Aldehydes and ketones (A-level only) — AQA A-Level Chemistry
Test yourself on Aldehydes and ketones (A-level only) with AQA A-Level practice questions.
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Aldehydes and ketones (A-level only) explained
The carbonyl carbon of an aldehyde carries a hydrogen atom, so an oxygen can be inserted into that C–H bond without disturbing the carbon skeleton: RCHO + [O] → RCOOH.
Read the full explanation
No water is formed, because nothing is removed from the molecule. Warming with acidified potassium dichromate(VI) under reflux does the job, the solution turning from orange to green. A ketone has two alkyl groups on its carbonyl carbon and no hydrogen, so the same oxidation would require a carbon–carbon bond to break and does not happen. That difference in ease of oxidation is what the mild oxidising agents Tollens' reagent and Fehling's solution exploit when they distinguish an aldehyde from a ketone.
Chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.
Both tests work by offering a mild oxidising agent that only an aldehyde can reduce. Tollens' reagent is made fresh by adding aqueous ammonia to silver nitrate solution until the precipitate redissolves, giving the ion [Ag(NH₃)₂]⁺. Warmed in a water bath with an aldehyde, the silver ion is reduced to silver, which deposits as a mirror on the tube wall; a ketone gives no change. Fehling's solution contains copper(II) ions held in alkaline solution and is deep blue. Warmed with an aldehyde, copper(II) is reduced to copper(I) and a red precipitate of copper(I) oxide appears; with a ketone it stays blue. In each case the aldehyde is itself oxidised to the carboxylic acid or its salt.
These reduction reactions are examples of nucleophilic addition.
Sodium tetrahydridoborate(III), NaBH₄, in aqueous solution supplies hydride ion, H⁻, which has a lone pair and a negative charge and therefore behaves as a nucleophile. The carbonyl group is polar because oxygen is more electronegative than carbon, so the carbon is δ⁺ and is the point of attack. The hydride's lone pair forms a bond to that carbon while the π electrons of the C=O shift onto the oxygen, giving a negatively charged alkoxide intermediate; this then takes a hydrogen ion from the water, completing the hydroxyl group. Two atoms have been added across the C=O and nothing has been removed, and the first step was attack by a nucleophile, so the mechanism is nucleophilic addition. Aldehydes give primary alcohols and ketones give secondary alcohols.
The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce hydroxynitriles.
Potassium cyanide provides the cyanide ion, a nucleophile with a lone pair on its carbon. It attacks the δ⁺ carbonyl carbon, the π electrons moving onto the oxygen to give a negatively charged intermediate; dilute acid added afterwards protonates that oxygen to a hydroxyl group. The product is a hydroxynitrile, with the OH and the CN on the same carbon, and it has one more carbon than the starting material, which makes the reaction useful for lengthening a chain. Ethanal gives 2-hydroxypropanenitrile; propanone gives 2-hydroxy-2-methylpropanenitrile. Note the naming rule: the nitrile carbon is carbon one, so the hydroxyl group ends up on carbon two.
Aldehydes and unsymmetrical ketones form mixtures of enantiomers when they react with KCN followed by dilute acid.
The carbonyl carbon is trigonal planar, so the cyanide ion can approach from above or below the plane with equal probability. In the product, that carbon carries four groups: hydroxyl, nitrile, and the two original groups. If these two are different, the carbon is chiral. Equal approach from both faces produces equal amounts of the two enantiomers, forming an optically inactive racemate. However, if the original groups are identical, no chiral centre forms. Methanal (two hydrogens) and symmetrical ketones like propanone (two methyls) are key exceptions; their reaction with KCN/H+ yields achiral products that do not exhibit optical isomerism.
The hazards of using KCN.
Potassium cyanide is acutely toxic by swallowing, by skin contact and through the gas it releases. The serious hazard in this preparation is that cyanide salts liberate hydrogen cyanide, a volatile and extremely toxic gas, on contact with acid, including the dilute acid added in the second stage of the reaction and the acid in the stomach. Control the risk rather than just listing equipment: work in a fume cupboard so any hydrogen cyanide is removed, wear gloves because the solid and its solution are toxic on skin contact, use the smallest quantity that will do, add the acid only when the addition step is complete, and dispose of residues in the designated container rather than down the sink.
Students should be able to: write overall equations for reduction reactions using [H] as the reductant outline the nucleophilic addition mechanism for reduction reactions with NaBH (the nucleophile – should be shown as H ) write overall equations for the formation of hydroxynitriles using HCN outline the nucleophilic addition mechanism for the reaction with KCN followed by dilute acid explain why nucleophilic addition reactions of KCN, followed by dilute acid, can produce a mixture of enantiomers.
