Aldehydes and Ketones – Notes

(a) Formation of Aldehydes and Ketones by Oxidation of Alcohols

Aldehydes are formed by the oxidation of primary alcohols using acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄) or acidified potassium permanganate (KMnO₄/H₂SO₄).

• Reaction: Primary alcohol → Aldehyde + H2O

• Example: Ethanol → Ethanal

Aldehydes can be further oxidised to carboxylic acids, so distillation is used to isolate them.

Ketones are formed by the oxidation of secondary alcohols using the same oxidizing agents.

• Reaction: Secondary alcohol → Ketone + H2O

• Example: Propan-2-ol → Propanone

Ketones cannot be oxidized further under normal conditions.

(c) Reduction of Aldehydes and Ketones Using NaBH₄

Sodium borohydride (NaBH₄) is a mild reducing agent used to convert aldehydes and ketones back to alcohols.

Reaction with Aldehydes: Produces primary alcohols

• Example: Ethanal + [H] → Ethanol

Reaction with Ketones: Produces secondary alcohols

• Example: Propanone + [H]→ Propan-2-ol

NaBH₄ provides hydride ions (H⁻), which attack the carbonyl carbon, leading to reduction.

(e) Reaction of Aldehydes and Ketones with 2,4-Dinitrophenylhydrazine (2,4-DNPH) and its Use as a Test

2,4-DNPH (Brady’s reagent) reacts with aldehydes and ketones to form orange/yellow precipitates (hydrazones).

Reaction Mechanism:

• The C=O carbonyl group reacts with 2,4-DNPH to form a condensation product.

• Example: Ethanal + 2,4-DNPH → Orange precipitate

• Use as a Test:

  • The formation of a yellow/orange precipitate confirms the presence of a carbonyl group (C=O).

  • The melting point of the purified derivative can be compared to known values to identify specific aldehydes or ketones.

(f) Triiodomethane (Iodoform) Test

Used to identify compounds containing the CH₃CO— group or its precursors (e.g., ethanol).

• Reagent: I₂ in NaOH

• Positive result: Formation of a yellow precipitate of triiodomethane (iodoform, CHI₃) with an antiseptic smell.

Reaction Example:

• With a methyl ketone (e.g., propanone):

Propanone + 3I₂ + 4NaOH → CHI₃ (yellow ppt) + carboxylate salt + 3NaI + 3H2O

• With ethanol (as it can be oxidized to ethanal, which has CH₃CO—):

Ethanol → Ethanal → CHI₃ (yellow ppt)

• Use in Identifying Carbonyl Compounds:

  • Positive test confirms the presence of a CH₃CO— group in ketones and aldehydes.

  • Common positive compounds: Ethanal, methyl ketones, ethanol, and secondary alcohols that can be oxidized to methyl ketones.