18.2    Reactions of Aldehydes and Ketones

Reactivity of carbonyl compounds

• The C=O bond of the carbonyl group is highly polarised due to oxygen atom being more electronegative

 

• This causes the slightly positive carbon atom to be susceptible to nucleophilic attacks. Nucleophiles are something that carries a negative charge

 

• Therefore, carbonyl compounds will undergo nucleophilic addition.

 

• Summary of reactions carbonyl compounds undergo:
  i) Reduction
  ii) Oxidation
  iii) Reaction with hydrogen cyanide, HCN.

 

1. Reduction

• Reagent     : Lithium tetrahydridoaluminate, LiAlH4 or   sodium tetrahydridoborate, NaBH4
  Condition : For LiAlH4 – in dry ether
  For NaBH4 – in aqueous alcoholic solution Product                   : Aldehyde – primary alcohol
  Ketone    – secondary alcohol

• LiAlH4 and NaBH4 are acting as reducing agents as well as providing the nucleophile, H⁻. This is a redox reaction as well as a nucleophilic addition

• For aldehydes, primary alcohols are formed upon reduction. Take ethanal as an example:

• For ketones, secondary alcohols are formed upon reduction. Take propanone as an example:

• Note:
  1. Due to the reactivity of LiAlH4, it cannot be used in the presence of water or alcohol. It must be carried out in solution in a carefully dried ether such as ethoxyethane(diethyl ether).

 

Oxidation

• Reagent     : Acidifted potassium dichromate(VI), K2Cr2O7    or acidifted potassium  manganate(VI), KMnO4
  Condition : Heat under  reflux
  Product    : Aldehyde – Carboxylic acid
  Ketone    – Will not be oxidised.

 

• Aldehydes will be oxidised to carboxylic acids. Take ethanal as an example:

 

• Ketones will not be oxidised by acidified K2Cr2O7 or KMnO4. The only exception is phenylethanone

 

Reaction with hydrogen cyanide, HCN

• Reagent   : Sodium/potassium cyanide, NaCN and a little sulfuric acid, H2SO4
  Condition : Room temperature
  Product                   : Hydroxynitriles
• Hydrogen cyanide is not used alone because it is a poisonous Instead, it is produced from the reaction between sodium/potassium cyanide and sulfuric acid. The solution will contain hydrogen cyanide and some free cyanide ions.

• For both aldehydes and ketones, hydroxynitriles are
  1. For aldehydes, take ethanal as an example, 2-hydroxypropanenitrile is produced.
  2. For ketones, take propanone as an example, 2-hydroxy-2-methylpropanenitrile is produced

• The mechanism of this reaction – nucleophilic addition:
  1. The electron-deficient carbon atom is attacked by the nucleophile, CN⁻.
  2. The negative ion formed then picks up a hydrogen ion from hydrogen cyanide, or from the water

• Such a reaction will produce a mixture of different isomers. This is because carbonyl compounds are planar and the cyanide ion has equal chance of attacking from above or below the plane of the molecule