Oxidation of alcoholsAQA A-Level Chemistry: Revision notes
Section 1
Primary, secondary and tertiary alcohols
Alcohols are classified by the number of carbon atoms attached to the carbon that carries the OH group.
- Primary: the carbon bonded to OH is attached to one other carbon (or none), e.g. ethanol, propan-1-ol
- Secondary: attached to two other carbons, e.g. propan-2-ol
- Tertiary: attached to three other carbons, e.g. 2-methylpropan-2-ol
The class decides what happens when the alcohol is oxidised.
Section 2
The oxidising agent and its colour change
A suitable oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute sulfuric acid. Orange dichromate(VI) ions, Cr₂O₇²⁻, are reduced to green chromium(III) ions, Cr³⁺, when an alcohol or aldehyde is oxidised.
In equations for organic oxidation, the oxidising agent can be written as [O], which is acceptable. Primary and secondary alcohols and aldehydes turn the mixture orange to green; tertiary alcohols and ketones leave it orange.
A tertiary alcohol gives no colour change: the mixture stays orange. Do not write 'colourless'.
Section 3
Oxidising primary alcohols
A primary alcohol is oxidised first to an aldehyde and then, with further oxidation, to a carboxylic acid.
- CH₃CH₂OH + [O] → CH₃CHO + H₂O
- CH₃CHO + [O] → CH₃COOH
The method decides the product:
- Distillation gives the aldehyde. The aldehyde has a lower boiling point than the alcohol (no hydrogen bonding), so it is removed as it forms and cannot be oxidised further.
- Heating under reflux with excess oxidising agent gives the carboxylic acid. Vapours condense and return to the flask, so the aldehyde stays in the oxidising mixture until it is fully oxidised.
Do not just say the aldehyde 'evaporates more easily': say it has no hydrogen bonding, so its boiling point is lower than the alcohol's.
Section 4
Oxidising secondary and tertiary alcohols
A secondary alcohol is oxidised to a ketone:
- CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O
A ketone is not easily oxidised further, so heating under reflux or distilling gives the same product.
A tertiary alcohol has no hydrogen atom on the carbon carrying the OH group, so it is not easily oxidised and the dichromate(VI) stays orange.
Section 5
Distinguishing aldehydes from ketones
Aldehydes are easily oxidised but ketones are not, so mild oxidising agents distinguish them.
- Tollens' reagent (ammoniacal silver nitrate, warmed): an aldehyde gives a silver mirror as Ag⁺ is reduced to Ag; a ketone gives no change.
- Fehling's solution (blue copper(II) complex, warmed): an aldehyde gives a red precipitate of copper(I) oxide; a ketone stays blue.
Acidified dichromate(VI) does not tell an aldehyde from an alcohol, because primary and secondary alcohols and aldehydes all turn it green.
Must Know
- Primary alcohols give aldehydes then carboxylic acids; secondary alcohols give ketones; tertiary alcohols are not oxidised
- Acidified K₂Cr₂O₇: orange to green
- Distil for the aldehyde; reflux with excess oxidising agent for the acid
- Aldehydes: silver mirror with Tollens', red precipitate with Fehling's; ketones: no reaction
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Oxidation of alcohols
- A school technician is given three unlabelled liquids, each with the molecular formula C₄H₁₀O: butan-1-ol, butan-2-ol and 2-methylpropan-2-ol. She warms a sample of each with acidified potassium dichromate(VI) solution.State the colour of the mixture after warming for butan-2-ol and for 2-methylpropan-2-ol, and explain each result.2 marks
- Propan-1-ol can be converted into either propanal or propanoic acid using acidified potassium dichromate(VI), depending on how the apparatus is set up. In one experiment the alcohol is heated in a flask fitted with a distillation head and the product is collected as it forms. In a second experiment the mixture is heated under reflux with excess oxidising agent for a long time.Write two equations, using [O] to represent the oxidising agent, for the oxidation of propan-1-ol to propanal and then propanal to propanoic acid.2 marks
- A forensic chemist has two colourless liquids, pentanal and pentan-2-one, which both have the molecular formula C₅H₁₀O. She tests a sample of each with warm Fehling's solution and with warm Tollens' reagent.Describe the observations the chemist would make when each liquid is tested with Fehling's solution and with Tollens' reagent.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).