Oxidation of alcohols — AQA A-Level Chemistry
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Oxidation of alcohols explained
The class of an alcohol is decided only at the carbon carrying the hydroxyl group.
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Count the alkyl groups attached to that carbon: one gives a primary alcohol, two a secondary, three a tertiary. Methanol has none and is counted as primary. Nothing else matters, so the chain length, the total number of carbons and any branch further along are irrelevant. Butan-1-ol and 2-methylpropan-1-ol are both primary, despite the branch in the second; butan-2-ol is secondary; 2-methylpropan-2-ol is tertiary. The classification is worth getting right because it predicts everything that follows: primary alcohols can be oxidised twice, secondary once, tertiary not at all under the usual conditions.
Primary alcohols can be oxidised to aldehydes which can be further oxidised to carboxylic acids.
A primary alcohol has two hydrogen atoms on the carbon carrying the hydroxyl group, so it can be oxidised twice. The first step gives an aldehyde, CH₃CH₂OH + [O] → CH₃CHO + H₂O; the second gives a carboxylic acid, CH₃CHO + [O] → CH₃COOH. The method decides which you isolate. To stop at the aldehyde, add the oxidising agent slowly to the alcohol and distil the aldehyde off as it forms, since it boils lower than the alcohol, having no hydrogen bonding. To reach the carboxylic acid, heat under reflux with an excess of oxidising agent so the aldehyde is returned to the flask and oxidised again. Acidified dichromate turns from orange to green as it works.
Secondary alcohols can be oxidised to ketones.
A secondary alcohol has just one hydrogen on the carbon carrying the hydroxyl group, so it can be oxidised once and no further. Propan-2-ol gives propanone: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O. The ketone that results has no hydrogen on its carbonyl carbon, so oxidising it again would mean breaking a carbon–carbon bond, which acidified dichromate cannot do. That makes the reaction convenient: you can heat under reflux without any risk of going too far, then distil the ketone out afterwards. AQA has tested this directly by asking which statement is not true of both primary and secondary alcohols, the answer being that both are easily oxidised to carboxylic acids; secondary alcohols are not.
Tertiary alcohols are not easily oxidised.
In a tertiary alcohol the carbon carrying the hydroxyl group is bonded to three alkyl groups and no hydrogen. Oxidation by acidified dichromate works by removing hydrogen from that carbon and adding oxygen to it; with no hydrogen available, the only route to a carbonyl would be to break a strong carbon–carbon bond, which this reagent cannot do. So nothing happens: the mixture stays orange because the dichromate is not reduced to chromium(III). That negative result is itself a useful test, since primary and secondary alcohols both turn the solution green on warming. AQA writes "not easily oxidised" rather than never, because vigorous conditions or simple combustion will still destroy the molecule.
Acidified potassium dichromate(VI) is a suitable oxidising agent.
The oxidising agent is potassium dichromate(VI) solution acidified with dilute sulfuric acid. The orange dichromate ion, Cr₂O₇²⁻, in which chromium is in the +6 state, is reduced to the green Cr³⁺ ion: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. The acid supplies the hydrogen ions that half equation needs, so an unacidified solution will not work and the word acidified must appear in your answer. In organic equations you are allowed to write the oxidant simply as [O], which keeps the focus on the organic change. Because the colour change reports on whether oxidation happened, the same reagent doubles as the test-tube test for primary and secondary alcohols and for aldehydes.
Students should be able to: write equations for these oxidation reactions (equations showing [O] as oxidant are acceptable) explain how the method used to oxidise a primary alcohol determines whether an aldehyde or carboxylic acid is obtained use chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.
Three skills are combined here. Write oxidation equations with [O], remembering that turning an alcohol into a carbonyl compound releases water, but turning an aldehyde into a carboxylic acid does not. Explain the method: distil the aldehyde off as it forms to isolate it, or reflux with excess oxidant to reach the acid. Then distinguish the aldehyde from the ketone. Tollens' reagent, ammoniacal silver nitrate, gives a silver mirror with an aldehyde and no change with a ketone; Fehling's solution, a blue copper(II) complex, gives a red precipitate with an aldehyde and stays blue with a ketone. Both tests must be warmed in a water bath: AQA has set a candidate method in which cooling Fehling's solution was the deliberate error.
Your focus
- Classify a given alcohol as primary, secondary or tertiary by counting alkyl groups on the hydroxyl carbon.
- Draw one primary, one secondary and one tertiary alcohol with the molecular formula C₄H₁₀O.
- Predict from the classification what, if anything, the alcohol is oxidised to by acidified dichromate.
Show all 18 objectives
- Write equations using [O] for both oxidation steps of a named primary alcohol.
- Explain how the apparatus chosen determines whether an aldehyde or a carboxylic acid is isolated.
- Describe the colour change observed when acidified potassium dichromate(VI) oxidises a primary alcohol.
- Write the equation using [O] for the oxidation of a named secondary alcohol and name the product.
- Explain why the ketone formed cannot be oxidised further by acidified potassium dichromate(VI).
- Contrast the products obtained from oxidising a primary and a secondary alcohol under reflux.
- Explain why a tertiary alcohol is not readily oxidised by acidified potassium dichromate(VI).
- Predict and describe the observation when a tertiary alcohol is warmed with the reagent.
- Design a simple test-tube procedure that distinguishes a tertiary alcohol from a secondary one.
- Name the oxidising agent fully and state the colour change seen when it oxidises an alcohol.
