Theory: Organic Chemistry — Amines
1. AgCN vs KCN — Ambident Nucleophile CN⁻
CN⁻ is an ambident nucleophile (can attack via C or N). AgCN: Ag⁺ is bonded to C → Ag−C≡N. When C₂H₅Cl reacts with AgCN: electrophilic C of C₂H₅ attacks the N end of AgCN → product is C₂H₅NC (isocyanide). KCN: K⁺ is ionic → CN⁻ free → C end attacks alkyl halide → product is C₂H₅CN (nitrile). Memory: AgCN → isocyanide (NC); KCN → nitrile (CN).
2. Hofmann Bromamide Reaction
RCONH₂ + Br₂ + 4NaOH → RNH₂ + 2NaBr + Na₂CO₃ + 2H₂O. The amide loses one carbon (CO₂ lost as Na₂CO₃). Product amine has one less carbon than original amide. C₂H₅CONH₂ (propionamide, 3C) → C₂H₅NH₂ (ethylamine, 2C). This is the Hofmann degradation/rearrangement. It's a method to prepare primary amines with one fewer carbon. Mechanism: Br₂ + NaOH brominates N → N-bromoamide → rearrangement → isocyanate → hydrolysis → amine.
3. Carbylamine Reaction — Isocyanide Test
RNH₂ (primary amine) + CHCl₃ + 3KOH(alc.) → RNC (isocyanide) + 3KCl + 3H₂O. Isocyanide has the structure R−N⁺≡C⁻ (or R−N=C). Extremely foul, unbearable smell. This is the carbylamine test — ONLY primary amines give it. Secondary and tertiary amines do NOT give carbylamine. Useful for: identifying 1° amines, distinguishing from 2° and 3° amines. Mechanism: CHCl₃ + 3KOH → :CCl₂ (dichlorocarbene) → reacts with RNH₂ → RNC.
4. Isocyanide vs Nitrile — Structure Comparison
Nitrile (R−CN = R−C≡N): carbon end attached to R. IUPAC: alkanenitrile or carbonitrile. Prepared from: R-X + KCN, or aldoxime dehydration. Nitriles are hydrolysed to carboxylic acids: R-CN + H₂O → RCOOH. Isocyanide (R−NC = R−N⁺≡C⁻): nitrogen end attached to R. IUPAC: isocyanide or isonitrile. Prepared from: R-X + AgCN, or carbylamine reaction. Isocyanides have foul smell. Hydrolysed to: R-NC + H₂O → R-NH₂ + CO (isocyanide hydrolysis gives amine).
5. Summary of CN⁻ Reactions with R-X
R-X + KCN → R-CN (nitrile) — C attacks. R-X + AgCN → R-NC (isocyanide) — N attacks. Reason: in KCN, CN⁻ is free ionic → thermodynamically favours attack from harder end (C) in polar aprotic. In AgCN, Ag coordinates to C, making C unavailable → N attacks. Alternative memory: K'CN' gives 'CN' (nitrile); Ag'CN' gives 'NC' (isocyanide = reversed attachment).
6. Other Important Amine Reactions
Hinsberg test: distinguish 1°, 2°, 3° amines using benzenesulfonyl chloride (C₆H₅SO₂Cl): 1°amine → sulfonamide (soluble in NaOH). 2°amine → sulfonamide (INsoluble in NaOH). 3°amine → no reaction. Diazonium salt: ArNH₂ + NaNO₂ + HCl (0-5°C) → ArN₂⁺Cl⁻ (diazonium salt). Used in coupling reactions to make azo dyes. Aliphatic amines: diazonium unstable → gives alcohol.
7. Reduction of Nitriles
Nitriles can be reduced to primary amines: R-CN + 4[H] →(LiAlH₄ or H₂/Ni) R-CH₂-NH₂. This is a method to prepare primary amines with one more carbon than the original alkyl halide. Example: C₂H₅CN + 4[H] → C₂H₅CH₂NH₂ (propylamine). Nitriles are also hydrolysed: R-CN + H₂O →(H⁺ or OH⁻) R-CONH₂ (amide) → R-COOH (carboxylic acid). Two-step hydrolysis.
8. Amines Classification and Properties
1° amine (RNH₂): one H replaced in NH₃. 2° amine (R₂NH): two H replaced. 3° amine (R₃N): three H replaced. Basicity: 3° > 2° > 1° > NH₃ (in gas phase, inductive effect). But in water (aqueous): 2° > 1° > 3° > NH₃ (solvation effect — 3° has no H for H-bonding with water → less solvated → less basic). Aromatic amines less basic than aliphatic (lone pair delocalised into ring). Aniline pKb=9·4; methylamine pKb=3·4.
