Oxidation of alcohols (3.3.5.2)

How can primary alcohols be oxidised?

Primary alcohols can be oxidised to form aldehydes and further oxidised to form carboxylic acids.

They can be oxidised by excess acidified potassium dichromate ([O]/H⁺)

CH₃CH₂OH + [O] → CH₃COH + H₂O

CH₃COH + [O] → CH₃COOH

How does the method used to oxidise a primary alcohol determines whether an aldehyde or carboxylic acid is formed?

The primary alcohol must be oxidised with excess [O]/H⁺ and distilled to form an aldehyde

The primary alcohol must be oxidised with excess [O]/H⁺ under reflux to form a carboxylic acid

How can secondary and tertiary alcohols be oxidised?

Secondary alcohols - oxidised in the presence of [O]/H⁺ to form ketones - they cannot be further oxidised

CH₃CHOHCH₃ + [O] → CH₃CHOCH₃

Tertiary alcohols - not easily oxidised

How can primary, secondary and tertiary alcohols be distinguished between?

Add K₂Cr₂O₇/H⁺ to each solution:

Observations for 1° - solution changes colour from orange to green

Observations for 2° - solution changes colour from orange to green

Observations for 3° - solution remains orange

How can chemical tests be used to distinguish between aldehydes and ketones?

1) Warm with K₂Cr₂O₇/H⁺

Observation for aldehyde - solution changes colour from orange to green

Observation for ketone - solution remains orange

2) Warm gently with Tollen’s reagent

Observation for aldehyde - silver mirror precipitate will form

Observation for ketone - solution remains colourless

3) Heat with Fehling’s solution

Observation for aldehyde - a brick red precipitate will form

Observation for ketone - solution remains blue