Five linked skills. Write reduction equations with 2[H] as the reductant: CH₃CHO + 2[H] → CH₃CH₂OH, and CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃. Outline the NaBH₄ mechanism with the nucleophile drawn as H⁻: arrow from its lone pair to the δ⁺ carbon, arrow from the C=O bond to the oxygen, then an arrow from the lone pair on the resulting negative oxygen to H⁺ from water. Write the hydroxynitrile equation using HCN, for example CH₃CHO + HCN → CH₃CH(OH)CN. Outline the cyanide mechanism the same way, protonating with dilute acid at the end. Finally explain the enantiomers: the carbonyl carbon is planar, so attack from either face is equally likely.
Your focus
- Write the equation using [O] for the oxidation of a named aldehyde to its carboxylic acid.
- Explain why aldehydes are readily oxidised but ketones are not, referring to the carbonyl carbon.
- State the reagent, conditions and colour change for oxidising an aldehyde with dichromate.
Show all 21 objectives
- Describe how to carry out the Tollens' and Fehling's tests, including how the reagent is prepared and warmed.
- State the observation with an aldehyde and with a ketone for each test.
- Explain each test as a redox reaction, naming the species oxidised and the species reduced.
- Name the mechanism by which NaBH₄ reduces a carbonyl compound and justify both words in the name.
- Explain why the carbonyl carbon is the site attacked, referring to electronegativity.
- Predict the class of alcohol formed when a given aldehyde or ketone is reduced.
- Draw the nucleophilic addition mechanism for the reaction of a carbonyl compound with cyanide ion and dilute acid.
- Identify the reagent and conditions needed to convert a named aldehyde into a hydroxynitrile.
- Name the hydroxynitrile formed from a given aldehyde or ketone using correct locants.
- Explain why nucleophilic addition of cyanide to most aldehydes produces equal amounts of two enantiomers.
- Deduce whether the product of a given carbonyl compound with cyanide will be chiral.
- Explain why methanal and propanone give products that show no optical isomerism.
- State two specific hazards of using potassium cyanide and the precaution that controls each.
- Explain why adding dilute acid to a cyanide mixture is the most dangerous step in the preparation.
- Justify why a fume cupboard rather than eye protection alone is required for this reaction.
- Write balanced equations using 2[H] for the reduction of a named aldehyde and a named ketone.
- Draw the nucleophilic addition mechanism with H⁻ and with CN⁻, including the final protonation.
- Explain why the cyanide reaction gives a racemate while the borohydride reduction of propanone does not give a chiral product.
Aldehydes and ketones (A-level only) exam tips
Marking Points
- one mark for the equation RCHO + [O] → RCOOH with the correct carboxylic acid drawn or named
- one mark for naming acidified potassium dichromate(VI) and heating under reflux
- one mark for saying the aldehyde carbon carries a hydrogen, so no carbon–carbon bond need break
- one mark for contrasting the ketone, which is not oxidised under these conditions
- one mark for naming the reagent fully, Tollens' reagent or Fehling's solution, with warming in a water bath
- one mark for the positive observation, a silver mirror or a red precipitate
- one mark for stating that the ketone gives no change
- one mark for saying the aldehyde is oxidised while the silver or copper ion is reduced
- one mark for naming the mechanism as nucleophilic addition
- one mark for identifying H⁻ as the nucleophile that attacks the δ⁺ carbon
- one mark for labelling the C=O as δ⁺ on carbon and δ⁻ on oxygen, with electronegativity as the reason
- one mark for the second step, in which the negatively charged intermediate gains H⁺ from water
- one mark for naming the reagent as KCN, or NaCN, followed by dilute acid
- one mark for the curly arrow from the lone pair on the carbon of the cyanide ion to the δ⁺ carbonyl carbon
- one mark for the arrow from the C=O bond to the oxygen, giving the negatively charged intermediate
- one mark for the correct hydroxynitrile, with the hydroxyl and nitrile groups on the same carbon
- one mark for naming the product with the nitrile carbon numbered as carbon one
- one mark for stating that the carbonyl carbon is planar
- one mark for saying the cyanide ion attacks from above or from below with equal probability
- one mark for identifying the new chiral centre as a carbon with four different groups
- one mark for stating that equal amounts of the two enantiomers are formed, so the product is a racemate and optically inactive
- one mark for a named precaution, such as working in a fume cupboard
- one mark for the matching reason, such as the release of toxic hydrogen cyanide gas
- one mark for stating that cyanide releases hydrogen cyanide when it meets acid
- one mark for a second precaution with its reason, such as gloves because the solid and solution are toxic by skin contact
- one mark for the overall reduction equation using 2[H] as the reductant
- one mark for the arrow from the lone pair on H⁻, or on the carbon of CN⁻, to the carbonyl carbon
- one mark for the arrow from the C=O bond to the oxygen
- one mark for the negatively charged intermediate and the arrow from its lone pair to H⁺
- one mark for explaining the racemate by equal attack on either face of the planar carbonyl carbon
Examiner Tips
- 💡Count atoms across your equation; if water has appeared from nowhere, the equation is wrong.
