Oxidation of Alcohols

Quick Notes

• In organic chemistry, oxidation can be considered a carbon atom gaining a bond to an oxygen atom and/or losing a bond to a hydrogen atom.
• Alcohols are classified as:
• Primary (1°) C in C-OH is bonded to one other carbon or no carbons – Can be oxidised to aldehydes and carboxylic acids.

• Secondary (2°) C in C-OH is bonded to two other carbon atoms – Can be oxidised to ketones.

• Tertiary (3°) C in C-OH is bonded to three other carbons – Not easily oxidised.

• Oxidising agent: Acidified potassium dichromate(VI) (K₂Cr₂O₇/H₂SO₄). Colour change of orange to green occurs.
• Aldehydes can be identified using:
  • Fehling’s solution (blue solution to brick red precipitate)
  • Tollens’ reagent (silver precipitate forms – silver mirror).

Full Notes

In organic chemistry, oxidation describes a carbon atom in a molecule gaining a bond to a more electronegative atom (such as oxygen) and/or losing a bond to a less electronegative atom (such as hydrogen).

Primary and secondary alcohols can both be oxidised and new products formed. These reactions are very important in organic chemistry.

Oxidising agents are shown in reactions as [O] providing oxygen required.

Oxidation of Primary Alcohols (1° Alcohols)

Primary alcohols oxidise first to aldehydes, then to carboxylic acids using heat and acidified potassium dichromate (VI) as an oxidising agent.

Controlled conditions determine the final product:
Distillation produces aldehydes, preventing further oxidation.
Reflux allows full oxidation, producing a carboxylic acid.

Formation of Aldehyde (Distillation):
CH₃CH₂OH + [O] → CH₃CHO + H₂O

(Ethanol → Ethanal)

Distillation is needed to obtain the aldehyde because the aldehyde has a low boiling point and will evaporate once formed, leaving the reaction mixture as a vapour.

Formation of Carboxylic Acid (Reflux):
CH₃CHO + [O] → CH₃COOH
(Ethanal → Ethanoic Acid)

Reflux is needed to obtain the carboxylic acid because the aldehyde must be continually condensed and forced to re-enter the reaction mixture, enabling it to be further oxidised.

Observation: Orange Cr₂O₇²⁻ (dichromate) turns green (Cr³⁺ formed).

Oxidation of Secondary Alcohols (2° Alcohols)

Secondary alcohols oxidise to ketones.

Reaction requires reflux with acidified potassium dichromate(VI).

Equation:
CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
(Propan-2-ol → Propanone)

Observation: Orange Cr₂O₇²⁻ turns green.

Tertiary Alcohols (3° Alcohols) Are Not Oxidised

No hydrogen on the carbon attached to the OH group.

No reaction occurs with acidified potassium dichromate(VI).

Solution remains orange.

Distinguishing Aldehydes and Ketones

Aldehydes can be further oxidised to carboxylic acids.

Ketones cannot be further oxidised.

Fehling’s Test:

Reagent: Fehling’s solution (contains Cu²⁺ ions).

Aldehyde result: Brick-red precipitate (Cu₂O formed).
Ketone result: No visible change.

Tollens’ Test:

Reagent: Tollens’ reagent ([Ag(NH₃)₂]⁺ complex).

Aldehyde result: Silver mirror (Ag precipitate forms).
Ketone result: No visible change.

Summary

• Oxidation in organic chemistry can involve carbon gaining bonds to oxygen and/or losing bonds to hydrogen.
• Primary alcohols oxidise to aldehydes (by distillation) and further to carboxylic acids (by reflux) with acidified K₂Cr₂O₇/H₂SO₄.
• Secondary alcohols oxidise to ketones under reflux with acidified dichromate.
• Tertiary alcohols are not readily oxidised with acidified dichromate (solution stays orange).
• Fehling’s: aldehydes give a brick-red Cu₂O precipitate (ketones do not react)
• Tollens’: aldehydes give a silver mirror (ketones do not react)