REACTIONS OF THE CARBONYL GROUP IN ALDEHYDES AND KETONES

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.

A test for an aldehyde using Fehling’s 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. .

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.

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

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

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION MechanismNucleophilic addition Step 1 CN¯ 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 2 A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM

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

Many reducing agents will reduce ketones and aldehydes to alcohols. NaBH4 (sodium tetrahydroborate(III) ) generates the nucleophile H-, the hydride ion. Write the mechanism of the reaction of a ketone/aldehyde with H-.

Will NaBH4 react with an alkene? NO! H- is repelled by the electron density of C=C. CH2 = CHCHO + 2[H] ———> CH2 =CHCH2OH

CARBONYL COMPOUNDS - REDUCTION ExampleWhat are the products when Compound X is reduced? COMPOUND X H2 NaBH4

CARBONYL COMPOUNDS - REDUCTION ExampleWhat are the products when Compound X is reduced? COMPOUND X H2 NaBH4 C=O is polar so is attacked by the nucleophilic H¯ C=C is non-polar so is not attacked by the nucleophilic H¯

Oxidation Fehling test (Cu2+). Silver mirror test (Ag+)

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

THE SILVER MIRROR TEST Tollen’sReagent contains [Ag(NH3)2 ]+ When an aldehyde is warmed with Tollen’s reagent, metallic silver is formed. Aldehyde is oxidised to carboxylic acid and Ag+ reduced to metallic silver RCHO + [o] -> RCOOH oxidation [Ag(NH3)2]+ + e- -> Ag + 2NH3 reduction clip

FEHLING’S 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, Cu2O, is formed. Ketones do not react with Tollen’s Reagent or Fehling’s Solution video 2

REACTIONS OF THE CARBONYL GROUP IN ALDEHYDES AND KETONES