12 min readPathway sheet1-mark MCQ on a named reaction; 2 or 3-mark 'write the reaction' for Rosenmund, Stephen, Etard, Gattermann-Koch; often part of a conversion; also a NEET favourite

Preparation of aldehydes and ketones

Aldehydes and ketones come from oxidising alcohols, from alkenes and alkynes, and from named reactions: Rosenmund, Stephen, Etard, Gattermann-Koch and Friedel-Crafts acylation.

Rosenmund reductionRCOCl + H₂ → RCHO + HCl, over Pd on BaSO₄ (a poisoned catalyst)

Etard reactionToluene + CrO₂Cl₂ (chromyl chloride) in CS₂, then H₃O⁺ → benzaldehyde

Gattermann-Koch reactionBenzene + CO + HCl, with anhydrous AlCl₃ or CuCl → benzaldehyde

Friedel-Crafts acylationC₆H₆ + RCOCl, anhydrous AlCl₃ → C₆H₅COR + HCl (a ketone)

General methods for both aldehydes and ketones

1★
Oxidation of alcohols: RCH₂OH → (PCC, or CrO₃ in an anhydrous medium) RCHO; R₂CHOH → (K₂Cr₂O₇ + dil. H₂SO₄, or CrO₃) R₂CO
Primary alcohols give aldehydes, secondary alcohols give ketones. A mild oxidant such as PCC (pyridinium chlorochromate) stops a primary alcohol at the aldehyde stage
2
Dehydrogenation: RCH₂OH → (Cu, 573 K) RCHO + H₂; R₂CHOH → (Cu, 573 K) R₂CO + H₂
Alcohol vapour is passed over a heated metal catalyst (Cu or Ag). Suitable for volatile alcohols; used in industry
3
Ozonolysis of alkenes: CH₃CH=CHCH₃ → (i) O₃ (ii) Zn dust + H₂O → 2CH₃CHO
Each C=C carbon becomes a C=O carbon. (CH₃)₂C=CH₂ gives propanone and methanal. Zinc dust stops the products being oxidised further
4
Hydration of alkynes: HC≡CH + H₂O → (dil. H₂SO₄, HgSO₄) CH₃CHO
Ethyne gives ethanal. Other alkynes give ketones: CH₃C≡CH + H₂O → CH₃COCH₃

★ key step

Methods only for aldehydes

1★
Rosenmund reduction: C₆H₅COCl + H₂ → (Pd-BaSO₄) C₆H₅CHO + HCl
Acyl chloride to aldehyde. BaSO₄ 'poisons' the Pd so the aldehyde is not reduced further to an alcohol
2
Stephen reaction: R-C≡N → (SnCl₂ + HCl) R-CH=NH → (H₃O⁺) R-CHO
Stannous chloride and HCl reduce the nitrile to an imine, which water hydrolyses to the aldehyde
3
DIBAL-H reduction: R-C≡N → (i) AlH(i-Bu)₂ (ii) H₂O → R-CHO; CH₃(CH₂)₉COOC₂H₅ → (i) DIBAL-H (ii) H₂O → CH₃(CH₂)₉CHO
Diisobutylaluminium hydride reduces nitriles and esters to aldehydes and leaves a C=C double bond untouched: CH₃CH=CHCH₂CH₂CN gives CH₃CH=CHCH₂CH₂CHO
4★
Etard reaction: C₆H₅CH₃ → (CrO₂Cl₂ in CS₂) chromium complex → (H₃O⁺) C₆H₅CHO
Chromyl chloride oxidises the methyl group of toluene to CHO
5
C₆H₅CH₃ → (CrO₃, (CH₃CO)₂O, 273 to 283 K) C₆H₅CH(OCOCH₃)₂ → (H₃O⁺, heat) C₆H₅CHO
Acetic anhydride traps the aldehyde as benzylidene diacetate, so it is not oxidised on to benzoic acid
6
Side-chain chlorination: C₆H₅CH₃ + 2Cl₂ → (hν) C₆H₅CHCl₂ + 2HCl; then C₆H₅CHCl₂ + H₂O → (373 K) C₆H₅CHO + 2HCl
Benzal chloride is hydrolysed to benzaldehyde. This is a commercial method
7★
Gattermann-Koch reaction: C₆H₆ + CO → (HCl, anhydrous AlCl₃ or CuCl) C₆H₅CHO
Benzene (or a derivative) is treated with carbon monoxide and HCl

