Reactions of aldehydes and ketones Essay
Reactions of aldehydes and ketones
Ketones are versatile compounds which can be converted to a number of useful functional groups through reduction, nucleophilic addition or condensation reactions. Ketones and aldehydes are important series in preparation of other compounds and they are commonly prepared by oxidizing alcohol which is done in this experiment. Ketone also plays a very important part in organic synthesis. Ketones and aldehydes can be synthesised into many other chemicals. Reactions involving ketones include nucleophilic addition reactions to the carbon-oxygen double bond to form an -OH group in the compound with the addition of a nucleophilic group.
Reaction of carbonyl compounds 1. Using 2,4-dinitrophenylhydrazine This is used in our experiment. 2,4-DNP reacts with the carbonyl group for a condensation reaction with the elimination of a water molecule. Take propanone as an example, The product formed is a yellow colour precipitate, so we can easily distinguish the presence of C=O group. This can also help us to identify the carbonyl compound as the precipitate collected has a sharp melting point. By using the melting point test, we can find out the melting point of the crystals formed and compare the result with a data book to find out the carbonyl compound.
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- i?? RCOO- + 2 H2O + 2 e- The aldehyde acts as an reducing agent where [Ag(NH3)2]+ was reduced to Ag(S) , the formation of silver mirror. This reaction is very useful to extinguish aldehyde from ketone as ketone does not show this reaction. Silver mirror formed in a flask The colour of the product mixture after the reaction. 3. Fehling’ reagent Aldehydes are also oxidized by the Fehling’s solution. This reagent is also prepared freshly in the laboratory. It is made initially as two separate solutions, known as Fehling’s A and Fehling’s B.
Fehling’s A is a blue aqueous solution of copper(II) sulfate, while Fehling’s B is a clear solution of aqueous potassium sodium tartrate and a strong alkali (commonly sodium hydroxide). Equal volumes of the two mixtures are mixed together to get the final Fehling’s solution, which is a deep blue colour. In this final mixture, aqueous tartrate ions from the dissolved Rochelle salt chelate to Cu2+ (aq) ions from the dissolved copper(II) sulfate, as bidentate ligands giving the bistartratocuprate(II)4- complex.
The tartarate ions, by complexing copper prevent the formation of Cu(OH)2 from the reaction of CuSO4.2H2O and NaOH present in the solution. The Copper (II) ion is reduced to copper (I) oxide which is a red ppt, and in some cases, to copper metal (copper mirror). This is also useful to distinguish aldehyde from ketone and aromatic aldehyde as both ketone and aromatic aldehyde does not show any reaction. [O] 2Cu2+(aq) Cu2O(S) Left: Fehling’s reagent Right: Product mixture 4. Nucleophilic addition reaction In the experiment, we used hydrogensulphite as a nucleophile to attack the electron deficient carbon in the C=O. The reaction mechanism of this reaction is as follow:
(Where Nuc represents the nucleophilic species which, in this case, is hydrogensulphite ion) The final product was an alcohol. For the 2nd step of the reaction, water is already sufficient to provide the H+ ion as methoxide ion is more basic than hydroxide ion, so the methoxide ion will take a proton from water forming an alcohol and leaving a hydroxide ion. However, depending on the reactivity of the nucleophile, there are two possible general scenarios: Strong nucleophiles (anionic) add directly to the C=O to form the intermediate alkoxide.
The alkoxides are then protonated on work-up with dilute acid. Examples of such nucleophiles are: LiAlH4, NaBH4 (H-) Weaker nucleophiles (neutral) require that the C=O be activated prior to attack of the Nu. This can be done using an acid catalyst which protonates on the Lewis basic O and makes the system more electrophilic. Examples of such nucleophiles are : H2O, ROH, R-NH2 The protonation of a carbonyl gives a structure that can be redrawn in another resonance form that reveals the electrophilic character of the C since it is a carbocation. 5.
Oxidation with acidified dichromate (VI) solution Aldehyde can be further oxidized to carboxylic acid. The product formed will be a carboxylic acid. In the reduction of dichromate ion, the dichromate ion (orange) is turned to chromium ion (green). Ketone may undergo further oxidation to form carboxylic acid with hot acidified potassium permanganate and under reflux. This reaction has very high activation energy because this requires breaking the strong C-C bond. So, ketone usually does not react with acidified dichromate solution in normal conditions.
Therefore, acidified dichromate solution can be used to distinguish aldehyde from ketone. Cr2O72- Cr3+ Besides acidified dichromate solution, acidified potassium permanganate solution can also be used as an oxidizing agent. The equation of its reduction is: MnO4- + 8H+ +5e- Mn2+ + 4H2O However, this oxidation test cannot be used to confirm that the sample is a aldehyde as there are also many other compounds that can be oxidized by these 2 strong oxidizing agent such as ethene which turned to ethen-1,2-diol.
Subject: Chemical reaction,
University/College: University of Chicago
Type of paper: Thesis/Dissertation Chapter
Date: 12 July 2017
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