1. Reactions of aldehydes and ketones
A2 Unit 4.8 Further organic chemistry p
TDO Nov 2013

2. Revision: Isomers of C3H6O2
Draw an isomer of C3H6O2 that is:
an alkene with two hydroxyl groups
an ester
a carboxylic acid
an aldehyde with a hydroxyl group
a ketone with a hydroxyl group
Which of these molecules has a chiral carbon?

3. What do you need to know?
Can you describe these reactions of carbonyl compounds?
oxidation with Fehling’s or Benedict’s solution, Tollens’ reagent and acidified dichromate(VI) ions
reduction with lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether
nucleophilic addition of HCN in the presence of KCN, using curly arrows, lone pairs, dipoles and evidence of optical activity to show the mechanism
iodine in the presence of alkali

4. Brady’s reagent
Brady’s reagent is 2,4-dinitrophenylhydrazine dissolved in methanol and concentrated sulfuric acid.
A bright orange ppt is formed if the carbonyl group is present. This is purified using recrystallisation, then the melting point of the crystals is tested.
Answer Q1-3 on p109

5. Brady’s reagent Q1. Propanal
Q2. Boiling temperatures are affected by the atmospheric pressure and harder to measure accurately.
Q3. Possibly propanone. Impure compounds melt at lower temperatures and over a range.
Question – why is this method of identifying carbonyl compounds not widely used anymore?

6. Testing for aldehydes?
List the tests can we use to distinguish between an aldehyde and a ketone – refer to your practical notes and the text book.
Test
Reagents/ conditions
Observations
Reaction

7. Oxidation/reduction of aldehydes & ketones
What are aldehydes are oxidised to by acidified dichromate ions?
RCHO → ??
Why do aldehydes need to be refluxed for complete oxidation?
Aldehydes can be reduced by LiAlH4 dissolved in dry ether (or NaBH4 dissolved in ethanol). What are aldehydes and ketones reduced to?
CH3CHO + [H] →
CH3COCH3 + [H] →

9. Addition reactions of hydrogen cyanide
Aldehydes and ketones react with hydrogen cyanide (HCN) to make hydoxynitriles – the HCN molecule adds across the C=O bond.
e.g.
this is a two-step mechanism that can produce a racemic mix of optical isomers – see p112

10. Bassant: methanal
Sabry & Hassan: propanone
Sherif: butanal
Abdulaziz: butanone

11. Sabren & Uvini: methanal
Sakina & Eeshal: propanone
Kamashi & Asalah: butanal
Salma & Layla: butanone

12. Some carbonyls react with iodine
called the triiodomethane (iodoform) test
warm with iodine in alkaline solution
a yellow precipitate with an antiseptic smell is the positive result
only carbonyls that contain a methyl carbonyl group will react

13. What do you need to know?
Can you describe these reactions of carbonyl compounds?
oxidation with Fehling’s or Benedict’s solution, Tollens’ reagent and acidified dichromate(VI) ions
reduction with lithium tetrahydridoaluminate (lithium aluminium hydride) in dry ether
nucleophilic addition of HCN in the presence of KCN, using curly arrows, lone pairs, dipoles and evidence of optical activity to show the mechanism
iodine in the presence of alkali