Aldehydes Ketones

Aldehydes Ketones

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Aldehydes (R-CHO) and ketones (R-CO-R’) both contain the carbonyl group (C=O). In aldehydes the carbonyl carbon carries one hydrogen and one R group; in ketones it carries two R groups. Naming: aldehydes use the suffix -al, ketones use -one (lowest locant to C=O). The carbonyl carbon is electrophilic because oxygen pulls electron density, so these compounds undergo nucleophilic addition. Methanal (HCHO) is the most reactive carbonyl; reactivity order is HCHO > CH₃CHO > other aldehydes > ketones, governed by steric crowding and +I effect of alkyl groups. For CUET: memorise Tollens’ (silver mirror, aldehyde only) vs Fehling’s (red Cu₂O, aliphatic aldehyde only) vs 2,4-DNP (orange ppt, both). Three must-know reactions: aldol condensation (α-H needed), Cannizzaro (no α-H, e.g. HCHO, C₆H₅CHO), Clemmensen (Zn-Hg/HCl) and Wolff-Kishner (NH₂NH₂/KOH) both reduce C=O to CH₂.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nomenclature & Structure

In IUPAC naming the parent chain must include the C=O carbon. Aldehydes end in -al (methanal, ethanal, benzaldehyde) and ketones in -one with a locant (propan-2-one, butan-2-one). The C=O bond is polar (μ ≈ 2.3–2.7 D), with bond length ≈ 1.22 Å and sp² carbonyl carbon showing trigonal planar geometry (≈ 120° bond angle). The formyl group (-CHO) is the aldehyde functional group; the acyl group (RCO-) is the acyl fragment derived from a carboxylic acid.

Preparation Methods

• Oxidation of alcohols: PCC or Cu/Δ on primary (R-CH₂OH) yields aldehydes; on secondary (R-CHOH-R’) yields ketones. Strong oxidants (KMnO₄) over-oxidise aldehydes to acids.
• Ozonolysis of alkenes: terminal alkene gives formaldehyde; internal gives two carbonyl fragments.
• Rosenmund reduction: Pd/BaSO₄ + H₂ converts acyl chloride (RCOCl) to aldehyde without going further to alcohol.
• Friedel-Crafts acylation: RCOCl + AlCl₃ + benzene → aryl ketone (R-CO-C₆H₅).

Nucleophilic Addition Mechanism

The Nu⁻ attacks the electrophilic carbonyl carbon; electrons shift onto oxygen giving an alkoxide intermediate, which is protonated to a tetrahedral product.

R₂C=O + Nu⁻ → R₂C(O⁻)(Nu) → R₂C(OH)(Nu)

Common nucleophiles: HCN → cyanohydrin; NaHSO₃ → bisulfite adduct (crystalline, useful for separation); RMgX → alcohol after hydrolysis; NH₂OH → oxime; 2,4-DNP → orange 2,4-dinitrophenylhydrazone (identification test).

Reactivity Order

HCHO > CH₃CHO > RCHO > RCOR’ — formaldehyde is unhindered; alkyl groups donate electrons (+I) and block nucleophile access, lowering reactivity.

Distinguishing Tests

Test	Aldehyde	Ketone	Aromatic aldehyde
Tollens’ (AgNO₃/NH₃)	Silver mirror ✓	✗	✓
Fehling’s (Cu²⁺/tartrate)	Red Cu₂O ✓	✗	✗
2,4-DNP	Orange ppt ✓	Orange ppt ✓	✓

Aldol Condensation

Carbonyls with α-H (e.g. acetaldehyde, acetone) form an enolate in dilute NaOH; the enolate attacks another carbonyl giving a β-hydroxy carbonyl (aldol), which dehydrates on heating to an α,β-unsaturated carbonyl.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

α-Hydrogen and Enolisation

Hydrogens on the carbon α to C=O are acidic (pKa ≈ 17–20) because the resulting carbanion is stabilised by resonance with the carbonyl. This enolate is the key intermediate in aldol condensation, Claisen-Schmidt, Mannich, and haloform reactions. The iodoform test (I₂/NaOH) is positive for CH₃CO- groups (acetone, ethanol, secondary alcohols bearing CH₃CH(OH)-) and gives yellow CHI₃ plus a carboxylate.

Cannizzaro Reaction

Aldehydes without α-H (HCHO, C₆H₅CHO, (CH₃)₃C-CHO) undergo disproportionation in concentrated NaOH: one molecule is oxidised to the acid, another is reduced to the alcohol. With HCHO crossed Cannizzaro with non-enolisable aldehydes selectively reduces the latter (used to prepare benzyl alcohol from benzaldehyde).

Reduction Pathways

• Clemmensen (Zn-Hg/conc. HCl): reduces C=O to CH₂; acidic, used when substrate is acid-stable.
• Wolff-Kishner (NH₂NH₂/KOH, ethylene glycol, Δ): reduces C=O to CH₂ via hydrazone; basic, used when substrate is base-stable.
• NaBH₄: mild, reduces aldehydes and ketones to alcohols without touching C=C.
• LiAlH₄: stronger, also reduces esters and acids.

Worked Example (Aldol of Acetaldehyde)

2 CH₃CHO —NaOH(Δ)—> CH₃CH=CHCHO + H₂O (crotonaldehyde, E-but-2-enal). Intermediate β-hydroxybutyraldehyde (aldol) is not isolated under heating conditions.

Common Mistakes to Avoid

• Tollens’ vs Fehling’s trap: benzaldehyde gives Tollens’ silver mirror but no Fehling’s — Fehling’s works only for aliphatic aldehydes.
• 2,4-DNP and NaHSO₃ tests are positive for ketones too — they do not distinguish aldehyde from ketone.
• Writing aldol product directly as α,β-unsaturated carbonyl without the β-hydroxy intermediate loses a mark if the question asks for the addition product.
• Cannizzaro needs no α-H, not just any aldehyde — ethanal gives aldol, not Cannizzaro.
• In ketone numbering, C=O must receive the lowest locant: pentan-2-one, not pentan-4-one.

CUET-Specific Strategy

Aldehydes/Ketones contribute ~3% of CUET Chemistry (1–2 questions). Expect MCQs on (i) identifying the reagent (Tollens’, Fehling’s, 2,4-DNP), (ii) product of aldol/Cannizzaro/Clemmensen, (iii) IUPAC name from a structure, and (iv) reactivity ordering. Time per question ≈ 1 minute; revise reagent-specific outcomes rather than full mechanisms on exam eve.

Practice Prompts

1. Identify products when benzaldehyde is treated with concentrated NaOH.
2. Why does acetone fail to give a positive Fehling’s test but forms a cyanohydrin with HCN?

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