Aldehydes, Ketones & Carboxylic Acids

Syllabus Unit: Organic Chemistry

Why it matters

Chapter overview

Carbonyl chemistry runs on one electrophilic carbon (C=O), but subtle differences in what's attached to it — an H versus two alkyl groups, or an -OH that can ionise — create entirely different reactivity classes: aldehydes add nucleophiles fastest, ketones resist oxidation, and carboxylic acids are acidic enough to form resonance-stabilised anions. NEET tests this unit through named preparation methods (Rosenmund, Friedel-Crafts acylation), the nucleophilic addition mechanism, the reactivity ladder of acid derivatives, and the classic identification tests (Tollens', Fehling's, iodoform).

Chemistry Syllabus Organic Chemistry ~50% weightage

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1Acid ChlorideR-COCl starting material

2H2 / Pd-BaSO4 (poisoned)Catalyst deliberately weakened

3Stops at AldehydeNo further reduction to alcohol

4R-CHO ProductIsolated aldehyde

1Preparation of Aldehydes & Ketones

A poisoned catalyst, a mild oxidant, and a Lewis acid catalyst give three completely different, carefully controlled routes to a carbonyl group.

Learning Objectives

Describe Rosenmund reduction (acid chloride + H2, Pd/BaSO4 poisoned with S/quinoline) to prepare an aldehyde without over-reduction to the alcohol.
Explain preparation of aldehydes via controlled oxidation of 1° alcohols using PCC (pyridinium chlorochromate).
Describe preparation of ketones via Friedel-Crafts acylation of benzene (acid chloride/anhydride + AlCl3).
Explain ozonolysis of an alkene as a method to obtain aldehydes and/or ketones, depending on the substitution pattern at the double bond.

Key Terms

Rosenmund ReductionSelective hydrogenation of an acid chloride to an aldehyde, using H2 over a Pd/BaSO4 catalyst poisoned with sulfur or quinoline to prevent over-reduction to the alcohol.

PCC (Pyridinium Chlorochromate)A mild oxidising agent that converts a 1° alcohol cleanly to an aldehyde (stopping there, unlike strong oxidants that continue to the acid) and a 2° alcohol to a ketone.

Friedel-Crafts AcylationElectrophilic aromatic substitution where an acid chloride or anhydride reacts with benzene under AlCl3 catalysis to install a ketone (aryl alkyl ketone) group on the ring.

Ozonolysis (Carbonyl Prep)O3 addition to an alkene followed by Zn/H2O workup cleaves the C=C bond, giving two carbonyl compounds (aldehyde and/or ketone) depending on the substitution at each alkene carbon.

Aldehydes and ketones share the same functional group but often need very different preparation strategies, since aldehydes are easily over-oxidised or over-reduced past the stage you want.

Rosenmund reduction: stopping exactly at the aldehyde

R-COCl + H2 →(Pd/BaSO4, poisoned with S or quinoline) R-CHO. A fully active Pd catalyst would keep reducing all the way to the primary alcohol; deliberately poisoning it lowers its activity just enough to stop cleanly at the aldehyde.

Controlled oxidation with PCC

PCC oxidises a 1° alcohol only as far as the aldehyde (unlike acidic K2Cr2O7/KMnO4, which continue on to the carboxylic acid), because it is a milder oxidant that does not provide the aqueous conditions needed for the aldehyde to hydrate and be oxidised further.

Friedel-Crafts acylation: building aryl ketones

Benzene + R-COCl (or (RCO)2O) →(AlCl3) aryl alkyl ketone + HCl. The AlCl3 Lewis acid activates the acyl chloride into an acylium ion (R-C≡O+), a stabilised electrophile that substitutes onto the ring without the carbocation rearrangement problems seen in Friedel-Crafts alkylation.

Key PointOzonolysis reveals substitution pattern: a =CH2 terminal alkene carbon gives formaldehyde on cleavage; a =CHR carbon gives an aldehyde; a =CR2 carbon gives a ketone — working this out lets you deduce an unknown alkene's structure entirely from its ozonolysis products.