ChemistryCHEM 12
Carbonyl Compounds
Aldehydes, ketones, Aldol condensation, Cannizzaro reaction, Tollens, Fehling, iodoform test
Concept Core
Essential theory — everything NCERT tests on Carbonyl Compounds
STRUCTURE & REACTIVITY
C=O polar (δ+C, δ-O). Aldehydes (RCHO) more reactive than ketones (RCOR') — less steric hindrance, less electron donation by one alkyl group. Nucleophilic addition is main reaction type.
Boiling point: RCHO > RCOOH > ROH > RX > RH (relative to MW). Aldehydes have lower BP than corresponding alcohols (no H-bonding) but higher than alkanes (dipole-dipole).
DISTINCTION TESTS
Tollens' test: RCHO + Ag(NH₃)₂⁺ → RCOO⁻ + Ag (silver mirror). Aldehyde ONLY.
Fehling's solution: RCHO + Cu²⁺ → RCOO⁻ + Cu₂O (brick-red ppt). Aliphatic aldehydes; benzaldehyde fails.
Iodoform test: CH₃CO- or CH₃CH(OH)- + I₂/NaOH → CHI₃ (yellow ppt). Positive: ethanal, acetone, ethanol, propan-2-ol.
NAMED REACTIONS
Aldol condensation: base-catalysed. Aldehyde/ketone with α-H → β-hydroxy aldehyde/ketone → α,β-unsaturated (on heating). Ethanal: acetaldol → crotonaldehyde.
Cannizzaro reaction: aldehydes WITHOUT α-H (HCHO, benzaldehyde) with conc. KOH → one oxidised (acid), one reduced (alcohol). HCHO: formic acid + methanol.
NUCLEOPHILIC ADDITION REACTIONS
HCN: forms cyanohydrin. NH₂OH: forms oxime. N₂H₄: forms hydrazone. C₆H₅NHNH₂: phenylhydrazone. LiAlH₄ reduces to alcohol. NaBH₄: milder reductant.
Clemmensen reduction: C=O → CH₂ (Zn-Hg/HCl). Wolff-Kishner: C=O → CH₂ (N₂H₄ then KOH/glycol). Used to remove carbonyl from ring systems.
Fact & Formula Vault
High-yield facts, numbers, and formulas
Distinction Tests
Tollens: all aldehydes → Ag mirror
Fehling: aliphatic RCHO → Cu₂O (br-red)
Fehling fails: benzaldehyde, ketones
Iodoform: CH₃CO- group → CHI₃ (yellow)
Named Reactions
Aldol: α-H required, base catalyst
Cannizzaro: no α-H, conc. KOH
Clemmensen: C=O → CH₂ (acidic)
Wolff-Kishner: C=O → CH₂ (basic)
Iodoform Positive
Ethanal (CH₃CHO)
Acetone (CH₃COCH₃)
Ethanol (CH₃CH₂OH)
Propan-2-ol (secondary alcohol with CH₃CO after oxidation)
Worked Examples
NEET-style questions solved step-by-step
EASYBenzaldehyde gives Tollens' test positive but NOT Fehling's test. Why?▾
Benzaldehyde gives Tollens' test positive but NOT Fehling's test. Why?
Benzaldehyde is an aromatic aldehyde. Fehling's (ionic Cu²⁺ complex) is only reduced by aliphatic aldehydes. Tollens' (Ag(NH₃)₂⁺) is reduced by ALL aldehydes including aromatic.
MEDIUMWhich compound gives iodoform test positive: acetone, benzaldehyde, or methanol?▾
Which compound gives iodoform test positive: acetone, benzaldehyde, or methanol?
Acetone (CH₃COCH₃). Contains CH₃C=O group → gives yellow iodoform (CHI₃) precipitate with I₂/NaOH. Benzaldehyde has C₆H₅CO- (no CH₃ on carbonyl). Methanol: HCHO — no CH₃ group.
HARDFormaldehyde undergoes Cannizzaro reaction but NOT Aldol condensation. Why?▾
Formaldehyde undergoes Cannizzaro reaction but NOT Aldol condensation. Why?
Formaldehyde (HCHO) has no α-hydrogen — no C-H adjacent to C=O. So Aldol (requires α-H) is impossible. Cannizzaro requires NO α-H → HCHO satisfies this → disproportionation.
Mistake DNA
Common NEET traps for this chapter
⚠ Tollens vs Fehling scope
Tollens: ALL aldehydes (including benzaldehyde). Fehling: ONLY aliphatic (NOT benzaldehyde, NOT ketones).
✓ Fix: Tollens > Fehling in scope. Both fail for ketones.
⚠ Cannizzaro requires no alpha-H
Aldehydes WITH α-H do Aldol, NOT Cannizzaro. Cannizzaro: HCHO, (CH₃)₃CCHO, C₆H₅CHO — all without α-H.
✓ Fix: No α-H → Cannizzaro. With α-H → Aldol.
⚠ Iodoform test vs iodine test
Iodoform test (I₂/NaOH) tests for CH₃CO- or CH₃CH(OH)-. Iodine test (I₂/KI solution) tests for starch. Different tests, different reagents.
✓ Fix: Iodoform = CHI₃ (yellow ppt). Starch test = blue-black colour.
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
2–3 Q/year. Tollens/Fehling/iodoform distinctions, Aldol vs Cannizzaro conditions, iodoform positive compounds, Clemmensen vs Wolff-Kishner.
High-Yield
Benzaldehyde: Tollens ✓, Fehling ✗. Iodoform: CH₃CO- group. Cannizzaro: no α-H + conc. KOH. Aldol: α-H + dil. base.
Strategy
Learn Tollens/Fehling/iodoform as a 3-test table. Know which compounds give iodoform positive. Aldol vs Cannizzaro: the α-H presence determines which reaction.
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