Oxidising alcohols to form a carbonyl

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Primary and secondary alcohols are oxidised by common oxidising agents such as potassium manganate(VII), KMnO4, or acidified sodium dichromate(VI), Na2Cr2O7 (acidified with dilute sulfuric acid).

If you gently heat a primary alcohol in the presence of an oxidising agent, the hydroxyl group forms a carbonyl group at the end of the molecule – we have made an aldehyde.

However, unless you distil off the aldehyde as it is formed it will be oxidised further forming a carboxylic acid functional group.

If we wanted to make a carboxylic acid, we would reflux a primary alcohol with excess potassium dichromate(VI) in the presence of sulfuric acid, and either distill off the product after approximately 15 minutes or reflux until the reaction is complete.

e.g. CH3CH2CH2OH + 2[O] ⇾ CH3CH2COOH + H2O

In both reactions, the oxidising agent is itself reduced which is signalled by a change in colour, from an orange solution to a green solution in the case of Cr2O72-(aq).

Secondary alcohols are oxidised to a ketone (the carbonyl group is in the middle of the carbon chain in the molecule) when heated under reflux with an oxidising agent. Heating under reflux for a period of time ensures that the reaction goes to completion. Once again, the oxidising agent is reduced as this is a redox reaction.

Tertiary alcohols are not oxidised as the reaction would involve breaking a C-C bond rather than a C-H bond.

Practice questions

  1. A student sets out to produce 3-methylpentanal in a high yield. Explain how they would best achieve this.
  2. There are four structural isomers of C4H10O containing an alcohol functional group.

(a) Draw the skeletal formula of each isomer and name it.

(b) Compare and contrast the oxidation of these isomers. Include the reaction conditions, the name and functional group of the product, and write a full balanced equation for each reaction.

Answers

  1. Gently warm 3-methylpentan-1-ol with acidified potassium dichromate(VI) and distill off the aldehyde product as soon as it forms.
  2. (a)

(b) 2-methylpropan-2-ol is a tertiary alcohol and cannot be oxidised.

Butan-1-ol, 2-methylpropan-1-ol and butan-2-ol can be oxidised using the oxidising agent acidified potassium chromate(VI).

If butan-1-ol is warmed with acidified Cr2O72-(aq) and the product distilled off as soon as it forms, the product is an aldehyde, butanal. If it is refluxed with Cr2O72-(aq) in the presence of sulfuric acid the product is butanoic acid, a carboxylic acid.

CH3CH2CH2CH2OH + [O] ⇾ CH3CH2CH2CHO + H2O

CH3CH2CH2CH2OH + 2[O] ⇾ CH3CH2CH2COOH + H2O

If 2-methylpropan-1-ol is warmed with acidified Cr2O72-(aq) and the product distilled off as soon as it forms, the product is an aldehyde, 2-methylpropanal. If it is refluxed with excess acidified Cr2O72-(aq) the product is 2-methylpropanoic acid, a carboxylic acid.

CH3CH(CH3)CH2OH + [O] ⇾ CH3CH(CH3)CHO + H2O

CH3CH(CH3)CH2OH + 2[O] ⇾ CH3CH(CH3)COOH + H2O

If butan-2-ol is refluxed with acidified Cr2O72-(aq) the product is butan-2-one, a ketone.

CH3CH2CH(OH)CH3 + [O] ⇾ CH3CH2COCH3 + H2O