Carbonyl Compounds (Aldehydes & Ketones)

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Carbonyl Compounds — Key Facts for SLMC Medical (Sri Lanka)

• The C=O carbonyl group is the defining feature of aldehydes and ketones
• Aldehyde: carbonyl carbon bonded to at least one H (R–CHO); highest priority aldehyde is formaldehyde (HCHO)
• Ketone: carbonyl carbon bonded to two carbon atoms (R–CO–R); simplest ketone is acetone (CH₃COCH₃)
• Aldehydes are generally more reactive than ketones due to less steric hindrance
• ⚡ Exam tip: Nucleophilic addition reactions, Tollen’s test, and Fehling’s test are high-yield for SLMC

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Carbonyl Compounds — SLMC Medical (Sri Lanka) Study Guide

The Carbonyl Group

The C=O double bond consists of:

• One σ-bond (formed by sp² orbital overlap, planar at ~120°)
• One π-bond (formed by p orbital overlap above and below the plane)
• The oxygen atom carries a partial negative charge (δ⁻) and the carbon carries partial positive charge (δ⁺)

This polarization makes the carbonyl carbon electrophilic (susceptible to nucleophilic attack).

Nomenclature

Aldehydes

• IUPAC: replace –e of alkane with –al (e.g., methanal, ethanal)
• Formaldehyde (methanal): HCHO
• Acetaldehyde (ethanal): CH₃CHO
• No number needed for C₁ aldehyde; for C₂+ aldehyde, the carbonyl carbon is always C-1

Ketones

• IUPAC: replace –e of alkane with –one; number the chain to give carbonyl the lowest number
• Propanone (acetone): CH₃COCH₃
• Butanone: CH₃COC₂H₅
• For ketones with 5+ carbons, position number is required (e.g., pentan-2-one)

Physical Properties

Property	Aldehydes	Ketones
Boiling point	Lower than corresponding alcohol; increases with chain length	Similar trend
Water solubility	Formaldehyde + acetaldehyde miscible; decreases with chain length	Acetone miscible; decreases with size
Odor	Formaldehyde: pungent; others: fruity	Acetone: fruity; others: pleasant

Formaldehyde (methanal) is a colorless gas with a pungent odor. It is supplied as a 37% aqueous solution called formalin, used as a preservative and disinfectant.

Acetone is the simplest and most important ketone — a colorless liquid with BP 56.5°C. It is widely used as a solvent and innail polish remover.

Chemical Properties

1. Nucleophilic Addition

The π-bond of C=O breaks to allow nucleophilic attack. The carbonyl carbon accepts the nucleophile, and the π electrons go to O (forming O⁻), which then gets protonated.

a) Addition of HCN

• R–CHO + HCN → R–CH(OH)–CN (cyanohydrin)
• Acetone + HCN → acetone cyanohydrin
• This reaction is catalyzed by base or acid
• Cyanohydrins are important intermediates in organic synthesis

b) Addition of Grignard Reagents (RMgX)

• R–CHO + RMgX → R₂CH–OH (secondary alcohol after hydrolysis)
• HCHO + RMgX → R–CH₂–OH (primary alcohol)
• R₂C=O + RMgX → R₃C–OH (tertiary alcohol)
• This is one of the most important carbon-carbon bond-forming reactions

c) Addition of Alcohols (Acetal/Ketal Formation)

• Aldehyde + 2R’OH ⇌ acetal + H₂O (acid catalyzed)
• Ketone + 2R’OH ⇌ ketal + H₂O
• Acetals and ketals are used as protecting groups for carbonyl compounds

2. Oxidation Reactions

Test	Reagent	Aldehyde Result	Ketone Result
Tollen’s test	AgNO₃ + NH₃ (Ag(NH₃)₂⁺)	Silver mirror (Ag⁰) — positive	No reaction — negative
Fehling’s test	Fehling’s solution (Cu²⁺ tartrate)	Red Cu₂O precipitate	No reaction — negative
Benedict’s test	Benedict’s solution (Cu²⁺ citrate)	Red Cu₂O precipitate	No reaction — negative
KMnO₄	Cold dilute KMnO₄	Decolorizes	Decolorizes

Tollen’s reagent (diamminesilver(I) ion): Aldehydes reduce Ag⁺ → Ag⁰ (silver mirror); ketones do NOT react. Fehling’s/Benedict’s: Aldehydes reduce Cu²⁺ → Cu⁺ (red Cu₂O); ketones do NOT react.

These tests distinguish aldehydes from ketones.

3. Reduction Reactions

Reagent	Product
NaBH₄ (mild)	Primary alcohol (from aldehyde) / Secondary alcohol (from ketone)
LiAlH₄ (strong)	Same as above
H₂/Pt	Same as above
Clemmensen (Zn-Hg/HCl)	Alkane (converts C=O to CH₂)
Wolff-Kishner (N₂H₄/KOH)	Alkane

4. Reactions with Ammonia and Amines

• Aldehyde + NH₃ → H(OH)C–NH₂ (hemiaminal) → unstable
• Aldehyde + primary amine → imine (Schiff base) + H₂O
• Aldehyde + secondary amine → enamines

5. Haloform Reaction

Methyl ketones (CH₃CO–R) react with I₂/NaOH to give iodoform (yellow precipitate of CHI₃):

• CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃↓ + CH₃COONa + 3NaI + 3H₂O
• This also works for acetaldehyde (CH₃CHO)
• Iodoform test distinguishes methyl ketones from other ketones

6. Specific Aldehyde Reactions

Polymerization of formaldehyde:

• Formaldehyde polymerizes in water to form paraformaldehyde (solid) and formalin (solution)
• Formalin (37% formaldehyde) is used as a preservative

Clinical and Medical Relevance

• Formaldehyde is carcinogenic (nasal squamous cell carcinoma); used in fixatives for histology/pathology specimens
• Acetone accumulates in diabetic ketoacidosis (DKA) — breath smells fruity (“acetone breath”)
• Benzaldehyde (C₆H₅CHO): the simplest aromatic aldehyde; found in bitter almonds; amygdalin releases HCN
• Acetaldehyde is the toxic metabolite of ethanol metabolism — causes hangover symptoms; implicated in alcoholism liver damage
• Chloral hydrate (trichloroacetaldehyde): a sedative-hypnotic drug; the oldest synthetic sedative
• Ketone bodies: acetoacetate, β-hydroxybutyrate, acetone — produced during starvation/DKA; important in medical biochemistry

Common SLMC Exam Traps

• Tollen’s, Fehling’s, and Benedict’s tests are ALL negative for ketones — only aldehydes give positive results
• The haloform reaction requires a methyl ketone (CH₃CO–) or acetaldehyde structure — not all ketones give this test
• Grignard reagents react with CO₂ to give carboxylic acids (adds one more carbon to the chain) — useful synthetic method
• Acetone cyanohydrin is important in synthetic organic chemistry but is highly toxic (releases HCN)

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