6.5 1. Oxidising Alcohols Comparing Oxidation This activity compares the oxidation reactions of the three types of alcohol. Acidified dichromate is a mild oxidising agent. Its reaction with alcohols depends on whether the alcohol is a primary, secondary or tertiary alcohol. Experiment Acidified dichromate solution was added to a primary, secondary and tertiary alcohol and the results recorded in the table: Name of Alcohol Type of Alcohol Colour change observed Butan-1-ol Primary Orange to green Butan-2-ol Secondary Orange to green 2-methylpropan-2-ol tertiary No change Primary and secondary alcohols can be oxidised by acidified dichromate solution. Tertiary alcohols can not be oxidised. 2. The Oxidation Reaction This activity explains the changes which occur when different types of alcohol are oxidised. Oxidation can take place by the loss of hydrogen from a molecule or by the gain of oxygen. Primary alcohols are oxidised in two stages: Stage One: In the first stage, oxidation is by loss of hydrogen. H O H H C C H + [O] H H ethanol H O H C C H + H2O H ethanal The product of this oxidation is an alkanal (aldehyde). Stage Two: In the second stage, oxidation is by gain of oxygen. H O H C C H + [O] H ethanal H O H C C O H H ethanoic acid The product of this oxidation is an alkanoic acid. Secondary Alcohols: Secondary alcohols only undergo one stage of oxidation. For secondary alcohols, oxidation is by loss of hydrogen. H O H H C C H + [O] H C H H H propan-2-ol H O H H C C C H + H2O H H propanone The product of this oxidation is an alkanone (ketone). All of the products of oxidation of alcohols contain a C=O group. The C=O group is known as a carbonyl group. Tertiary alcohols cannot undergo this type of oxidation. 3. Oxidising with Copper(II) Oxide This activity investigates another method for oxidising primary and secondary alcohols. Copper(II) oxide will readily give up its oxygen to form copper. The liberated oxygen can be used to oxidise alcohols. Experiment: Both alcohols had been oxidised since the copper oxide (black) had formed copper (brown). The pH paper only changed to red in the tube containing the primary alcohol. Only the primary alcohol forms an acid.