CARBONYL COMPOUNDS

OXIDATION of Primary & Secondary Alcohols

• oxidation of primary alcohols and aldehydes

• Oxidising agents: acidified K₂Cr₂O₇ or acidified KMnO₄ , both need reflux condition (heating)

• Oxidising agents themselves get reduced → causing solutions to change color:

+ In K₂Cr₂O_7, the orange dichromate ions (Cr₂O₇^2-) → green Cr³⁺ ions

+ In KMnO₄, purple maganate ions (MnO₄^-) → colorless Mn²⁺ ions

(1) Primary alcohol [o] → (2) Aldehyde [o] → (3) Carboxylic acid

(1) Secondary alcohol [o] → (2) Ketones

[o]: acidified K₂Cr₂O₇ or acidified KMnO₄

SYNTHESIS of Aldehydes

(1) Warm primary alcohol is slowly added to the oxiding agent

(2) The formed aldehyde has a lower boiling point than the alcohol → distilled off as soon as it forms

(3) Condenses into liquid and collected

→ primary alcohol + [o] (heat) → aldehydes + water

SYNTHESIS of Ketones

(1) warm secondary alcohol is slowly added to the oxidising agent

(2) The ketone cannot be further oxidised → it does not need to be distilled off straight away after it has been formed.

→ secondary alcohol + [o] (heat)→ ketones + water

REDUCTION of Aldehydes & Ketones

• to form Primary Alcohol

• Reducing Agents or [H]: NaBH₄ OR LiAlH₄

• Aldehydes + [H] → Primary Alcohol

• Ketones + [H] → Secondary Alcohol

Aldehydes & Ketones with HCN (Hydrogen cyanide)

• NUCLEOPHILLIC ADDITION

• CN^- acts as nucleophile and adds across the C=O bond

• Catalyst: KCN

• Condition: Heat

• Product: Hydroxynitriles (nitriles containing a hydroxy, -OH, group)

Mechanism of nucleophilic addition of HCN with aldehydes and ketones

Test for Carbonyl compound

• Reagent: 2,4-dinitrophenylhydrazine (2,4-DNPH)

• Condensation - H2O is eliminated

• Positive = deep-orange ppt, can be purified by recrystallization

Identifying Carbonyls: Fehling’s solution

• Agent: Fehling’s solution

• Clear blue, alkaline solution containing copper(II) ions acts as oxidising agent

• Oxidisation

• Warmed with aldehydes → aldehydes oxidised to carboxylic acid → Cu²⁺ is reduced to Cu⁺

• Positive: turns opaque, red ppt formed

• Ketones cannot be oxidised → negative result

Identifying carbonyl: Tollen’s reagent

• aqueous alkaline solution of silver nitrate in excess ammonia

• Oxidation

• When warmed with aldehydes, aldehyde is oxides → carboxylic acids → the Ag⁺ are reduced to Ag atoms

• Positive = silver ‘mirror’

• Ketones cannot be oxidised → negative result