemental silver precipitates out of solution, occasionally onto the inner surface of the reaction vessel, giving a characteristic "silver mirror". The carboxylate ion on acidification will give its corresponding carboxylic acid. The carboxylic acid is not directly formed in the first place as the reaction takes place under alkaline conditions. The general equation for the overall reaction is shown below: RCHO + 2[Ag(NH 3)2]+ + OH- → RCOO- + 2Ag(s) + 2NH 4+ +2NH3 The ionic equations for the reactions are as follows:







Schiff’s Test. The Schiff test is an early organic early organic chemistry name chemistry name reaction developed by Hugo by Hugo Schiff and is relatively general chemical general chemical test for detection of many organic aldehydes organic aldehydes that has also found use in the staining of biological tissues. The Schiff reagent is the reaction product of a dye formulation

such

as

fuchsin and sodium

bisulfite; pararosaniline bisulfite; pararosaniline (which

an aromatic an aromatic methyl methyl group) andnew andnew fuchsin (which is uniformly mono-methylated

lacks

ortho to

the

dye's amine functionalities) are dye alternatives alternatives with comparable detection chemistry. chemistry. For this experiment, 1 – 1 – 3 3 drops of Schiff ’s reagent is added into 2 test tubes which contains Unknown A and B respectively. Both test tubes are shaken gently. For Unknown A, the light purple colour of the Schiff ’s reagent changes into magenta with no emulsion. For Unknown B, the light purple colour of the Schiff ’s reagent changes into dark purple with an emulsion. Theoretically, Theoretically, an aldehyde reacts with Schiff ’s reagent to produce a magenta colour which is a positive result while ketones do not react with Schiff ’ Schiff ’s reagent. Therefore, it can be said that there was an error in this experiment which may be caused by an expired Schiff ’ Schiff ’s reagent or accidentally mixing other substance into the test tube or the test tube is not cleaned properly and thoroughly. Precautions. Throughout the experiment, experiment, several precautions are taken and noted. One of them is to wear lab coats and protective gloves to prevent the substances and reagent used in the experiment to spill into the clothes and hands in case the experiment is not properly conducted as some of them are hard for the stain to wear off. Besides that, the wastes of the experiment (used substances and reagents) should be discarded in a special beaker provided by the lab instructor. This is because some of the substances and reagents may for other harmful substances. One example is the Tollens ’ reagent. The reagent should be acidified with dilute acid after the test before it is discarded to prevent the formation of the highly explosive silver nitride. Another one is to make sure the test tubes which are going to









CONCLUSION In conclusion, it can be deduced that Unknown A is a ketone while Unknown B is an aldehyde by using qualitative analysis from Brady ’s Test, Fehling’ Fehling’s Test, Tollens’ Tollens’ Test and Schiff ’s Test. Unknown B which is an aldehyde is more reactive than Unknown A which is a ketone. This is caused by the functional group of the aldehyde which contains a hydrogen atom and an alkyl group that makes it less stable than ketone which does not have the hydrogen atom but instead 2 alkyl groups.







QUESTIONS 1. What causes the silver mirror to form in carbonyl compounds compounds in Tollens’ Tollens’ Test? The formation of silver mirror in Tollens’ Test is caused by the reduction of silver nitrate ions, [Ag(NH 3)2]+ into grey metallic silver compound by aldehyde which is a reducing agent. 2. Explain the iodoform iodoform test on carbonyl carbonyl compounds. compounds. The aldehyde or ketone is mixed with aqueous sodium hydroxide. After that, iodinepotassium iodide solution is added to the mixture. The yellow precipitate that is formed give an indication that both aldehyde and ketone reacts with the iodinepotassium iodide solution to produce acetaldehyde and methyl ketone respectively. 3. What other test can be carried carried out to differentiate aldehydes and ketones? Another test that can be carried out to differentiate aldehydes aldehydes and ketones is chromic acid test (Bordwells Reagent or Jones Reagent). The chromic acid (CrO3) will change in colour from orange to brown to greenish-blue when it oxidises an aldehyde. As for ketones, the colour of the chromic acid will remain orange as ketones do not oxidise.







REFERENCES 1. CHEMISTRY FOR MATRICULATION MATRICULATION SEMESTER SEMESTER 2 FOURTH EDITION, Tan Yin Toon, Oxford Fajar Sdn. Bhd. 2013. 2. http://www.chemguide.co.uk/orga http://www.chemguide.co.uk/organicprops/carbo nicprops/carbonyls/background nyls/background.html .html 3. http://spot.pcc.edu/~md http://spot.pcc.edu/~mdeming/102/La eming/102/Labs/CH102_L bs/CH102_Lab_5_Alde ab_5_Aldehydes_and hydes_and_Ketones.pdf _Ketones.pdf 4. http://myweb.brooklyn.liu.edu/law http://myweb.brooklyn.liu.edu/lawrence/che4x/e4a rence/che4x/e4aldket.pdf ldket.pdf 5. http://employees.oneonta.edu/knaue http://employees.oneonta.edu/knauerbr/chem226 rbr/chem226/226expts/226_ /226expts/226_expt09_pro.pd expt09_pro.pdff 6. http://faculty.swosu.edu/william http://faculty.swosu.edu/william.kelly/pdf/ketone.p .kelly/pdf/ketone.pdf df 7. http://www.chem.umass.edu/~samal/269/aak.pdf 8. http://www.wiu.edu/users/mftkv/Che http://www.wiu.edu/users/mftkv/Chemistry102/oxid mistry102/oxidationaldehy ationaldehydes.html des.html 9. http://stainsfile.info/StainsFile/stain/sc http://stainsfile.info/StainsFile/stain/schiff/schiffwhatis.htm hiff/schiffwhatis.htm 10. http://www.scribd.com/doc/963797 http://www.scribd.com/doc/96379782/CHEM-II-5 82/CHEM-II-5 11. http://en.wikipedia.org/wiki/Ca http://en.wikipedia.org/wiki/Carbonyl#Ca rbonyl#Carbonyl_compou rbonyl_compounds nds

CHM1024 Report 5 : Reactions of Aldehydes and Ketones

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