Frequently Asked Questions
1. Why does AgCN give isocyanide but KCN gives nitrile? ⌄
AgCN: Ag⁺ is a soft metal ion that coordinates strongly to C (soft ligand). So in AgCN, the C end of CN is 'blocked' by Ag. When R-X reacts: the N end (now more available/nucleophilic) attacks the alkyl halide. Product: R-N≡C (isocyanide). KCN: K⁺ is a hard, ionic metal — doesn't coordinate covalently to CN⁻. CN⁻ is free. Thermodynamically, C is the preferred nucleophilic end (harder HOMO on C in HSAB theory context) → C attacks R-X → R-CN (nitrile).
2. What is the Hofmann rearrangement product from C₂H₅CONH₂? ⌄
C₂H₅CONH₂ (propionamide, 3 carbons) + Br₂ + NaOH → C₂H₅NH₂ (ethylamine, 2 carbons). One carbon is lost as CO₂ (forms Na₂CO₃). The amine formed has ONE LESS carbon than the original amide. This is the key feature of Hofmann degradation: RCONH₂ → RNH₂ (R goes from n+1 carbons to n carbons). It's used to step down the carbon chain.
3. What is Z and why does it smell foul? ⌄
Z = C₂H₅NC (ethyl isocyanide). Isocyanides (R-N≡C) are notoriously foul-smelling compounds — described as one of the most unpleasant odours in chemistry. The foul smell is used as a diagnostic criterion in the carbylamine test. The structure R−N⁺≡C⁻ has a dative N→C bond. Isocyanides are used as ligands in coordination chemistry and as building blocks in Ugi reactions (multicomponent synthesis in drug discovery).
4. How does the carbylamine reaction produce isocyanide? ⌄
Mechanism: CHCl₃ + 3KOH → :CCl₂ (dichlorocarbene) + KCl + 2H₂O. :CCl₂ + RNH₂ → RNH-CCl₂ → [RN=CCl₂] + HCl → [RN≡CCl] + KCl → RNC + KCl. Three equivalents of KOH are used (one to generate carbene, two to remove HCl). The reaction requires: (1) primary amine (−NH₂ group needed), (2) CHCl₃ (chloroform), (3) KOH (base), (4) heating. The isocyanide's foul smell confirms completion.
5. How to distinguish nitrile from isocyanide? ⌄
Structure: R-CN (nitrile) vs R-NC (isocyanide). Properties: isocyanides have characteristic unbearable foul smell; nitriles have a mild pleasant smell. Chemical tests: Hydrolysis of isocyanide gives amine (R-NC + H₂O → R-NH₂ + CO); nitrile gives amide then carboxylic acid. Reduction: both give amines but different products. AgNO₃: both give precipitate if Ag⁺ present (not selective). IR: N≡C stretch at different frequencies (isocyanide ~2100-2200 cm⁻¹, nitrile ~2200-2260 cm⁻¹).
6. What other compounds have foul smells in chemistry? ⌄
H₂S (rotten egg), isocyanides (worst smell in lab), ethyl mercaptan (C₂H₅SH, skunk smell, added to natural gas), thiols generally (rotten, musty), putrescine and cadaverine (decomposing flesh), skatole (faeces), butyric acid (rancid butter), trimethylamine (rotten fish). In contrast: pleasant smells come from esters (fruit smells), terpenes (citrus, pine), vanillin (vanilla). The carbylamine test is always described as giving an 'unpleasant' or 'offensive' odour — a distinguishing feature in NEET questions.
7. What is the mechanism of Hofmann rearrangement? ⌄
RCONH₂ + Br₂ → RCONHBr (N-bromoamide) + HBr. RCONHBr + NaOH → RCONBr⁻Na⁺ → [RCO-N:] intermediate (nitrene-like, actually concerted) where CO and N migrate → RNCO (isocyanate). RNCO + 2NaOH → RNH₂ + Na₂CO₃. Key: R migrates from C to N (rearrangement). The carbon chain shortens by 1. The reaction is named after August Wilhelm von Hofmann (1818-1892).
8. What are industrial uses of nitriles and isocyanides? ⌄
Nitriles: acrylonitrile (CH₂=CHCN) → polyacrylonitrile (acrylic fibres like Orlon), ABS plastic (Acrylonitrile-Butadiene-Styrene), adiponitrile → nylon 6,6. Acetonitrile (CH₃CN): common laboratory solvent. Isocyanides (R-NC): used in Passerini reaction and Ugi reaction — multicomponent reactions to build complex drug-like molecules rapidly. Methyl isocyanate (CH₃NC, MIC): tragically known from Bhopal disaster (1984) — toxic industrial chemical (note: MIC = methyl isocyanate, different from methyl isocyanide CH₃NC).