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 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. .

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

5. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic 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 1

6. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic 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

7. CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION
Mechanism Nucleophilic 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

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.
NaBH4 (sodium tetrahydroborate(III) ) generates the nucleophile H-, the hydride ion.
Write the mechanism of the reaction of a ketone/aldehyde with H-.

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

12. CARBONYL COMPOUNDS - REDUCTION
Example What are the products when Compound X is reduced?
COMPOUND X H2
NaBH4

13. CARBONYL COMPOUNDS - REDUCTION
Example What are the products when Compound X is reduced?
COMPOUND X H2
NaBH4
Hyrogenation: not in main text but in summary questions
C=O is polar so is attacked by the nucleophilic H¯
C=C is non-polar so is not attacked by the nucleophilic H¯

14. Silver mirror test (Ag+)
Oxidation
Fehling test (Cu2+).
Silver mirror test (Ag+)
Do practical. Try both with both tests.

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

16. THE SILVER MIRROR TEST
Tollen’s Reagent 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
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17. 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
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