- Write the half equation showing dichromate(VI) reduced to chromium(III) and explain why acid is needed.
- Use [O] to write a balanced equation for the oxidation of a named alcohol.
- Decide, when writing an oxidation equation with [O], whether water is a product: it is when an alcohol is oxidised, but not when an aldehyde becomes a carboxylic acid.
- Find the fault in a given method for distinguishing an aldehyde from a ketone, such as Fehling's solution used without warming, and state the observation it would wrongly produce.
- Describe a test-tube procedure, with observations, that distinguishes propanal from propanone.
Oxidation of alcohols exam tips
Marking Points
- one mark for counting the alkyl groups attached to the carbon bearing the hydroxyl group
- one mark for the correct classification of a named or drawn alcohol
- one mark for a correctly drawn example of each class with the same molecular formula
- one mark for linking the class to the product obtained on oxidation
- one mark for the first equation using [O], with water shown as a product
- one mark for the second equation using [O], with no water formed at that step
- one mark for distilling the aldehyde off as it forms, so it is not oxidised further
- one mark for heating under reflux with excess oxidising agent to obtain the carboxylic acid
- one mark for the equation using [O], with water as the second product
- one mark for naming the ketone product, such as propanone from propan-2-ol
- one mark for stating that the ketone is not oxidised further by this reagent
- one mark for the reason, that the carbonyl carbon carries no hydrogen atom
- one mark for stating that the carbon bearing the hydroxyl group has no hydrogen atom attached
- one mark for saying oxidation would require a carbon–carbon bond to break
- one mark for the observation that the solution remains orange
- one mark for using that observation to identify the alcohol as tertiary
- one mark for naming acidified potassium dichromate(VI), stating that it is acidified
- one mark for the colour change from orange to green
- one mark for identifying the change as chromium going from the +6 to the +3 oxidation state
- one mark for using [O] correctly in a balanced organic equation
- one mark for a balanced equation using [O] with water shown only where it is formed
- one mark for saying distillation as the aldehyde forms gives the aldehyde and reflux with excess oxidant gives the acid
- one mark for Tollens' reagent giving a silver mirror with an aldehyde and no change with a ketone
- one mark for Fehling's solution turning from blue to a red precipitate, warmed in a water bath
Examiner Tips
- 💡Circle the carbon holding the hydroxyl group first, then count only what is bonded to that carbon.
- 💡Draw the structure out fully before classifying; names alone invite mistakes.
- 💡Use the classification to predict the oxidation product, since the two are almost always tested together.
- 💡Balance [O] equations by counting oxygen last; each [O] supplies exactly one oxygen atom.
- 💡State the apparatus, distillation or reflux, as part of any answer about which product forms.
- 💡Explain the low boiling point of the aldehyde by its lack of hydrogen bonding.
- 💡Check the carbonyl carbon in your product: no hydrogen on it means the oxidation has finished.
- 💡Name aldehydes with the ending -al and ketones with -one, and check which the carbon skeleton allows.
- 💡Use reflux freely for a secondary alcohol; there is no over-oxidation to guard against.
- 💡Record "remains orange" as a positive piece of evidence; leaving the box blank scores nothing.
- 💡Say which hydrogen is missing, the one on the carbon holding the hydroxyl group.
- 💡Pair this test with the classification question, since the two are usually asked together.
- 💡AQA treats a reagent as something you could take out of a bottle, so name the acid as well as the dichromate.
- 💡Learn the two colours as a pair, orange and green, and always say which is the starting colour.
- 💡Use [O] to balance organic oxidation equations, then check oxygen and hydrogen separately.
- 💡Warm gently in a water bath, never over a flame, because the organic liquids are flammable.
- 💡Give the observation for both compounds being distinguished, including the one that does not react.
- 💡Match the method to the product asked for before writing any equation.
Common Mistakes
- classifying from the locant in the name, so calling butan-2-ol secondary but 2-methylpropan-1-ol secondary as well
- counting hydrogen atoms on the carbon instead of alkyl groups
- calling methanol secondary or tertiary because it has no alkyl group, when it is primary
- letting a branch elsewhere in the chain change the classification
- omitting the water from the first oxidation so the equation does not balance
- swapping the methods, saying reflux gives the aldehyde and distillation gives the acid
- explaining that the aldehyde distils off because it is more soluble, rather than because it boils lower
- adding water to the second equation, when no hydrogen is removed from the aldehyde
- oxidising the ketone on to a carboxylic acid, which acidified dichromate cannot do
- leaving the water out of the equation
- saying distillation is needed to stop the reaction at the ketone
- naming the product of oxidising propan-2-ol as propanal rather than propanone
- claiming a tertiary alcohol can never be oxidised at all, when it still burns
- explaining the lack of reaction by the bulk of the three alkyl groups instead of the missing hydrogen
- expecting a green solution, or recording no observation at all rather than "stays orange"
- confusing the negative dichromate result with a negative Tollens' or Fehling's result
- writing "potassium dichromate" without acidified, which loses the reagent mark
- giving the colour change as orange to blue, or yellow to green
- saying the dichromate is reduced to chromium metal rather than to Cr³⁺
- calling the dichromate a catalyst while also using it as the oxidising agent
- cooling the Fehling's or Tollens' tube instead of warming it, an error AQA has specifically penalised
- describing the Fehling's precipitate as brown or green rather than red
- expecting a positive Tollens' result from a ketone
- adding water to the aldehyde to carboxylic acid equation, where none is produced