2 Oxidation of aldehydes

Oxidation of Aldehydes RCHO + [O]  RCO2H C + [O]  O C H H C H H H H C C O H O H C H H H H Full Equation for oxidation 3CH3CHO + Cr2O7 2- + 8H+  3 CH3CO2H + 4H2O + 2Cr3+ Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Aldehydes can also be oxidised using Fehling’s solution or Tollen’s Reagent. These are used as tests for the presence of aldehyde groups Tollen’s Reagent CH3CHO + 2Ag+ + H2O  CH3COOH + 2Ag + 2H+ Reagent: Fehling’s Solution containing blue Cu 2+ ions. Conditions: heat gently Reaction: aldehydes only are oxidised by Fehling’s Solution into a carboxylic acid and the copper ions are reduced to copper(I) oxide . . Observation: Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react. Fehling’s solution CH3CHO + 2Cu2+ + 2H2O  CH3COOH + Cu2O + 4H+ Reagent: Tollen’s Reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3 )2 ]+ . Conditions: heat gently Reaction: aldehydes only are oxidised by Tollen’s reagent into a carboxylic acid and the silver(I) ions are reduced to silver atoms Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no change. Reaction: aldehyde  carboxylic acid Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: heat under reflux