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REACTIONS OF THE CARBONYL GROUP IN ALDEHYDES AND KETONES L.O.: The mechanism of nucleophilic addition reactions of carbonyl compounds How carbonyl compounds.

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Presentation on theme: "REACTIONS OF THE CARBONYL GROUP IN ALDEHYDES AND KETONES L.O.: The mechanism of nucleophilic addition reactions of carbonyl compounds How carbonyl compounds."— Presentation transcript:

1 REACTIONS OF THE CARBONYL GROUP IN ALDEHYDES AND KETONES L.O.: The mechanism of nucleophilic addition reactions of carbonyl compounds How carbonyl compounds react when oxidised or reduced.

2 A test for an aldehyde using Fehlings solution. a) In a clean test tube mix together equal volumes of Fehling's solution A and Fehling's solution B. The resultant Fehling's test reagent should be a clear dark blue solution. b) Add 5 drops of this test reagent to 1 cm3 of sodium carbonate solution in a test tube containing a few anti- bumping granules and then add 1 cm3 of ethanal (or propanal) to this same test tube. c) Warm the test tube gently for approximately two minutes in a beaker half filled with hot water and then gradually bring the beaker of water to boiling and maintain this temperature for a few minutes. d) Using the test tube holder, carefully lift the test tube out of the boiling water and allow its contents to stand for several minutes..

3 Tollens silver mirror test Prepare a sample of Tollens reagent by adding 5 drops of sodium hydroxide solution to 2 cm3 of silver nitrate solution in a test tube. To this test tube add just enough dilute ammonia solution to dissolve the brown precipitate completely. Using a beaker of hot water (50 oC to 60 oC), gently warm approximately 5 cm3 of this test reagent in a test tube. Add 10 drops of the aldehyde to the warmed test reagent in the test tube. Wait a few minutes and note what happens.

4 Carbonyl groups consists of a carbon- oxygen double bond The bond is polar due to the difference in electronegativity

5 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION MechanismNucleophilic addition Step 1CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O STEP 1

6 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION MechanismNucleophilic addition Step 1CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2A pair of electrons is used to form a bond with H + Overall, there has been addition of HCN STEP 2STEP 1

7 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION MechanismNucleophilic addition Step 1CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2A pair of electrons is used to form a bond with H + Overall, there has been addition of HCN STEP 2STEP 1

8 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM

9 CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Watch out for the possibility of optical isomerism in hydroxynitriles CN¯ attacks from above CN¯ attacks from below

10 Many reducing agents will reduce ketones and aldehydes to alcohols. NaBH 4 (sodium tetrahydroborate(III) ) generates the nucleophile H -, the hydride ion. Write the mechanism of the reaction of a ketone/aldehyde with H -.

11 Will NaBH 4 react with an alkene? NO! H - is repelled by the electron density of C=C. CH 2 = CHCHO + 2[H] > CH 2 =CHCH 2 OH

12 CARBONYL COMPOUNDS - REDUCTION ExampleWhat are the products when Compound X is reduced? NaBH 4 H2H2 COMPOUND X

13 CARBONYL COMPOUNDS - REDUCTION ExampleWhat are the products when Compound X is reduced? C=O is polar so is attacked by the nucleophilic H¯ C=C is non-polar so is not attacked by the nucleophilic H¯ NaBH 4 H2H2 COMPOUND X

14 Oxidation Fehling test (Cu 2+ ). Silver mirror test (Ag + )

15 Is it more difficult to oxidise a ketone than an aldehyde? Why

16 THE SILVER MIRROR TEST Tollens Reagent contains [Ag(NH 3 ) 2 ] + When an aldehyde is warmed with Tollens reagent, metallic silver is formed. Aldehyde is oxidised to carboxylic acid and Ag + reduced to metallic silver RCHO + [o] -> RCOOH oxidation [Ag(NH 3 ) 2 ]+ + e - -> Ag + 2NH 3 reduction clip

17 FEHLINGS SOLUTION It contains a copper(II) complex ion giving a blue solution. On warming, it will oxidise aldehydes. The copper(II) is reduced to copper(I) and a red precipitate of copper(I) oxide, Cu 2 O, is formed. Ketones do not react with Tollens Reagent or Fehlings Solution video 2


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