MHT-CET Chemistry · Aldehydes, Ketones and Carboxylic Acids

Preparation of Aldehydes and Ketones

Aldehydes come from acid chlorides (Rosenmund, H₂/Pd–BaSO₄), from nitriles (Stephen, SnCl₂/HCl; or DIBAL-H) and from toluene (Etard, CrO₂Cl₂); ketones come from acid chlorides with dialkylcadmium, from nitriles with a Grignard reagent, and Grignard reagents themselves come from R–X and magnesium in dry ether.

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Why this matters

45 PYQs, 2 HARD — the largest page in the chapter and pure name-to-reagent recall. Of the thirty set from 2021 to 2025, fourteen are the three aldehyde routes (Rosenmund's reagent, Stephen's product from benzonitrile or isopropyl cyanide, DIBAL-H keeping a C=C); twelve are the Grignard and cadmium routes (dimethylcadmium + acetyl chloride → propanone appears five times; benzonitrile + PhMgBr → benzophenone; which carbonyl gives a 2° alcohol); four are Etard and two reaction-figure rows. Three cards.

Concept 1 of 3: Rosenmund, Stephen and DIBAL-H: Three Ways to an Aldehyde

Each route stops at the aldehyde by design. Rosenmund hydrogenates an acid chloride over palladium POISONED with BaSO₄ so the aldehyde is not reduced further. Stephen reduces a nitrile with SnCl₂/HCl to an imine salt that water hydrolyses to the aldehyde. DIBAL-H does the same job on a nitrile at low temperature and leaves a C=C untouched.

Definition

Rosenmund: $\text{RCOCl} \xrightarrow{\text{H}_2,\ \text{Pd-BaSO}_4} \text{RCHO}$ . Benzoyl chloride → benzaldehyde. Reagent R = H₂/Pd–BaSO₄ (not DIBAL-H, not CO/HCl).
Stephen: $\text{RCN} \xrightarrow{\text{SnCl}_2/\text{HCl}} \text{RCH=NH·HCl} \xrightarrow{\text{H}_3\text{O}^+} \text{RCHO} + \text{NH}_4\text{Cl}$ . Benzonitrile → benzaldehyde; isopropyl cyanide → 2-methylpropanal.
DIBAL-H $\text{AlH(i-Bu)}_2$ : nitrile → aldehyde, C=C survives — pent-3-enenitrile → pent-3-enal; hex-3-enenitrile → hex-3-enal. Also ester → aldehyde.
Gattermann–Koch: benzene + CO/HCl, AlCl₃ → benzaldehyde (NOT CrO₃/Ac₂O, which is the Etard-like oxidation of toluene).

Three aldehyde routes

\text{RCOCl} \xrightarrow{\text{H}_2/\text{Pd-BaSO}_4} \text{RCHO};\quad \text{RCN} \xrightarrow{\text{SnCl}_2/\text{HCl};\ \text{H}_3\text{O}^+} \text{RCHO};\quad \text{RCN} \xrightarrow{\text{DIBAL-H};\ \text{H}_3\text{O}^+} \text{RCHO}

Worked example

Give the product of (i) propanoyl chloride under Rosenmund conditions and (ii) butanenitrile by the Stephen reaction.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 10th May Shift 2 · Q93Moderate

Example 1 · Aldehydes, Ketones and Carboxylic Acids · Preparation Methods of Aldehydes and Ketones

Identify the product obtained when benzonitrile is reduced by stannous chloride in presence of hydrochloric acid followed by acid hydrolysis.

Assigning DIBAL-H to Rosenmund

DIBAL-H reduces NITRILES and esters; Rosenmund is hydrogen over poisoned palladium on an ACID CHLORIDE. Both give aldehydes, and the paper lists both reagents in one question.

Drill 13 more on rosenmund, stephen and dibal-h: three ways to an aldehyde

Concept 2 of 3: Grignard and Dialkylcadmium: Ketones and Alcohols

A Grignard reagent (R–MgX, from R–X and magnesium in DRY ether — water kills it) adds to any carbonyl: methanal gives a primary alcohol with one more carbon, other aldehydes a secondary alcohol, ketones a tertiary one; with a nitrile it gives a ketone after hydrolysis, and with dry ice an acid. Dialkylcadmium, made from the Grignard and CdCl₂, is too weak to attack ketones, so with an acid chloride it stops at the ketone.

Definition

Grignard: magnesium metal + alkyl halide in dry ether. Not Zn, not Mg(OH)₂, not aqueous.
$\text{HCHO} + \text{RMgX} \to \text{RCH}_2\text{OH}$ (1°, one carbon more); $\text{CH}_3\text{CHO} \to$ 2° alcohol; $\text{CH}_3\text{COCH}_3 \to$ 3° alcohol. Secondary alcohol from CH₃CHO, not from HCHO or a ketone.
Nitrile: $\text{C}_6\text{H}_5\text{CN} + \text{C}_6\text{H}_5\text{MgBr} \xrightarrow{\text{H}_2\text{O}} \text{benzophenone}$ . Dry ice: $\text{CH}_3\text{MgBr} + \text{CO}_2 \to \text{CH}_3\text{COOMgBr} \to$ ethanoic acid.
Cadmium: $2\text{CH}_3\text{MgBr} + \text{CdCl}_2 \to (\text{CH}_3)_2\text{Cd}$ (A); $(\text{CH}_3)_2\text{Cd} + 2\text{CH}_3\text{COCl} \to 2\text{CH}_3\text{COCH}_3$ (propanone) + CdCl₂. Substrate for propanone = ethanoyl chloride; benzoyl chloride + $(\text{CH}_3)_2\text{Cd}$ → acetophenone (benzophenone would need $(\text{C}_6\text{H}_5)_2\text{Cd}$ ).

