9701 Chemistry · Topics 17 & 31 · AS + A Level
Aldehydes and Ketones Cheat Sheet — A Level Chemistry 9701
Carbonyls are the third mechanism you must be able to draw in full, and the topic where distinguishing tests earn easy marks. This sheet covers nucleophilic addition of HCN with the cyanide attack shown properly, reduction with NaBH₄, the Tollens’, Fehling’s and 2,4-DNPH tests, and the tri-iodomethane test applied to methyl ketones.
✅ 16 worked sections
🎓 CAIE 9701 syllabus-mapped
⭐ 4.8 rated tutors
What’s on this cheat sheet
Carbonyl Compounds
01 · The carbonyl group
C=O is trigonal planar at 120°, one σ plus one π bond. Oxygen is far more electronegative, so the bond is strongly polar with δ+ carbon.
That δ+ carbon is attacked by nucleophiles — the opposite of an alkene, whose electron‑rich C=C attracts electrophiles.
02 · Aldehyde or ketone?
Aldehyde RCHO — the carbonyl is at the end of the chain, so it carries a hydrogen and is easily oxidised.
Ketone RCOR′ — the carbonyl is inside the chain with no hydrogen attached, so it resists oxidation. That single difference drives every test below.
03 · Tests to know
| Reagent | Aldehyde | Ketone |
|---|---|---|
| 2,4‑DNPH | orange ppt | orange ppt |
| Tollens’ | silver mirror | none |
| Fehling’s | brick‑red ppt | none |
| K₂Cr₂O₇ / H⁺ | orange → green | stays orange |
2,4‑DNPH detects any carbonyl; the melting point of the purified orange derivative identifies which one. Tollens’ and Fehling’s then separate aldehyde from ketone.
04 · Reduction
RCHO + 2[H] → RCH₂OH (primary alcohol)
RCOR′ + 2[H] → RCH(OH)R′ (secondary alcohol)
Exactly the reverse of alcohol oxidation, so the two reactions run a loop you can use in synthesis.
05 · Oxidation
RCHO + [O] → RCOOH with acidified K₂Cr₂O₇ under reflux. Ketones do not react — there is no hydrogen on the carbonyl carbon to remove.
Methanal is the exception among aldehydes in being oxidised all the way to CO₂ under vigorous conditions.
06 · Addition of HCN
Reagent: NaCN with dilute acid — HCN itself is a toxic gas, and the cyanide must be generated in situ.
The product is a hydroxynitrile: RCHO + HCN → RCH(OH)CN. This lengthens the carbon chain by one, and the nitrile can then be hydrolysed to a hydroxy acid.
07 · Nucleophilic addition mechanism
2 An intermediate with a negatively charged oxygen forms.
3 That oxygen takes H⁺ from HCN or water, giving the –OH and regenerating CN⁻.
Two curly arrows in step 1: one from the lone pair on CN⁻ to the carbon, one from the C=O π bond to the oxygen.
08 · Racemic products A2
The carbonyl is planar, so the nucleophile attacks either face with equal probability. If the product has a chiral carbon the two enantiomers form in equal amounts — a racemic mixture, optically inactive.
09 · The tri‑iodomethane test
I₂ with NaOH(aq), warm → pale yellow CHI₃ precipitate.
Positive for ethanal and any methyl ketone (CH₃CO–), and for alcohols oxidisable to them. Propanone gives it; propanal does not — a fast way to separate the two.
10 · Worked example — identify the compound
2,4‑DNPH → a carbonyl compound
No mirror → ketone, not aldehyde
Yellow CHI₃ → contains CH₃CO–
→ propanone, CH₃COCH₃
11 · Making carbonyls
Aldehyde — oxidise a primary alcohol with acidified K₂Cr₂O₇ and distil the product off as it forms, before it is oxidised further.
Ketone — reflux a secondary alcohol with the same reagent; no over‑oxidation is possible, so reflux is safe.
12 · Equations to know
CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃
CH₃CHO + [O] → CH₃COOH
CH₃CHO + HCN → CH₃CH(OH)CN
CH₃CH(OH)CN + 2H₂O + H⁺ → CH₃CH(OH)COOH + NH₄⁺
13 · Reagents at a glance
| Reagent | What it contains |
|---|---|
| Tollens’ | [Ag(NH₃)₂]⁺ in aqueous ammonia |
| Fehling’s | Cu²⁺ complexed in alkali |
| 2,4‑DNPH | 2,4‑dinitrophenylhydrazine |
| tri‑iodomethane | I₂ with NaOH(aq) |
Both Tollens’ and Fehling’s work by the aldehyde reducing the metal ion: Ag⁺ → Ag as a mirror, Cu²⁺ → Cu₂O as a brick‑red solid.
14 · Where carbonyls sit in synthesis
They are the junction between alcohols and acids: reduce to go one way, oxidise to go the other.
Adding HCN is one of only two ways in the syllabus to lengthen a carbon chain, and the hydroxynitrile it makes can be hydrolysed to a hydroxy acid such as 2‑hydroxypropanoic acid.
15 · Worked example — chain lengthening
1 · NaCN with dilute acid → (CH₃)₂C(OH)CN
2 · dilute HCl, reflux → (CH₃)₂C(OH)COOH
The nitrile carbon becomes the –COOH, so the product has one carbon more than the ketone. The product here is achiral, but a similar route from an aldehyde would give a racemic mixture.
Marks lost here
— Saying 2,4‑DNPH distinguishes aldehydes from ketones; it detects both.
— Calling the HCN reaction electrophilic addition. The carbonyl carbon is δ+, so the attack is nucleophilic.
— Forgetting that the hydroxynitrile has one more carbon than the starting material.
— Missing the racemic outcome when a chiral centre is created.
Get all 25 topic cheat sheets for 9701 Chemistry
One free PDF pack, AS + A Level, straight to WhatsApp.
Aldehydes and Ketones — Frequently Asked Questions
How do you distinguish an aldehyde from a ketone?
Tollens’ reagent gives a silver mirror with an aldehyde and no change with a ketone. Fehling’s solution turns from blue to a brick-red precipitate with an aliphatic aldehyde only. Both work because aldehydes are readily oxidised to carboxylic acids and ketones are not.
Why do carbonyls undergo nucleophilic addition?
The C=O bond is strongly polarised because oxygen is much more electronegative than carbon, leaving the carbonyl carbon δ+. A nucleophile attacks that carbon and the π electrons shift onto the oxygen, giving an alkoxide that is then protonated.
What does 2,4-DNPH tell you?
An orange precipitate confirms a carbonyl group — aldehyde or ketone. The melting point of the purified derivative can then be compared with a data table to identify exactly which carbonyl compound it was.
