Preparation Methods

Aldehydes and ketones are among the most important carbonyl-containing functional groups in organic chemistry. Aldehydes possess a carbonyl group (C=O) at the terminal carbon of a chain, while ketones have the carbonyl flanked by two carbon substituents. Both classes are pivotal intermediates in synthesis, pharmaceuticals, and biological processes such as glycolysis. Understanding how to prepare them selectively is a foundational skill in organic chemistry. Several named reactions have been developed specifically to synthesise aldehydes and ketones with high selectivity. These include ozonolysis (cleavage of alkenes), the Rosenmund reduction (partial reduction of acyl chlorides), the Stephen reduction (reduction of nitriles), the Etard reaction (oxidation of methylbenzenes), and Friedel-Crafts acylation (electrophilic aromatic substitution). Each method targets a different starting material and employs distinct reagents and conditions. Choosing the correct preparation method depends on the available starting material, the desired product, and the functional-group tolerance of other parts of the molecule. For example, if an aldehyde must be prepared without over-reduction to an alcohol, the Rosenmund or Stephen reduction is preferred over a direct reduction. Mastering these five methods equips students to plan multi-step syntheses logically and efficiently.

Key takeaways
  • Ozonolysis
  • Rosenmund Reduction
  • Stephen Reduction
  • Etard Reaction
  • Friedel-Crafts Acylation

Ozonolysis

Ozonolysis cleaves the C=C double bond of an alkene using ozone (O₃) followed by a reductive workup (Zn/H₂O or Me₂S) or oxidative workup (H₂O₂). In reductive workup, each carbon of the double bond becomes a carbonyl: terminal carbons (=CH₂) give formaldehyde (HCHO), internal CH= carbons give aldehydes (RCHO), and internal CR= carbons give ketones (RCOR'). Mechanism: ozone adds across the double bond to form a molozonide (1,2,3-trioxolane), which rearranges to an ozonide (1,2,4-trioxolane). Reductive workup then cleaves the ozonide. Example: 2-butene (CH₃CH=CHCH₃) + O₃ then Zn/H₂O → 2 molecules of ethanal (CH₃CHO).

Rosenmund Reduction

The Rosenmund reduction converts an acyl chloride (RCOCl) to an aldehyde (RCHO) using hydrogen gas over a poisoned palladium-on-barium sulfate catalyst (Pd/BaSO₄). The catalyst is 'poisoned' with sulfur or quinoline to prevent over-reduction to the primary alcohol. The mechanism is a surface-catalysed hydrogenolysis: H₂ adsorbs onto the Pd surface; the acyl chloride coordinates; HCl is eliminated and RCHO desorbs. Because the catalyst is deliberately deactivated, reduction stops at the aldehyde stage. Example: benzoyl chloride (C₆H₅COCl) + H₂ →(Pd/BaSO₄) benzaldehyde (C₆H₅CHO) + HCl.

Stephen Reduction

The Stephen reduction converts a nitrile (RCN) to an aldehyde (RCHO) via a two-step sequence. First, anhydrous stannous chloride (SnCl₂) and dry HCl reduce the nitrile in ethereal solution to form an imine–tin complex (aldimine stannichloride salt, [RCH=NH₂]⁺[SnCl₃]⁻ type). Hydrolysis of this intermediate with water gives the aldehyde. Since nitriles are easily prepared from primary alkyl halides via SN2 with CN⁻, the Stephen reduction provides a route to aldehydes one carbon longer than the starting halide. Example: benzonitrile (C₆H₅CN) →(SnCl₂/HCl, then H₂O) benzaldehyde (C₆H₅CHO).

Etard Reaction

The Etard reaction oxidises a methyl group directly attached to a benzene ring (ArCH₃, a toluene derivative) to an aromatic aldehyde (ArCHO) using chromyl chloride (CrO₂Cl₂) in carbon tetrachloride (CCl₄) as solvent. A complex brown chromium intermediate (the Etard complex) forms first; hydrolysis with water liberates the aldehyde. The reaction is selective for the first oxidation state, avoiding over-oxidation to a carboxylic acid. Example: toluene (C₆H₅CH₃) + 2 CrO₂Cl₂ → [complex] →H₂O benzaldehyde (C₆H₅CHO). Note: the reagent is corrosive and toxic, so modern laboratories often prefer alternative oxidants.

