Carbonyls
1. What are Carbonyls?
Definition: organic compounds containing a C=O bond.
| Type | Structure | Board examples |
|---|---|---|
| Aldehyde | R–CHO : C=O carbon is bonded to one H (always H) and R; R can be anything (alkyl, aryl, H). The CHO is always at the end of the chain (C1) — never in the middle. | methanal HCHO · ethanal CH₃CHO · propanal CH₃CH₂CHO · butanal · 2-methylpropanal CH₃CH(CH₃)CHO · 2-methylbutanal · 3-methylbutanal |
| Ketone | R–CO–R′ : C=O carbon bonded to two carbon groups (R, R′ cannot be H). Always in the middle of the chain. | propanone CH₃COCH₃ · butanone CH₃COCH₂CH₃ · pentan-3-one · pentan-2-one · 3-methylpentan-2-one |
2. Physical Properties
- Low solubility in water (esp. longer chains): they have only temporary and permanent dipole forces — no H on O to donate H-bonds (small ones, e.g. methanal, ethanal, propanone, do dissolve a little by H-bonding to water).
- Lower boiling points than alcohols of similar Mr: no hydrogen bonding between their own molecules (only permanent + temporary dipoles).
3. Reaction ① — Addition of HCN (Nucleophilic Addition)
Reagent: HCN with NaCN or KCN (CN⁻ catalyses it). Product: a hydroxynitrile (cyanohydrin). Name: the C bearing OH and CN: nitrile gets C1.
| Carbonyl | Equation | Product |
|---|---|---|
| ethanal | CH₃CHO + HCN → CH₃CH(OH)CN | 2-hydroxypropanenitrile |
| propanone | CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN | 2-hydroxy-2-methylpropanenitrile |
| butanone | CH₃COC₂H₅ + HCN → CH₃C(OH)(CN)CH₂CH₃ | 2-hydroxy-2-methylbutanenitrile |
Mechanism: nucleophilic addition
- Key points to show: δ+ on C, δ– on O; arrow from the lone pair on C of CN⁻ to Cδ⁺; arrow from the C=O π bond to O; O⁻ with lone pair; arrow from O⁻ to Hδ⁺ of HCN; arrow showing H–CN breaking.
- The C=O carbon is planar ⇒ CN⁻ can attack from above or below with equal probability.
- Ethanal (and other aldehydes, unsymmetrical ketones) gives a product with a chiral centre* ⇒ a racemic mixture (50 : 50 of 2 optical isomers) is made, which has no optical activity overall. Propanone gives no chiral C.
4. Reaction ② — Reduction
- Aldehydes are reduced to primary alcohols; ketones to secondary alcohols.
- Reagents: LiAlH₄ in dry ether · NaBH₄ (aq) · H₂ + Ni catalyst + heat (≈200 °C). Show hydride as [H] in equations.
5. Reaction ③ — Oxidation (and the Tests)
- Aldehydes are oxidised to carboxylic acids.
- Ketones NEVER oxidise (no H on the C=O carbon).
| Reagent | Aldehyde | Ketone | Note |
|---|---|---|---|
| Acidified K₂Cr₂O₇ | orange → green | no change (stays orange) | also 1° and 2° alcohols |
| Tollens’ reagent | silver mirror / silver–grey precipitate (Ag⁺ → Ag) | no reaction | warm; “[Ag(NH₃)₂]⁺ in NaOH” |
| Fehling’s reagent | brick-red precipitate (Cu²⁺ → Cu₂O) | no reaction | warm; Cu²⁺ blue → red ppt |
6. Reaction ④ — 2,4-DNPH (Brady’s test)
Test for aldehydes AND ketones (i.e. for the C=O group): both give a yellow/orange precipitate with 2,4-dinitrophenylhydrazine. (Mark schemes accept “orange” or “yellow”.)
7. Reaction ⑤ — Triiodomethane (Iodoform) Test
Reagent: I₂(aq) + NaOH(aq) (alkaline aqueous iodine), warm. A yellow precipitate of CHI₃ (triiodomethane) with:
- methyl ketones: CH₃–CO–R;
- secondary methyl alcohols: CH₃–CH(OH)–R (oxidised to the methyl ketone first);
- also ethanal (CH₃CHO) and ethanol (CH₃CH₂OH) — the only aldehyde and 1° alcohol that do.
