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Reactions of aldehydes and ketones. The purpose of this experiment is to compare some reactions of ethanal and propanone.

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Introduction

Date: 8/11/2011 Exp. No.: 25 Title: Reactions of aldehydes and ketones Aim: The purpose of this experiment is to compare some reactions of ethanal and propanone. Introduction: In this experiment, ethanal and propanone were chosen as they are relatively safe. The structures of the 2 compounds are as follow: Ethanal Propanone In this experiment, we are going to investigate the following reactions of these 2 compounds: 1. Addition 2. Condensation 3. Oxidation 4. Iodoform reaction Apparatus and chemicals: Apparatus: Safety spectacles, hot water bath, cold water bath, test tubes, beakers Chemicals: Saturated hydrosulphite solution , 2,4-dinitrophenylhydrazine solution, 0.1M potassium dichromate solution, 1M sulphuric acid, Fehling's reagent (Fehling solution A + Fehling solution B), 0.05M Silver nitrate solution, 2M sodium hydroxide solution, 2M ammonia solution, iodine solution 10% (in KI(aq)) Procedures: Part 1: Addition reaction with sodium hydrogensulphite 1. About 2 cm3 of saturated sodium hydrogensulphite solution was added into a test tube. 2. The test tube was put into a cold water bath. 3. Similar volume of ethanal was added drop by drop into the test tube. 4. The experiment was repeated using propanone instead of ethanal. Part 2: Condensation reaction with 2,3-dinitrophenylhydrazine 1. About 1 to 2 drops of ethanal was added into a test tube. 2. The test tube was put into a warm water bath. ...read more.

Middle

However, when doing this test, we should only add 1 to 2 drops of ethanal/ propanone into the test tube. If too much carbonyl compound was added into the test tube, it will dissolve the product formed as ethanal and propanone are both good polar and nonpolar solvent. So, we cannot see any precipitate if too much are added into the test tube. Yellow precipitate was formed 2. Tollens' reagent (Distinguish aldehyde from ketone) The formula of this reagent is Ag(NH3)2+. As this reagent is not very stable, it must be prepared freshly in laboratory. To prepare the reagent, aqueous ammonia can be added in a continuous fashion directly to silver nitrate solution. At first, silver oxide will be formed and precipitate out, but as more ammonia solution is added the precipitate dissolves and the solution becomes clear as diamminesilver(I) is formed. At this point the addition of the ammonia should be stopped. This reagent is used in the silver mirror test. In this test, when there is the presence of aldehyde group, there would be formation of silver mirror. The equation of this reaction is as below [Ag(NH3)2]+ (aq) + e- � Ag (s) + 2 NH3 (aq) RCHO (aq) + 3 OH- � RCOO- + 2 H2O + 2 e- The aldehyde acts as an reducing agent where [Ag(NH3)2]+ was reduced to Ag(S) ...read more.

Conclusion

Finally a tri iodo methyl carbonyl derivate is formed. The next step is a nucleophilic attack by hydroxide on the carbonyl carbon atom. A carbon - carbon bond cleavage occurs and a triiodomethanide ion departs. The triiodomethanide ion is unusually stable. Its negative charge is dispersed by the three negative iodine atoms. In the last step a proton transfer takes place between carboxylic acid and triiodomethanide ion to form ultimately carboxylate ion and iodoform. Other method of distinguishing carbonyl compound Using IR spectrum Using the IR spectrum, if the compound contains a carbonyl (C=O) group, there will be a stretch in the wave number 1670-1820 with high intensity. Below is the IR spectrum of propanone In the test with Tollens' reagent, we need to use a very clean test tube. Why? This is because if the test tube contains tap water or other impurities, there may be precipitate formed when silver ion is added into the test tube as tap water contain a small amount of Cl-. Moreover, if the test tube is not clean, the surface of the inner wall of the test tube may not be smooth, so the silver may not be evenly distributed on the wall of the test tube which makes silver mirror not able to be formed. Reference: http://www.chemguide.co.uk/organicprops/alcohols/oxidation.html http://www.docbrown.info/page07/redox3.htm http://chemistry.boisestate.edu/people/richardbanks/organic/nomenclature/ethernomenclature1.htm http://www.newworldencyclopedia.org/entry/Ketone http://www.lookchem.com/Chempedia/Basic-Chemical/Chemical-Reaction/8524.html http://www.demochem.de/D-Tollens-e.htm http://www.chemguide.co.uk/organicprops/carbonyls/addelim.html http://www.jce.divched.org/JCESoft/CCA/CCA5/MAIN/1ORGANIC/ORG12/TRAM12/B/0380011/MOVIE.HTM http://www.demochem.de/D-Fehling-e.htm http://quest.videofoundry.co.nz/biology/Fehling's_solution.pdf http://www.demochem.de/D-Iodof-e.htm http://en.wikipedia.org/wiki/Iodoform http://www.chemguide.co.uk/organicprops/alkenes/kmno4.html http://www.chemguide.co.uk/organicprops/carbonyls/iodoform.html http://chemvista.org/Iodoform%20reaction.html http://www.hull.ac.uk/chemistry/spectroscopy/infrared_spectrum.php?res=&molecule=propanone http://www.creative-chemistry.org.uk/alevel/module3/documents/N-ch3-16.pdf http://chemistry.about.com/od/factsstructures/ig/Chemical-Structures---A/Acetaldehyde-or-Ethanal.htm http://www.chemspider.com/Chemical-Structure.172.html http://www.chemguide.co.uk/mechanisms/addelim/whatis.html http://en.wikipedia.org/wiki/Nucleophilic_addition http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch17nucleophilicaddition.html http://www.oup.com/uk/orc/bin/9780199277896/01student/learning_outcomes/ch23.pdf ?? ?? ?? ?? ...read more.

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This is a fantastic, extremely detailed account with excellent chemistry used throughout. I would highly recommend it to anyone studying the identification of aldehydes or ketones

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Marked by teacher Brady Smith 14/07/2012

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