2 Oxidation PAG7

Oxidation reactions of the alcohols Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols to oxidise. The exact reaction, however, depends on the type of alcohol, i.e. whether it is primary, secondary, or tertiary, and on the conditions. Partial Oxidation of Primary Alcohols Reaction: primary alcohol  aldehyde Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: (use a limited amount of dichromate) warm gently and distil out the aldehyde as it forms: C O C H H H H An aldehyde’s name ends in –al It always has the C=O bond on the first carbon of the chain so it does not need Ethanal an extra number Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Write the oxidation equations in a simplified form using [O] which represents O from the oxidising agent When writing the formulae of aldehydes in a condensed way write CHO and not COH e.g.CH3CH2CHO N Goalby chemrevise.org 2 propan-1-ol propanal + [O] + H2O CH3CH2CH2OH + [O]  CH3CH2CHO + H2O OH + [O] O + H2O C O C H H C H H H H C O H H H C H H C H H H  Full Oxidation of Primary Alcohols Reaction: primary alcohol  carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulphuric acid Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished) C C O O H H C H H H H Propanoic acid propan-1-ol Propanoic acid + 2 [O] + H2O Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion CH3CH2CH2OH + 2[O]  CH3CH2COOH + H2O C O H H H C H H C H H H C C O H O H C H H H H OH + 2[O] + H2O O OH Oxidation of Secondary Alcohols Reaction: secondary alcohol  ketone Reagent: potassium dichromate(VI) solution and dilute sulphuric acid. Conditions: heat under reflux C C C O H H H H H H Propanone Ketones end in -one When ketones have 5C’s or more in a chain then it needs a number to show the position of the double bond. E.g. pentan-2-one propan-2-ol Propanone C H H C H C H H H O H H C C C O H H H H H H + [O] + H2O There is no further oxidation of the ketone under these conditions. Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Tertiary alcohols cannot be oxidised at all by potassium dichromate: This is because there is no hydrogen atom bonded to the carbon with the OH group