- 💡Reflux is the condition for making the acid, since nothing needs to be removed as it forms.
- 💡Use the hydrogen on the carbonyl carbon as your one-line explanation for every aldehyde and ketone comparison.
- 💡Always state the condition, warm in a water bath, alongside the reagent; it carries a mark of its own.
- 💡Give the observation for both compounds, the aldehyde and the ketone.
- 💡Say what happens to the metal ion as well as to the organic compound if the question says explain.
- 💡AQA requires the nucleophile to be shown as H⁻, so write the charge and the lone pair.
- 💡Name the product class as well as the mechanism: primary alcohol from an aldehyde, secondary from a ketone.
- 💡Check you have added two atoms in total across the C=O and removed none.
- 💡Write reagent and condition as a pair: KCN followed by dilute acid, warmed.
- 💡Show the charge on the intermediate oxygen; it is often a separate mark.
- 💡Count carbons before and after; the product must have one more than the carbonyl compound.
- 💡Check the four groups on the new carbon before claiming enantiomers; two identical groups (e.g., in methanal) means none.
- 💡Use the three-step chain: planar carbon, attack from either side, equal amounts of enantiomers.
- 💡Finish by saying the mixture is optically inactive, since that is often the final mark.
- 💡Always pair a precaution with the hazard it controls; AQA marks them as a pair.
- 💡Name the toxic gas as hydrogen cyanide and say what releases it.
- 💡Avoid generic safety points that would apply to any experiment.
- 💡Balance every [H] equation by counting hydrogens; carbonyl reductions almost always need 2[H].
- 💡Draw the lone pair and the charge on the nucleophile before drawing any arrow from it.
- 💡Keep the two mechanisms parallel in your revision; only the nucleophile and the final acid differ.
Common Mistakes
- including water as a product of the aldehyde oxidation, when no atoms leave the molecule
- saying ketones are oxidised more slowly rather than not at all by this reagent
- naming the product of oxidising ethanal as ethanol rather than ethanoic acid
- omitting the word acidified from the name of the oxidising agent
- cooling the tube instead of warming it, a method error AQA has specifically penalised
- describing the Fehling's precipitate as brown or green rather than red
- recording a "negative colour change" for the ketone instead of simply no change
- giving the Tollens' product as silver oxide rather than silver metal
- naming the mechanism nucleophilic addition–elimination, which belongs to the acyl chlorides
- showing NaBH₄ itself as the attacking species rather than H⁻
- putting the δ⁺ on the oxygen of the carbonyl group
- forgetting that the second hydrogen comes from water, not from the reducing agent
- giving HCN when the question asks for the reagent for the mechanism, where AQA wants KCN (or NaCN) followed by dilute acid; HCN is correct only in the overall equation
- starting the curly arrow at the negative charge rather than at the lone pair on carbon
- bonding the cyanide through its nitrogen atom instead of its carbon
- numbering the chain from the hydroxyl end, so the product is named as a 1-hydroxy compound
- saying the product is chiral without explaining that attack occurs equally from both faces
- claiming methanal or propanone gives a racemate, when their products lack a chiral centre due to identical groups
- arguing that one enantiomer forms in excess because it is more stable
- describing the planar species as a carbocation, when here it is the neutral carbonyl carbon
- giving a precaution with no reason, or a reason that does not match the precaution given
- describing potassium cyanide as flammable or corrosive rather than toxic
- treating only the solid as hazardous, when the solution and the gas released are the greater danger
- offering "wear goggles" as the whole answer, when eye protection is assumed in every practical
- writing [H] once instead of 2[H], so the equation does not balance
- drawing NaBH₄ or CN with no charge as the attacking species instead of H⁻ and CN⁻
- starting a curly arrow at the negative sign rather than at a lone pair, which AQA does not credit
- omitting the final protonation step, leaving a charged product