2.9.3 Oxidation of Alcohols

Oxidation of alcohols

• Primary alcohols can be oxidised to form aldehydes which can undergo further oxidation to form carboxylic acids
• Secondary alcohols can be oxidised to form ketones only
• Tertiary alcohols do not undergo oxidation
• The oxidising agents of alcohols include acidified K₂Cr₂O₇ 
• Acidified potassium dichromate(VI), K₂Cr₂O₇, is an orange oxidising agent 
  • Acidified means that that the potassium dichromate(VI) is in a solution of dilute acid (such as dilute sulfuric acid) 
  • For potassium dichromate(VI) to act as an oxidising agent, it itself needs to be reduced
  • This reduction requires hydrogen (H⁺) ions which are provided by the acidic medium
    • When alcohols are oxidised the orange dichromate ions (Cr₂O_7^2-) are reduced to green Cr³⁺ ions
• The primary alcohol is added to the oxidising agent and warmed
• The aldehyde product has a lower boiling point than the alcohol reactant so it can be distilled off as soon as it forms 
• If the aldehyde is not distilled off, further refluxing with excess oxidising agent will oxidise it to a carboxylic acid
• Since ketones cannot be further oxidised, the ketone product does not need to be distilled off straight away after it has been formed

Oxidation Stages of Primary Alcohols

Oxidation of propan-2-ol by acidified K₂Cr₂O₇ to form a ketone

• The presence of an aldehyde group (-CHO) in an unknown compound can be determined by the oxidising agents Fehling’s and Tollens’ reagents

Fehling’s solution

• Fehling’s solution is an alkaline solution containing copper(II) ions which act as the oxidising agent
• When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Cu²⁺ ions are reduced to Cu⁺ ions 
  • In the alkaline conditions, the carboxylic acid formed will be neutralised to a carboxylate ion (the -COOH will lose a proton to become -COO^- )
  • The carboxylate ion (-COO^-) will form a salt with a positively charged metal ion such as sodium (-COO^-Na⁺)

• The clear blue solution turns opaque due to the formation of a red precipitate, copper(I) oxide
• Ketones cannot be oxidised and therefore give a negative test when warmed with Fehling’s solution

The copper(II) ions in Fehling’s solution are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to copper(I) ions in the Cu₂O precipitate

Tollens’ reagent

• Tollens' reagent is an aqueous alkaline solution of silver nitrate in excess ammonia solution
  • Tollen’s reagent is also called ammoniacal silver nitrate solution
• When warmed with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the Ag⁺ ions are reduced to Ag atoms
  • In the alkaline conditions, the carboxylic acid will become a carboxylate ion and form a salt

• The Ag atoms form a silver ‘mirror’ on the inside of the tube
• Ketones cannot be oxidised and therefore give a negative test when warmed with Tollens’ reagent

The Ag⁺ ions in Tollens’ reagent are oxidising agents, oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms

Different practical techniques

• Because of the easier oxidation of aldehydes compared to alcohols, two different techniques are used
  • Heating under reflux
  • Distillation with addition

Heating under reflux

• This technique is used when we want full oxidation
  • Producing a carboxylic acid for a primary alcohol
  • Producing a ketone for a secondary alcohol

Apparatus set up for heating under reflux

• This set up means any products of oxidation remain in the reaction mixture
• Products which boil off condense in the vertical condenser then return to the heating flask

Distillation with addition

• This technique is used when we do not want to complete oxidation
  • To obtain an aldehyde rather than carboxylic acid for primary alcohol

Apparatus set up for distillation with addition

• Only the oxidising agent is heated whilst the alcohol is slowly added
• When the aldehyde is formed it immediately distils off as it has a much lower boiling point than the alcohol used to make it
• The aldehyde is then collected in the reciever