8. Summary — Test Table
| Reagent | Observation | Positive for |
|---|---|---|
| Acidified K₂Cr₂O₇ (warm) | orange → green | 1° alcohol, 2° alcohol, aldehyde |
| Tollens’ / Fehling’s | silver mirror / brick-red ppt | aldehydes only |
| 2,4-DNPH | orange/yellow ppt | aldehydes and ketones (any C=O) |
| I₂ / NaOH | yellow ppt CHI₃ | CH₃CO– or CH₃CH(OH)– (methyl ketone, 2° methyl alcohol, ethanal, ethanol) |
| Br₂(aq) | brown → colourless | alkenes |
| Na | effervescence (H₂) | –OH (alcohol, acid) |
| PCl₅ | steamy HCl fumes | –OH |
| NaHCO₃ | effervescence (CO₂) | carboxylic acid |
9. Past-Paper MCQs (board)
| Q | Question | Working / answer |
|---|---|---|
| 1 | HCN product with a chiral centre | CH₃CHO + HCN ⇒ CH₃CH(OH)CN: C bonded to H, OH, CH₃, CN ⇒ chiral ⇒ A ✓. B is pentan-3-one (two identical ethyl groups ⇒ product not chiral); methanal gives CH₂(OH)CN (no chiral C) |
| 4 | ethanal → 3-carbon acid, intermediate? | HCN adds, then hydrolysis ⇒ intermediate CH₃CH(OH)CN ⇒ D ✓ |
| 5 | burnt-sugar compound CH₃COCHO tested with 2,4-DNPH and Fehling’s | 2,4-DNPH: both C=O (Q ketone + R aldehyde); Fehling’s: aldehyde only (R) ⇒ C ✓ |
| 6 | propanone + HCN, then acid hydrolysis: final product | (CH₃)₂C(OH)CN → (CH₃)₂C(OH)CO₂H ⇒ D ✓ |
| 8 | pair distinguished by Tollens’ test | one aldehyde + one non-aldehyde ⇒ CH₃CHO and CH₃COCH₃ ⇒ A ✓ |
| 9 | P: cis–trans + red-brown ppt with Fehling’s | needs a CHO (Fehling’s) and a C=C with different groups on each carbon ⇒ D ✓ (board circles D); B is a ketone (no Fehling’s) |
| 10 | X: orange → green with Cr₂O₇²⁻; 1 mol X reacts with 2 mol HCN | 1° alcohol / 2° alcohol / aldehyde (oxidised) and 2 C=O groups to add HCN ⇒ dialdehyde ⇒ D ✓ (OHCCH₂CH₂CHO) |
| 12 | glycol HOCH₂CH₂OH — oxidation product that won’t react with Na | no OH left ⇒ OHC–CHO = C₂H₂O₂ ⇒ A ✓ |
| Q | Question | Working / answer |
|---|---|---|
| 17 | reaction forming ibuprofen (aryl–CH(CH₃)CO₂H) | oxidise 1° alcohol with warm acidified K₂Cr₂O₇ ⇒ B ✓ |
| 19 | alcohol → product that reacts with 2,4-DNPH but NOT Fehling’s | ketone ⇒ 2° alcohol ⇒ butan-2-ol ⇒ B ✓ |
| 20 | P (ketoester) + phenylhydrazine ⇒ Q | condensation at the ketone C=O: C=N–NH–Ph ⇒ D ✓ |
| 22 | malic acid in two steps from NCCH₂CH(OH)CN | step 1 HCN/NaCN adds (carbonyl → hydroxynitrile), step 2 dil. H₂SO₄ hydrolyses nitriles ⇒ B ✓ |
| 26 | hept-4-enal + H₂/Ni or NaBH₄ | H₂/Ni reduces both C=C and C=O ⇒ A ✓ CH₃(CH₂)₅CH₂OH |
| 30 | carvone + hot conc. MnO₄⁻ ⇒ CO₂ + X (9 C); max 2,4-DNPH per X | X has 3 C=O (ketones/aldehydes) ⇒ C (3) ✓ |
| 33 | civetone: no reaction with? | ketone + alkene: reacts with 2,4-DNPH ✓, HBr ✓, NaBH₄ ✓; not Fehling’s (not an aldehyde) ⇒ B ✓ |
| 36 | tartaric acid in 2 steps from OHC–CHO | HCN/NaCN then H₂SO₄(aq) ⇒ B ✓ |
| Q | Question | Working / answer |
|---|---|---|
| 42 | Y + warm dichromate ⇒ product with 2,4-DNPH but no Tollens’ | ketone product ⇒ 2° alcohol ⇒ butan-2-ol ⇒ B ✓ |
| 47 | ethanal → HCN, NaCN → F → hot dil. H₂SO₄ → G → CH₃OH/conc. H₂SO₄ → H | CH₃CH(OH)CN → CH₃CH(OH)COOH → methyl ester CH₃CH(OH)COOCH₃ ⇒ B ✓ |
| 64 | pineapple ester from butanal (NaBH₄ → X; Cr₂O₇²⁻ → Y) | X = butan-1-ol, Y = butanoic acid ⇒ butyl butanoate CH₃(CH₂)₂CO₂(CH₂)₃CH₃ ⇒ B ✓ |
| 80 | citral + excess H₂/Ni → V; hot conc. MnO₄⁻ reflux → W | V = saturated alcohol; oxidised to the carboxylic acid ⇒ B ✓ (the CH₂OH → COOH; no C=C left) |
| 92 | G: chiral, positive iodoform, no Tollens’ | 3-hydroxybutan-2-one: CH₃COCH(OH)CH₃ — chiral C (H, OH, CH₃, COCH₃) and CH₃CO ⇒ D ✓ |
10. Long CIE-style Questions (board)
| Reagent / observation | Compounds |
|---|---|
| acidified K₂Cr₂O₇ boiling — green | A, B, C |
| hot acidified KMnO₄ ⇒ ethanoic acid | C, E (E gives 2 CH₃COOH; C oxidised to CH₃COOH) |
| H₂ / Pt — hydrogen absorbed | C, D, E (C=O and C=C) |
| Fehling’s — brown-red ppt | C |
| 2,4-DNPH — orange ppt | C, D |
| Br₂ in inert solvent decolourised | E |
(a)(i) CH₃CHO + HCN → CH₃CH(OH)CN (displayed). (ii) nucleophilic addition. (iii) mechanism: δ+ C, δ– O; arrow from CN⁻ lone pair to Cδ⁺; π bond → O; O⁻ attacks Hδ⁺ of HCN; CN⁻ regenerated. (b)(i) CH₃CH(OH)CN + ½H₂SO₄ + 2H₂O → CH₃CH(OH)CO₂H + ½(NH₄)₂SO₄ · (ii) hydrolysis.