★ key step

Methods only for ketones

1
Make the dialkylcadmium: 2RMgX + CdCl₂ → R₂Cd + 2Mg(X)Cl
From a Grignard reagent and cadmium chloride
2★
From acyl chlorides: 2R'COCl + R₂Cd → 2R'COR + CdCl₂; for example 2CH₃COCl + (CH₃)₂Cd → 2CH₃COCH₃ + CdCl₂
3
From nitriles: CH₃CH₂C≡N + C₆H₅MgBr → (ether) imine salt → (H₃O⁺) C₆H₅COCH₂CH₃
The Grignard reagent adds to C≡N; hydrolysis gives a ketone, here 1-phenylpropan-1-one (propiophenone)
4★
Friedel-Crafts acylation: C₆H₆ + CH₃COCl → (anhydrous AlCl₃) C₆H₅COCH₃ + HCl
Benzoyl chloride, C₆H₅COCl, gives benzophenone in the same way

★ key step

Rosenmund vs Stephen: both give aldehydes

Rosenmund reductionStarts from an acyl chloride, RCOCl. Reagent: H₂ over Pd-BaSO₄. By-product: HCl.

Stephen reactionStarts from a nitrile, RCN. Reagent: SnCl₂ + HCl, then hydrolysis (H₃O⁺). Goes through an imine, RCH=NH.

Etard and Gattermann-Koch also give benzaldehyde: Etard starts from toluene, Gattermann-Koch from benzene.

Named reactions at a glance

Reaction	Start	Reagents and conditions	Product
Rosenmund reduction	RCOCl	H₂, Pd-BaSO₄	RCHO
Stephen reaction	RCN	SnCl₂ + HCl, then H₃O⁺	RCHO
DIBAL-H reduction	RCN or RCOOR'	(i) DIBAL-H (ii) H₂O	RCHO
Etard reaction	Toluene	CrO₂Cl₂ in CS₂, then H₃O⁺	Benzaldehyde
Gattermann-Koch reaction	Benzene	CO + HCl, anhydrous AlCl₃ or CuCl	Benzaldehyde
Ozonolysis	Alkene	(i) O₃ (ii) Zn + H₂O	Aldehydes and/or ketones
Dialkylcadmium route	R'COCl	R₂Cd	Ketone R'COR
Friedel-Crafts acylation	Benzene	RCOCl, anhydrous AlCl₃	Aryl ketone C₆H₅COR

In real life

The flavour of bitter almonds is mainly benzaldehyde, and industry makes it from toluene by side-chain chlorination and hydrolysis. Dehydrogenation of alcohol vapour over hot copper is another method used in factories.

Exam angle

2 marks: “Write the Rosenmund reduction. Why is the palladium catalyst poisoned with BaSO₄?” C₆H₅COCl + H₂ → (Pd-BaSO₄) C₆H₅CHO + HCl. BaSO₄ lowers the activity of Pd, so the aldehyde formed is not reduced further to an alcohol.

3 marks: “How will you obtain benzaldehyde from (i) toluene (ii) benzene (iii) benzoyl chloride? Give reagents and conditions.” (i) Etard: CrO₂Cl₂ in CS₂, then H₃O⁺ (or Cl₂/hν then H₂O at 373 K). (ii) Gattermann-Koch: CO + HCl, anhydrous AlCl₃/CuCl. (iii) Rosenmund: H₂, Pd-BaSO₄.

3 marks: “Write equations for: (i) Stephen reaction (ii) Friedel-Crafts acylation of benzene with ethanoyl chloride (iii) hydration of ethyne.” (i) RCN + SnCl₂/HCl → RCH=NH → (H₃O⁺) RCHO. (ii) C₆H₆ + CH₃COCl → (anhydrous AlCl₃) C₆H₅COCH₃ + HCl. (iii) HC≡CH + H₂O → (dil. H₂SO₄, HgSO₄) CH₃CHO.

Remember it

Start decides the name: acyl chloride → Rosenmund, nitrile → Stephen, toluene → Etard, benzene + CO → Gattermann-Koch, benzene + RCOCl → Friedel-Crafts ketone.