The carbonyl group: structure
A carbonyl group is a carbon joined to an oxygen by a double bond, C=O.
- The carbon is sp² hybridised. Its three sigma bonds lie in one flat plane at about 120°. The fourth (pi) bond sits above and below this plane.
- Oxygen is more electronegative (pulls electrons harder) than carbon. So the bond is polar: C is δ+ and O is δ−. We can also draw a resonance form C⁺–O⁻.
- Aldehyde: the C=O carbon has at least one H: R–CHO (for example HCHO, CH₃CHO, C₆H₅CHO).
- Ketone: the C=O carbon has two carbon groups: R–CO–R′ (for example CH₃COCH₃).
Naming
IUPAC names: replace the final 'e' of the alkane by -al for aldehydes (ethanal) and -one for ketones (propanone, butan-2-one). Common names: formaldehyde, acetaldehyde, acetone, benzaldehyde, acetophenone.
Preparation of aldehydes and ketones
Methods for both
- Oxidation of alcohols: primary alcohol → aldehyde; secondary alcohol → ketone. A mild oxidant like PCC stops at the aldehyde. Strong KMnO₄ would go on to the acid.
- Dehydrogenation: alcohol vapour over hot Cu (573 K) loses H₂.
- Ozonolysis of alkenes: O₃ then Zn/H₂O cuts the C=C into two C=O.
- Hydration of alkynes: ethyne + water (HgSO₄, dil. H₂SO₄) → ethanal; other alkynes give ketones.
Aldehydes only
- Rosenmund reduction: acyl chloride + H₂ over Pd on BaSO₄ (poisoned) → aldehyde.
- From nitriles and esters: Stephen reaction (SnCl₂ + HCl, then water) or DIBAL-H.
- From toluene: Etard reaction (CrO₂Cl₂) or CrO₃ in acetic anhydride → benzaldehyde; also side-chain chlorination then hydrolysis.
- Gatterman–Koch: benzene + CO + HCl (AlCl₃/CuCl) → benzaldehyde.
Ketones only
- From acyl chlorides with dialkylcadmium (R₂Cd).
- From nitriles with a Grignard reagent, then hydrolysis.
- Friedel–Crafts acylation: benzene + CH₃COCl (AlCl₃) → acetophenone.
Physical properties
- Methanal is a gas; ethanal is a very volatile liquid; others are liquids or solids.
- Boiling points are higher than hydrocarbons and ethers of similar mass (because of dipole–dipole attraction), but lower than alcohols (they cannot hydrogen-bond with each other: no O–H).
- Small ones (methanal, ethanal, propanone) mix with water because they form H-bonds with water. Solubility falls as the carbon chain grows.
- Smaller aldehydes have sharp smells; bigger ones and many ketones smell pleasant (fruits, flowers).
Nucleophilic addition reactions
A nucleophile is a particle with a spare electron pair (it 'loves the nucleus' / positive centre). It attacks the δ+ carbon from above the flat plane. Carbon changes from sp² (flat) to sp³ (tetrahedral), and the O⁻ then picks up H⁺.
Why aldehydes react faster than ketones
- Size (steric): a ketone has two bulky groups that crowd the carbon. An aldehyde has a small H.
- Electronic: alkyl groups push electrons towards C, making it less δ+. Two alkyl groups (ketone) push more than one.
- Order: HCHO > CH₃CHO > CH₃COCH₃. Aromatic aldehydes (C₆H₅CHO) are less reactive than aliphatic ones because the ring shares electrons with C=O.
Main additions
- HCN (base catalyst) → cyanohydrin.
- NaHSO₃ → crystalline bisulphite adduct (used to separate aldehydes).
- Alcohols (dry HCl) → hemiacetal, then acetal. Ketones with ethylene glycol → ketal.
- Grignard reagent then water → alcohols (HCHO gives 1°, other aldehydes 2°, ketones 3°).
- Ammonia derivatives (H₂N–Z): addition then loss of water, giving C=N–Z: hydroxylamine → oxime, hydrazine → hydrazone, phenylhydrazine → phenylhydrazone, 2,4-DNP → orange 2,4-DNP derivative, semicarbazide → semicarbazone.
Reduction and oxidation
Reduction
- NaBH₄ or LiAlH₄ (or H₂/Ni): aldehyde → primary alcohol; ketone → secondary alcohol.
- Clemmensen (Zn–Hg + conc. HCl) and Wolff–Kishner (NH₂NH₂, then KOH in ethylene glycol, heat): C=O → CH₂ (gives a hydrocarbon).
Oxidation
- Aldehydes are oxidised even by mild oxidants to carboxylic acids, because the H on the C=O carbon is easy to remove.
- Tollens test (ammoniacal AgNO₃): aldehyde gives a silver mirror.
- Fehling test (Cu²⁺ in alkaline tartrate): aliphatic aldehyde gives a red-brown Cu₂O precipitate. Aromatic aldehydes do not give it.
- Ketones need strong oxidants and hot conditions; the carbon chain breaks.
- Haloform (iodoform) reaction: compounds with CH₃CO– (and ethanal) give a yellow CHI₃ precipitate with I₂ + NaOH.
Reactions due to α-hydrogen
The α-carbon is the carbon next to C=O. Its H atoms are α-hydrogens. They are slightly acidic because the ion left behind (the enolate) spreads its negative charge onto oxygen by resonance.
- Aldol reaction: with dilute NaOH, two molecules having α-H join. 2 CH₃CHO → CH₃CH(OH)CH₂CHO (3-hydroxybutanal). On heating it loses water → CH₃CH=CHCHO (but-2-enal). This whole change is aldol condensation.
