Classifying Alcohols (AQA A Level Chemistry)

Classification of Alcohols

• Primary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to one other carbon atom (or alkyl group)

• Secondary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to two other carbon atoms (or alkyl groups)

• Tertiary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to three other carbon atoms (or alkyl groups)

Classifying primary, secondary and tertiary alcohols and alcohols with more than one alcohol group

• Only primary and secondary alcohols can get oxidised when mildly oxidised with acidified K₂Cr₂O₇

  • Primary alcohols get mildly oxidised to aldehydes

  • Secondary alcohols get mildly oxidised to ketones

• Tertiary alcohols do not undergo oxidation with acidified K₂Cr₂O₇

• Therefore, only the oxidation of primary and secondary alcohols will change the colour of K₂Cr₂O₇ solution as the orange Cr₂O₇^2- ions are reduced to green Cr³⁺ ions

Only primary and secondary alcohols can be oxidised, turning the orange solution green

No colour change is observed with a tertiary alcohol