Oxidation of Alcohols (AQA A Level Chemistry): Revision Note

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

• Common oxidising agents for alcohols include acidified potassium dichromate(VI), K₂Cr₂O₇ and acidified potassium manganate(VII), KMnO₄

Oxidation Reaction Conditions

• The primary alcohol is added to a limited amount of oxidising agent and gently 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 removed, further heating under reflux with excess oxidising agent will oxidise it to a carboxylic acid

• Ketones, however, cannot be further oxidised, so the ketone product does not need to be distilled off immediately after it has formed

Simple Distillation

Heating Under Reflux

• Heating under reflux is a laboratory technique used in chemistry to heat reactions at a constant temperature (boiling point) for extended periods without losing volatile solvent or reactants

• A vertical condenser cools vapours, returning them to the flask

• It is critical to never seal the top of the condenser to avoid pressure buildup and explosions.

Oxidation Products

• Aldehydes and ketones are carbonyl compounds that contain a C=O group.

• They can be prepared by oxidising primary and secondary alcohols, respectively.

Oxidising agents

Acidified potassium dichromate(VI)

• Acidified potassium dichromate(VI) is an orange oxidising agent

  • “Acidified” means that the dichromate is dissolved in a dilute acid, such as dilute sulfuric acid

  • For potassium dichromate(VI) to act as an oxidising agent, it must itself be reduced

  • This reduction requires hydrogen ions (H⁺), which are supplied by the acidic medium

  • When alcohols are oxidised, the orange dichromate ions (Cr₂O₇^2-) are reduced to green Cr³⁺ ions

Acidified potassium manganate(VII)

• Acidified potassium manganate(VII) is a purple oxidising agent

  • As with potassium dichromate(VI), it must be in an acidic medium to allow reduction to occur

  • When alcohols are oxidised, the purple manganate ions (MnO₄^-) are reduced to colourless Mn²⁺ ions 

Testing for Oxidation Products

• 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 is neutralised to a carboxylate ion, as the –COOH group loses a proton to form –COO⁻

  • The carboxylate ion then forms a salt with a positively charged metal ion, such as sodium, producing a –COO^-Na⁺ salt

• During the reaction, the clear blue solution turns brick red due to the formation of a copper(I) oxide precipitate

• Ketones cannot be oxidised and therefore give a negative result when warmed with Fehling’s solution

  

Tollens’ reagent

• Tollens’ reagent is an aqueous alkaline solution of silver nitrate in excess ammonia and is also known as ammoniacal silver nitrate

• When warmed in a water bath with an aldehyde, the aldehyde is oxidised to a carboxylic acid, and the Ag⁺ ions are reduced to silver atoms

  • In the alkaline conditions, the carboxylic acid formed is neutralised to a carboxylate ion and forms a salt

• The silver atoms deposit on the inside of the test tube, forming a characteristic silver “mirror”

• Ketones cannot be oxidised and therefore give a negative result when warmed with Tollens’ reagent