(b)(i) Br₂ test ⇒ alkene; (ii) Na test ⇒ alcohol (or carboxylic acid). (c)(i) a positive result in test 6 would show a carbonyl group; (ii) tests 5 + 6 together: the product of oxidation has no C=O reaction with 2,4-DNPH ⇒ it is a carboxylic acid, so H is a primary alcohol (an aldehyde or ketone product would have given a positive). (d) cis–trans: C=C with different groups on each carbon; the π bond prevents rotation. (e) H = but-2-en-1-ol CH₃CH=CHCH₂OH — draw cis and trans, displayed.
(a) Br₂(aq) decolourised ⇒ alkene. (b) Na ⇒ H₂ and C₄H₅O₂Na; ethanoic acid + conc. H₂SO₄ ⇒ ester C₆H₈O₃ ⇒ alcohol. (c) 2,4-DNPH + Tollens’ silver mirror ⇒ aldehyde.
(d) Z (no cis–trans): CH₂=C(OH)–CH₂–CHO (as drawn on board). (e) Na ⇒ sodium salt; ethanoic acid ⇒ ester (O–CO–CH₃). (f) Tollens’ ⇒ CH₂=C(OH)CH₂COOH; 2,4-DNPH ⇒ the hydrazone (C=N–NH–Ar). (g) but-2-enoic acid, cis isomer: CH₃ and COOH on the same side of the C=C — draw it displayed.
(a)(i) CH₂Cl₂ + NaOH(aq) then AgNO₃: white precipitate, AgCl. (ii) CH₃OH + PCl₅: steamy fumes of HCl. (iii) CH₃OH + Na: effervescence, H₂.
(c) further isomers of C₂H₄O₂: X = CH₃COOH, Y = HOCH=CHOH (cis/trans), Z = OHC–CH₂OH. (d) NaHCO₃: X (acid) — effervescence of CO₂; Tollens’: Z — silver mirror. (e) Y shows stereoisomerism: draw cis and trans (HOCH=CHOH).
(a) sodium ⇒ alcohol; bromine water ⇒ alkene; alkaline aqueous iodine ⇒ methyl ketone or secondary methyl alcohol.
(b) V = CH₃CH(OH)CH=CH₂ (but-3-en-2-ol: positive iodoform); W = CH₃CH=CHCH₂OH (but-2-en-1-ol: primary alcohol, no CH₃CH(OH)). (c) V shows optical isomerism (chiral C bonded to H, OH, CH₃, CH=CH₂) — draw 2 mirror images with wedge/dash. (d) X (cold dilute KMnO₄ on the C=C of both V and W) = CH₃CH(OH)CH(OH)CH₂OH, C₄H₁₀O₃ — the same neutral triol from either compound.
- Aldehyde R–CHO (end of chain, –al); ketone R–CO–R′ (middle, –one). Lower b.p. than alcohols (no H-bonding); little solubility.
- HCN + NaCN/KCN: nucleophilic addition ⇒ hydroxynitrile (+1 C); CN⁻ attacks Cδ⁺ from above/below ⇒ racemic mixture (if product chiral); nitrile → COOH by acid hydrolysis.
- Reduction: LiAlH₄/dry ether, NaBH₄, H₂/Ni 200 °C: aldehyde → 1° alcohol, ketone → 2° alcohol (NaBH₄/LiAlH₄ leave C=C).
- Oxidation: aldehydes → acids; ketones never. K₂Cr₂O₇ orange→green; Tollens’ silver mirror; Fehling’s brick-red (aldehyde only).
- 2,4-DNPH: orange/yellow ppt with any C=O (condensation, loses H₂O). I₂/NaOH: yellow CHI₃ with CH₃CO– / CH₃CH(OH)–.
- Deduction questions: combine tests — Br₂ (alkene), Na (OH), Tollens’ (aldehyde), iodoform (methyl ketone), DNPH (C=O), chirality, cis–trans.