- Cross aldol: two different carbonyl compounds give a mixture of four products (fewer if one has no α-H).
- Cannizzaro reaction: aldehydes with no α-H (HCHO, C₆H₅CHO) with concentrated NaOH: one molecule is oxidised (acid salt), another reduced (alcohol).
Other reactions
Aromatic aldehydes/ketones undergo electrophilic substitution (nitration) at the meta position because C=O pulls electrons from the ring.
Uses of aldehydes and ketones
- Formalin (40% methanal): preserving biological specimens; making Bakelite and other resins.
- Ethanal: making acetic acid, ethyl acetate, and some plastics.
- Acetone and butanone: common solvents (nail-polish remover, paints).
- Benzaldehyde (almond smell), vanillin, cinnamaldehyde: flavours and perfumes.
- Many sugars (glucose) and hormones (testosterone, progesterone) contain C=O.
Try it: predict, then check
In the 3D free-play step, before you pick each compound, predict: aldehyde or ketone? How many α-H? Tollens yes or no? Iodoform yes or no? Then pick it and check. Score yourself out of 5 compounds × 4 = 20.
At home (with an adult): smell a drop of nail-polish remover (acetone) on cotton and notice it dries very fast: small ketones are volatile because they have no O–H to hold molecules together.
Exam corner
The unit carries about 8 marks in CBSE. Common questions: name reactions (Rosenmund, Stephen, Etard, Clemmensen, Wolff–Kishner, aldol, Cannizzaro), chemical tests to tell two compounds apart, reactivity order towards nucleophilic addition, conversions in 2–3 steps, and identifying a compound from its reactions.
Key formulas and definitions
- Aldehyde: R–CHO; ketone: R–CO–R′
- R–CHO + HCN → R–CH(OH)CN (cyanohydrin)
- R–CHO + [O] → R–COOH (Tollens: Ag mirror; Fehling: red Cu₂O)
- >C=O + Zn–Hg/HCl → >CH₂ (Clemmensen)
- 2 CH₃CHO → CH₃CH(OH)CH₂CHO (aldol, dil. NaOH)
- 2 HCHO + conc. NaOH → CH₃OH + HCOONa (Cannizzaro)
- Reactivity: HCHO > CH₃CHO > CH₃COCH₃
Worked examples
1. How many α-hydrogens are there in butan-2-one, CH₃COCH₂CH₃?
Step 1: The C=O carbon is C-2. Step 2: Its neighbours are C-1 (CH₃, 3 H) and C-3 (CH₂, 2 H). Step 3: 3 + 2 = 5. Answer: 5 α-hydrogens.
2. Arrange in increasing order of reactivity towards HCN: CH₃COCH₃, HCHO, CH₃CHO.
Step 1: More alkyl groups = more crowding and less δ+ on carbon = slower. Step 2: HCHO has 0 alkyl, CH₃CHO has 1, CH₃COCH₃ has 2. Answer: CH₃COCH₃ < CH₃CHO < HCHO.
3. Give a chemical test to tell propanal from propanone.
Step 1: Propanal is an aldehyde (has H on C=O); propanone is a ketone. Step 2: Warm each with Tollens reagent. Step 3: Propanal gives a silver mirror; propanone does not. (Also: propanone gives yellow iodoform; propanal does not.)
4. Write the aldol product of propanal and the product after heating.
Step 1: The α-carbon of CH₃CH₂CHO is the CH₂. Base removes an H from it: CH₃CH⁻CHO. Step 2: This attacks the C=O of a second propanal. Step 3: Aldol: CH₃CH₂CH(OH)CH(CH₃)CHO (3-hydroxy-2-methylpentanal). Step 4: Heat removes water: CH₃CH₂CH=C(CH₃)CHO (2-methylpent-2-enal).
5. What happens when benzaldehyde is heated with concentrated NaOH? Why not aldol?
Step 1: C₆H₅CHO has no α-H (the C=O carbon is joined to the ring carbon, which has no H). Step 2: So aldol is not possible; Cannizzaro occurs. Step 3: 2 C₆H₅CHO + NaOH → C₆H₅CH₂OH + C₆H₅COONa. Answer: benzyl alcohol and sodium benzoate.
6. A compound C₃H₆O gives an orange precipitate with 2,4-DNP and a yellow precipitate with I₂/NaOH, but no silver mirror. Identify it.
Step 1: 2,4-DNP positive → it has C=O. Step 2: No Tollens → it is not an aldehyde, so it is a ketone. Step 3: Iodoform positive → it has CH₃CO–. Step 4: The only C₃ ketone is CH₃COCH₃. Answer: propanone (acetone).
7. Convert ethanal to propan-2-ol.
Step 1: We need to add one CH₃ to the C=O carbon: use CH₃MgBr. Step 2: CH₃CHO + CH₃MgBr → CH₃CH(OMgBr)CH₃. Step 3: Add water (H⁺): CH₃CH(OH)CH₃. Answer: propan-2-ol, a secondary alcohol.
Common mistakes
- Thinking ketones give the Tollens test. Only aldehydes (and a few special compounds) give the silver mirror.
- Counting the H on the aldehyde C=O carbon as an α-H. α-H is on the NEXT carbon; HCHO and C₆H₅CHO have none.
- Saying aldehydes have higher boiling points than alcohols. Alcohols are higher because they hydrogen-bond with each other.
- Mixing up Clemmensen (C=O → CH₂) with NaBH₄ reduction (C=O → CH–OH).