Connect the polar carbonyl group to aldehyde oxidation, identification tests and reduction with sodium borohydride.
Locate the carbonyl carbon
An aldehyde has a carbonyl carbon bonded to at least one hydrogen: RCHO, with methanal HCHO as the simplest case. A ketone has that carbon bonded to two carbon-containing groups: RCOR′. The functional groups are –CHO and >C=O respectively. Not every compound with C=O is an aldehyde or ketone: carboxylic acids, esters and amides also contain carbonyl groups.
Use –al for an aldehyde and –one with a position number when needed for a ketone. CH₃CH₂CHO is propanal; CH₃COCH₂CH₃ is butan-2-one. The carbonyl carbon counts as part of the chain. The C=O bond is polarised towards oxygen because oxygen is more electronegative, leaving carbon δ⁺ and oxygen δ⁻. A nucleophile attacks the electron-deficient carbon.
Explain physical properties with the right interactions
Simple aldehydes and ketones have London forces and permanent dipole–dipole attractions. They lack an O–H or N–H donor, so pure samples cannot hydrogen-bond to themselves in the usual model. Their oxygen lone pairs can accept hydrogen bonds from water, explaining appreciable water solubility of smaller members. Solubility generally decreases as the non-polar hydrocarbon part grows.
Compare similar-sized molecules when explaining boiling points. Hydrogen bonding usually makes the corresponding alcohol boil higher than a carbonyl compound; molecular size and shape still matter. Do not state that a carbonyl oxygen cannot hydrogen-bond merely because the molecule cannot donate an H bond.
Aldehydes oxidise under mild conditions
Heating an aldehyde with acidified potassium dichromate(VI) gives a carboxylic acid, with orange Cr₂O₇²⁻ becoming green Cr³⁺. Use sulfuric acid to acidify the reagent. Ordinary ketones do not react with this mild test; harsher conditions can break carbon–carbon bonds, so “ketones can never be oxidised” is too broad.
Aldehyde oxidation retains the number of carbon atoms. The shorthand [O] represents an oxidising equivalent, not a bottle of atomic oxygen. Under alkaline aldehyde-test conditions, the organic oxidation product is the carboxylate rather than predominantly free carboxylic acid.
State the reagent and a visible observation
Tollens’ reagent contains [Ag(NH₃)₂]⁺; silver(I) gains electrons to form silver metal. In Fehling’s solution copper(II) is reduced to copper(I) oxide. Fehling’s is not a universal test for every aromatic aldehyde, and other reducing substances can also respond to aldehyde tests: draw conclusions within the supplied candidate set.
Use fresh separate portions. A dichromate change cannot by itself distinguish an aldehyde from an oxidisable alcohol. Do not replace a requested observation with “oxidation happens”. Tollens’ reagent must be freshly prepared and promptly disposed of through the laboratory’s procedure, never stored or allowed to dry.
| Reagent and conditions | Aldehyde result | Ketone result |
|---|---|---|
| Tollens’ reagent, warm gently | Silver mirror or grey/silver metal deposit | No visible change |
| Fehling’s solution, heat gently | Brick-red Cu₂O precipitate for typical aliphatic aldehydes | No visible change |
| K₂Cr₂O₇ with dilute H₂SO₄, warm | Orange solution turns green | No visible change in this mild test |
NaBH₄ supplies the hydride nucleophile
Aqueous sodium borohydride, NaBH₄, reduces aldehydes to primary alcohols and ketones to secondary alcohols under mild conditions; aqueous ethanol can be used where needed to dissolve the organic reactant. The exam mechanism represents the transferred hydride as H⁻ with its electron pair, not H⁺ or a hydrogen radical.
First draw a full-headed curly arrow from the hydride electron pair to carbonyl carbon and another from the C=O π bond to oxygen. The tetrahedral intermediate has an O⁻ group and a new C–H bond. Then O⁻ accepts a proton from water or another appropriate proton source, giving the alcohol. Proton transfer from water also requires an arrow from its O–H bond back to that water oxygen.
June 2023 Paper 2 Q03.3 assessed the hydride arrow, C=O electron movement, intermediate and mechanism name. A correct final alcohol does not replace a requested mechanism.
Diagram placeholder
Hydride addition to butan-2-one to add
Labels to include:
- Cδ⁺=Oδ⁻ of CH₃COCH₂CH₃
- H⁻ with lone pair
- Arrow H lone pair → carbonyl C
- Arrow C=O π bond → O
- CH₃C(H)(O⁻)CH₂CH₃ intermediate
- Arrow O⁻ lone pair → H of water; water O–H bond → water O
- Butan-2-ol product and OH⁻ after water protonation
The carbonyl carbon has three bonds before attack and four single bonds afterwards. Retain every carbon and the negative charge on intermediate oxygen. The two newly added hydrogens end up on carbon and oxygen.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Name CH₃CH₂COCH₂CH₃ and identify its functional group.Show answer
Pentan-3-one; it is a ketone. The carbonyl carbon is attached to two ethyl groups.
Q2. Why can propanone dissolve in water even though pure propanone does not hydrogen-bond to itself?Show answer
Its oxygen lone pairs accept hydrogen bonds from water O–H groups. Propanone has no suitable O–H/N–H donor for hydrogen bonding between its own molecules.
Q3. How would Tollens’ reagent distinguish propanal from propanone?Show answer
Warm separate samples with fresh Tollens’ reagent. Propanal gives silver metal as a mirror or deposit; propanone gives no visible change under the test conditions.
Q4. What is the organic product when ethanal reacts with Fehling’s solution?Show answer
Ethanoate, CH₃COO⁻, because the test mixture is alkaline. Cu₂O forms as a brick-red precipitate.
Q5. Describe the first two electron-pair movements in reduction by NaBH₄.Show answer
The hydride electron pair moves to carbonyl carbon; the C=O π electrons move to oxygen. This makes a C–H bond and an O⁻ tetrahedral intermediate, which is then protonated.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 organic chemistry specification — 3.3.8 coverage and required skills.
- Chemrevise: Aldehydes and ketones — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 2 mark scheme — Q03.3 p15 and report Q03.3 p3: hydride nucleophile, arrow origins, intermediate and addition mechanism. Q06.1–06.3 p25 and report pp4–5: reagents, observations and interpreting candidate-specific tests.
- AQA June 2023 Paper 2 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.