Cadmium route to ketones

\text{R}_2\text{Cd} + 2\,\text{R'COCl} \to 2\,\text{R'COR} + \text{CdCl}_2

Worked example

Plan butanone from an acid chloride and an organocadmium, and name the Grignard partner that turns propanenitrile into pentan-3-one.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift II · Q71Moderate

Example 2 · Aldehydes, Ketones and Carboxylic Acids · Preparation Methods of Aldehydes and Ketones

Identify the product '

B

' in the following sequence of reactions. Methyl magnesium bromide

\xrightarrow{CdCl_{2}}

\ \xrightarrow{CH_{3}COCl}\ B

Stopping at dimethylcadmium

Dimethylcadmium is A, the intermediate. The question asks for B, after acetyl chloride — propanone. 'Dimethyl cadmium' is option (a) every time.

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Concept 3 of 3: Etard Reaction and Hydrolysis Routes

Toluene's methyl group is oxidised by chromyl chloride (CrO₂Cl₂) in CS₂ to a chromium complex — the Etard complex — which water hydrolyses to benzaldehyde. CrO₃ in acetic anhydride does the same via benzylidene diacetate. A gem-dihalide or the Etard complex both need only water for the final step to the carbonyl.

Definition

Etard: toluene $\xrightarrow{\text{CrO}_2\text{Cl}_2,\ \text{CS}_2}$ chromium complex (A) $\xrightarrow{\text{H}_3\text{O}^+}$ benzaldehyde (B). Reagent = chromyl chloride.
Toluene + $\text{CrO}_3/(\text{CH}_3\text{CO})_2\text{O}$ → benzylidene diacetate → benzaldehyde on hydrolysis.
Benzal chloride $\text{C}_6\text{H}_5\text{CHCl}_2$ + water (aq. KOH) → benzaldehyde; the reagent that finishes a gem-dihalide is H₂O.
Alkaline KMnO₄ would take the side chain all the way to benzoic acid — not the aldehyde.

Etard reaction

\text{C}_6\text{H}_5\text{CH}_3 \xrightarrow{\text{CrO}_2\text{Cl}_2/\text{CS}_2} \text{Etard complex} \xrightarrow{\text{H}_3\text{O}^+} \text{C}_6\text{H}_5\text{CHO}

Worked example

How would you get benzaldehyde from toluene without reaching benzoic acid, and what does the same toluene give with alkaline KMnO₄?

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 4th May Shift 2 · Q66Moderate

Example 3 · Aldehydes, Ketones and Carboxylic Acids · Preparation Methods of Aldehydes and Ketones

Identify the product 'B' in the following reaction. Toluene

\xrightarrow{\text{CrO}_2\text{Cl}_2/\text{CS}_2} A \xrightarrow{H_3O^+} B

Reading the Etard intermediate as benzal chloride

Chromyl chloride does not chlorinate the side chain; A is a chromium complex. The paper offers benzal chloride as option (a) for A's product; the answer to 'B' is benzaldehyde either way.

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Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

Rosenmund, Stephen and DIBAL-H: Three Ways to an Aldehyde
Three aldehyde routes
$\text{RCOCl} \xrightarrow{\text{H}_2/\text{Pd-BaSO}_4} \text{RCHO};\quad \text{RCN} \xrightarrow{\text{SnCl}_2/\text{HCl};\ \text{H}_3\text{O}^+} \text{RCHO};\quad \text{RCN} \xrightarrow{\text{DIBAL-H};\ \text{H}_3\text{O}^+} \text{RCHO}$
Grignard and Dialkylcadmium: Ketones and Alcohols
Cadmium route to ketones
$\text{R}_2\text{Cd} + 2\,\text{R'COCl} \to 2\,\text{R'COR} + \text{CdCl}_2$
Etard Reaction and Hydrolysis Routes
Etard reaction
$\text{C}_6\text{H}_5\text{CH}_3 \xrightarrow{\text{CrO}_2\text{Cl}_2/\text{CS}_2} \text{Etard complex} \xrightarrow{\text{H}_3\text{O}^+} \text{C}_6\text{H}_5\text{CHO}$

Watch out for (3)

Assigning DIBAL-H to Rosenmund
→ Rosenmund, Stephen and DIBAL-H: Three Ways to an Aldehyde
Stopping at dimethylcadmium
→ Grignard and Dialkylcadmium: Ketones and Alcohols
Reading the Etard intermediate as benzal chloride
→ Etard Reaction and Hydrolysis Routes

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