Friedel-Crafts Acylation

Friedel-Crafts acylation introduces a ketone group onto an aromatic ring. An acyl chloride (RCOCl) or acid anhydride ((RCO)₂O) reacts with an aromatic compound in the presence of a Lewis acid catalyst, most commonly anhydrous aluminium chloride (AlCl₃). The Lewis acid activates the acyl chloride to generate an acylium ion (RCO⁺) electrophile. This electrophile attacks the π system of the ring, forming a sigma complex (arenium ion), followed by proton loss to restore aromaticity. Unlike Friedel-Crafts alkylation, acylation does not suffer from carbocation rearrangements, and the electron-withdrawing ketone product deactivates the ring, ensuring monosubstitution. Example: benzene + CH₃COCl →(AlCl₃) acetophenone (C₆H₅COCH₃) + HCl.

The Key Equation

\ceC6H5COCl+H2−>[(Pd/BaSO4)][Δ]C6H5CHO+HCl\ce{C6H5COCl + H2 ->[(Pd/BaSO4)][\Delta] C6H5CHO + HCl}
  • \ce{C6H5COCl} — Benzoyl chloride (mol): The acyl chloride starting material; the carbonyl carbon bears a chlorine leaving group.
  • \ce{H2} — Hydrogen gas (mol): Reductant that supplies two hydrogen atoms; adsorbs onto the Pd catalyst surface.
  • \ce{Pd/BaSO4} — Poisoned palladium catalyst (—): Pd deposited on barium sulfate and deactivated with sulfur or quinoline to prevent over-reduction.
  • \ce{C6H5CHO} — Benzaldehyde (mol): The aldehyde product; the terminal carbonyl group is characteristic of aldehydes.
  • \ce{HCl} — Hydrogen chloride (mol): By-product formed when the chlorine leaving group is removed during hydrogenolysis.

Worked Example

Problem. A student has 2-methylpropene (isobutylene, (CH₃)₂C=CH₂) and wishes to use ozonolysis with reductive workup to identify its carbonyl products. Predict the two carbonyl compounds formed and explain your reasoning.

\ce(CH3)2C=CH2−>[(1) O3][(2) Zn/H2O](CH3)2C=O+HCHO\ce{(CH3)2C=CH2 ->[(1)\,O3][(2)\,Zn/H2O] (CH3)2C=O + HCHO}

Answer. The two products are acetone (propan-2-one) and formaldehyde (methanal).

Figure
Diagram
Friedel-Crafts Acylation MechanismStep 1: Acylium Ion FormationCH₃COCl + AlCl₃→CH₃C≡O⁺ + AlCl₄⁻(acylium ion)Step 2: Electrophilic Attack on Benzene Ringbenzeneπ electrons+CH₃—C≡O⁺electrophile→sigmacomplex(arenium)COCH₃Step 3: Proton Loss — Restore Aromaticityareniumion→aromaticringCOCH₃+ H⁺ (taken up by AlCl₄⁻ → AlCl₃ + HCl)Overall: C₆H₆ + CH₃COCl →(AlCl₃) C₆H₅COCH₃ + HCl
Origins · 1 minCharles Adolphe Wurtz and how this was discovered
CA
Charles Adolphe Wurtz
1817–1884 · French

Wurtz made foundational contributions to organic chemistry, including studies on aldehydes and the development of the Wurtz reaction for coupling alkyl halides. His work helped establish the systematic understanding of functional groups that underpins modern preparation methods for aldehydes and ketones. Wurtz was a fierce advocate of atomic theory at a time when many chemists still doubted the physical reality of atoms — he famously opened his textbook with the line 'Chemistry is a French science.'

  • Reactions of Aldehydes & Ketones — Once aldehydes and ketones are prepared, understanding their nucleophilic addition, oxidation, and reduction reactions is the natural next step.
  • Acyl Chlorides — Both Rosenmund reduction and Friedel-Crafts acylation use acyl chlorides as starting materials, so understanding their structure and reactivity is essential.
  • Reactions of Alkenes — Ozonolysis is an alkene reaction; a firm grasp of alkene structure and addition reactions provides the mechanistic context for understanding ozonide formation.
  • Electrophilic Aromatic Substitution — Friedel-Crafts acylation is a specific type of EAS; understanding the general mechanism of EAS (sigma complex formation and proton loss) is prerequisite knowledge.
  • Nitriles — The Stephen reduction starts from a nitrile; knowledge of nitrile synthesis from alkyl halides and their hydrolysis reactions contextualises this preparation method.